Ebook The practice of computing using python (2nd edition) Part 2

(BQ) Part 2 book The practice of computing using python has contents: Functions—QuickStart, lists and tuples, more on functions, dictionaries and sets, more program development, introduction to classes, more on classes, program development with classes, files and exceptions II,... and other contents.

Chapter 7 Lists and Tuples

Chapter 8 More on Functions

Chapter 9 Dictionaries and Sets

Chapter 10 More Program Development

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• 6

C H A P T E R

Functions -QuickStart

Function, the exercise, or executing of some office or charge

T Blount, Glossographia, 1656, earliest definition offunction

in the Oxford English Dictionary

YOU HAVE SEEN MANY EXAMPLES OF USING PYTHON BUILT-IN FUNCTIONS AND

methods In Section 4.3.1, we took at look at how functions work and how we could use them

to manipulate string objects In this chapter, you’ll learn how to create your own functions.The concept of a function should be familiar from its use in mathematics Functions

in programming languages share many of the characteristics of mathematical functions butadd some unique features as well that make them more useful for programming

One of the main advantages for using functions is that they support divide-and-conquerproblem solving Remember divide-and-conquer from Section 3.4.5? This technique en-courages you to break a problem down into simpler subproblems, solve those subproblems,and then assemble the smaller solutions into the overall solutions Functions are a way todirectly encode the “smaller subproblem” solution You’ll see more about this as we workthrough this chapter

6.1 W H A T I S A F U N C T I O N ?

In mathematics, a function defines the relationship between values Consider the function

f (x )⇒√x If you provide a particular value of x , e.g., x = 4, the function will perform

a calculation (here the square root operation) and return the associated value, e.g., 2

Mathematicians term the variable x the argument to the function and say that the function

returns the value 2.

It is possible for a function to have multiple arguments—for example, a function that

cal-culates multiplication requires two arguments: f (x , y ) ⇒ x ∗ y However, a mathematical

257

258 C H A P T E R 6 • F U N C T I O N S - Q U I C K S T A R T

function returns only one object Note that the returned object can be a compound object—

an object with multiple elements For example, when working with graph paper, each point

is represented by by its x and y coordinates, an ordered pair (x , y ) A function that returns an

ordered-pair object does indeed return a single value, but it is a value with multiple elements,

the x and y values An example of such a function would be the mirror function The mirror function swaps the x and y values of the ordered pair: f (x , y ) ⇒ (y , x) The notion that

the object a function returns can be a compound object is very useful, including in Python.Python functions share many of the characteristics of mathematical functions In par-ticular, a Python function:

rRepresents a single operation to be performed

rTakes zero or more arguments as input

rReturns one value (potentially a compound object) as output

A function is also important because it represents an encapsulation By encapsulation,

we mean that the details of an operation can be hidden, providing coarser operations that

we as programmers can use without having to understand the function’s internal details Afunction can represent the performance of an operation without making the reader plow

through the details of how the operation is actually performed.

Consider thesqrt function with respect to encapsulation There are many ways tocalculate a square root that vary in accuracy and speed Remember, we saw a particularapproach that we called the Babylonian square root approach in Section 3.5 Other imple-mentations exist However, each implementation, each method, represents the “square root”operation Each method takes in an argument and returns that object’s square root As long

as the result is correct, we need not be concerned with the details of how the operation isperformed That is encapsulation

6.1.1 Why Have Functions?

As you progress in learning to program, you’ll move from essential programming elements

to important programming elements Selection (if) and repetition (while) are essentialprogramming constructs It is difficult to write any program without using these two essential

features Functions, on the other hand, allow us to write better code in the sense that it is

more readable Also, because functions allow us to divide programs into smaller pieces, they

assist in divide-and-conquer problem solving In that way, functions make programs easier

to write Finally, once a function is written, it can be shared and used by other programmers

(including ourselves) Thus, functions provide a powerful construct that we can use to makeour programs easier to read, write, and maintain

From this point on, many of the programming elements we introduce will makesome tasks easier and will subsequently make writing and understanding (that is, reading)the program easier You could write programs without them, but those programs would bemore difficult to read, write, and maintain

6 2 • P Y T H O N F U N C T I O N S 259

In more detail, functions provide the following features, which help in programming:

r Divide-and-conquer problem solving: As we have already mentioned, functionsdivide programs into smaller pieces, an approach that corresponds nicely to a divide-and-conquer approach to problem solving (introduced in Section 3.4.5)

r Abstraction: Functions provide a higher-level interface to operation that the functionimplements By encapsulating details, functions provide a programmer with a high-levelview of the function’s operation that could be implemented in multiple ways—possibly

by someone else By analogy, consider how you drive a car It has a simple interface thathides considerable complexity Your car has many options—for example, fuel-injection,turbo, or many others Does the existence of these options change that basic interfaceyou have to the car—i.e., turn the wheel, hit the gas, press the brake? Do you understandhow fuel injection provides better performance for your car than a carburetor does? Doyou care? You don’t have to know the difference: you simply drive the car Abstractionmeans that the operation the function represents (drive the car) can be implemented

in many ways that do not affect the basic car interface The underlying operations canalso be changed (upgrade the engine) without changing the interface

r Reuse: Once a function has been created, it can be reused If you write a function thatlocates strings in a database, then anywhere that functionality is needed can use thatfunction The more such a function is needed, the “simpler” the code that uses it

r Sharing: Once your function is well tested, you can distribute it for use by other people.Those people can further test the function, refine its capabilities, and through improve-ment provide a service to everyone in the field Useful functions can be collected intomodules for sharing Sharing of modules is one of Python’s strengths, as programmersgenerously share modules in many diverse areas

r Security: You hear again and again of security breaches in code: phone companies,computer distributors, software companies, etc One way to battle security issues isthe use of functions Small pieces of code can be more easily vetted and security (andother issues) more easily addressed Once they have been approved, they can be used

to construct other secure components, which can also be reused, and so on Buildingsecurely from the bottom up is one approach to writing secure code

r Simplification and readability (duplication removal): Because a function provides anencapsulation, it can be used to simplify a program and make it more readable Anywherethat multiple lines of code might be needed to address a problem, a function can replacethose lines If the replacement can be done in multiple places, the result is simpler code

260 C H A P T E R 6 • F U N C T I O N S - Q U I C K S T A R T

definition is the second way we have seen to create a name associated with an object in

Python, the first being an assignment statement

Consider an example function that converts Celsius temperatures to Fahrenheit

rFirst we need a conversion formula: C ∗ 1.8 + 32

rMathematics has a function invocation:

but the Python definition looks quite different:

def f(celsius float):

return celsius float* 1.8 + 32

As in mathematics, C is called an argument of the function Thecelsius float

variable is termed a parameter of the function Upon invoking the function, the argument

C’s value is passed to the parameter valuecelsius float for use in the calculation.2More detail on the passing of values between arguments and parameters can be found inSection 8.1.1

A function definition begins with the keyword def The Python definition workssimilarly to an assignment statement By executing adefstatement, a new name is created

in the namespace and a new object, a function object, is associated with that name As wehave observed elsewhere in Python, everything is an object, and functions are no different.Thedefis a compound statement, meaning that it provides a suite of other Pythonstatements and expressions that are part of the function The suite of statements are whatwill constitute the calculation done by the function object One of the special keywordsthat can be used in functions is the return statement The return indicates a valuethat is returned as output from the function invocation A function’s operation ends after a

returnstatement is executed A function may have more than onereturnstatement, butthe first one that is executed will end the function’s operation (We have used the phrase “to

invoke a function,” but an equivalent and frequently used phrase is “to call a function.”)

The general form of a function is shown in Figure 6.1

Let’s create a function to do our temperature conversion and use it in a session.Note the parts of the function: thedefandreturn keywords as well as the parameter(celsius float) Finally, notice the indentation for the suite of statements, part of the

2 Note that to adhere to our naming convention, the argument C should have been named better, such as celsius float, but we

6 2 • P Y T H O N F U N C T I O N S 261

def function_name (parameter1, parameter2) : statement1

statement2 return value_to_return

Keyword indicating function is being defined.

Return statement indicates the value returned when the func- tion finishes.

Function suite:

contains code to perform some action

Indented.

Suite of the function follows the colon.

FIGURE 6.1 Function parts.

function definition We will discuss the special comment with triple quotes (""") later In

essence, it is a brief description of the function and is called a docstring.

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

262 C H A P T E R 6 • F U N C T I O N S - Q U I C K S T A R T

NameError: name 'new fn' is not defined

>>> new fn = "a string object"

>>> new fn

'a string object'

>>> new fn()

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

TypeError: 'str' object is not callable

It is the parentheses that mark the invocation, and the name adjacent to the parentheses

is the function invoked In the session, we converted 100◦C to 212◦F and 0◦C to 32◦F Ifthe function is not defined, or if the object associated with a name is not a function object,then an error occurs, as is shown in the session

a larger program

In short, the flow of control with functions is to flow from the invocation (call) inthe calling program, to the function itself, and then back to the calling program with

the function’s return value being made available to the calling program Control within a

function remains sequential: one statement after another along with local control statementssuch asifandwhile

For every program, there is usually one “main” part where execution begins After that,the flow of control is based on the order of both statements and functions In particularfor functions, operation of a function is determined by when it is invoked, not when it isdefined Functions can be defined anywhere in the program file, as long as they are defined

before they are invoked Functions must be defined before use because the function name

must be placed in the namespace before it can be called

Figure 6.2 shows an example of function control flow

6 3 • F L O W O F C O N T R O L W I T H F U N C T I O N S 263

Main program statement fahrenheit = celsius_to_fahrenheit(25) statement

statement

def celsius_to_fahrenheit(celsius): val = celsius * 1.8 + 32 return val

Function

Return

Call

FIGURE 6.2 Function flow of control.

6.3.1 Function Flow in Detail

Consider two programs: the caller, the program presently executing, and the function In this

example, the caller is the main program, the program where execution begins A caller cutes its statements until it encounters a function invocation,celsius to fahrenheit(25) in Figure 6.2 At that point, the caller temporarily suspends, and the functionbegins Thus, one program is suspended (the caller) waiting for a result from the nowexecuting function When the function finishes execution, the suspended caller receivesthe return value from the function and the main program (caller) resumes execution fromthat point

exe-Because the function is essentially a new program, the function gets its own namespacewhen it begins execution Any object that gets created and given a name (by assignment,

by adef, etc.) within the function is placed in the function’s separate namespace, not the

main program’s (caller’s) namespace If a new name is created in the function that is thesame as a name that existed in the caller’s namespace, the newly created association is used

in the function This protocol has some interesting consequences, which we will explore inSection 8.1.1

Parameter passing is done just before the calling program suspends The caller associatesits argument values to the corresponding function parameter in the function object InFigure 6.2, the argument value 25 is associated with the parametercelsius The parameter

is then defined in the function’s namespace and associated with the value of its correspondingargument After that, function operation begins

264 C H A P T E R 6 • F U N C T I O N S - Q U I C K S T A R T

Argument values are typically passed to parameter names in the order they are listed.3The names of the corresponding argument and parameter need not match Only the ordermatters: the first argument value is passed to the first parameter, the second argument value

to the second parameter, and so on The number of arguments and parameters must match.4Again, after values are passed, the function then begins execution

During function execution, if a return statement is executed, the function endsand the return value is provided to the caller (“return” arrow in Figure 6.2) For example,fahrenheit = celsius to fahrenheit(25)assigns the returned value to the mainprogram variablefahrenheitas shown in Figure 6.2 After the function ends, the callercontinues

Code Listing 6.2 is a sample program with a function that takes in a Celsius temperature

7 # main part o f the program

8 print("Convert Celsius to Fahrenheit.")

9 celsius float = float(input("Enter a Celsius temp: "))

10 # c a l l the conversion function

11 fahrenheit float = celsius to fahrenheit(celsius float)

12 # print the returned value

13 print(celsius float," converts to ",fahrenheit float," Fahrenheit")

>>> ================================ RESTART ================================

>>>

Convert Celsius to Fahrenheit.

Enter a Celsius temp: 100

100.0 converts to 212.0 Fahrenheit

>>> ================================ RESTART ================================

>>>

Convert Celsius to Fahrenheit.

Enter a Celsius temp: 0

0.0 converts to 32.0 Fahrenheit

3 Python also has other ways to pass argument values to parameters See Chapter 8.

4

6 3 • F L O W O F C O N T R O L W I T H F U N C T I O N S 265

Lines 3–5 define the function Notice def beginning the compound statement, theparameter (celsius float), and thereturnwith its associated expression withinthe function suite To repeat, thedefstatement is executed by Python and creates thefunction Once created, it can be called (invoked) by another program Remember, afunction must be defined (thedefstatement must be executed) before a program cancall the function

Lines 7–13 are the “main” program.

Line 9 prompts the user for input.

Line 11 invokes (calls) the function The value in the argument celsius float

is passed to the parameter celsius float Then control passes to the tion When the function ends, it returns a value that the main program assigns tofahrenheit float

func-Line 13 The main program continues with func-Line 13 and prints the results.

Check Yourself: Simple Functions Check

1. Given the functionmake odd, what are the results of the four function invocations(print statements)?

2. Write a functionmake even(n)

What happens if you use a string as an argument?

Why is the behavior different then withmake odd(n)?

6.3.3 Another Function Example

You have used thelenfunction to find the length of a string Let’s write our own version

of that function that performs the same operation We will name our functionlengthtoavoid confusion with the Python built-inlen

Here is an algorithm to find the length of a string, S

r Initialize a counter:count = 0

r Iterate through each character of the string using aforloop:for char in a str:

r When each character is encountered in the for loop, add 1 to the counter:count += 1

>>> question = "What is your quest?"

>>> length(question) # our function

19

>>> len(question) # the Python built −in function

19

With alengthfunction in hand, let’s modify it Suppose that instead of a count of

the number of characters in the string, we wanted a count of only the letters in the string.

That is, we will ignore characters other than lowercase alphabetic letters (such as numbers,spaces, punctuation, etc.) Our algorithm is similar:

rInitialize a counter.

rIterate through each character of the string using aforloop.

rFor each character in the string, if the character is a letter, add 1 to the counter.The change in the algorithm is small, but the implementation takes a little morethought How do we check “if the character is a letter”? One way to do that is to create

a variable associated with a string of all the lowercase letters We can then check if eachcharacter we encounter in the parametera strisin(rememberintests for membership)the lowercase letters However, Python has created for us a number of these strings that

we can test against As discussed in Section 4.6.2, we could use a variable from the stringmodule, the stringstring.ascii lowercase Finally, we must consider what to dowith uppercase letters We have a choice: we could ignore them (not include them in thecount), or we could include them by converting everya strcharacter to lowercase before

6 3 • F L O W O F C O N T R O L W I T H F U N C T I O N S 267

we test for membership We can handle that by making the character lowercase beforechecking to see whether it is in the string of letters We will call our modified functionletter count

Code Listing 6.4

1 import string

2

3 def letter count(a str):

4 """ Return the count o f l e t t e r s in a str """

6.3.4 Function Example: Word Puzzle

Let’s use strings and functions to solve a word puzzle

Find a word that contains the vowelsa, e, i, o, and u in that order in a string

To solve a word puzzle, we need a list of words

Reading a File of Words

Many word lists can be found on the Internet by searching for either “word lists” or

“dictionary,” usually with one word per line We can download a dictionary and save it in a

268 C H A P T E R 6 • F U N C T I O N S - Q U I C K S T A R T

file; let’s name the file dictionary.txt We have worked with files in Section 5.7.1 and will do

so in more detail in Chapter 14, but let’s review how to read the contents of a file

To read through the file, one line at a time, we do the following:

rOpen the file for reading.

rRead through the file one line at a time.

A Python program that reads through the whole file dictionary.txt, and simply prints

each line of the file is Code Listing 6.5:

Code Listing 6.5

# Print a l l words in a d i c t i o n a r y f i l e that has one word per l i n e

# open f i l e named " d i c t i o n a r y t x t " f o r reading ( ' r ' )

data file = open("dictionary.txt", 'r')

# i t e r a t e through the f i l e one l i n e at a time

for line str in data file:

print(line str)

Interestingly, this program prints the lines of the file in double-space format Why isthat?5

Searching a File of Words

Now that you remember how to read through a file of words, let’s try to solve our puzzle.First, we need to handle a common feature of a file’s strings: there are often somecharacters that we don’t want to deal with In this case, the dictionary file contains a carriage-return character at the end of each line, and it may contain stray characters such as spaces ortabs We can eliminate such characters using thestripstring method, one of the methodslisted in Section 4.3.4 If no arguments are provided tostrip, then a new string is returned

with whitespace removed from either end of the calling string The calling string is not

modified If a string argument is provided tostrip, only those characters in the argumentstring are removed An example of its use isa str.strip('., '),which returns a newstring that strips commas, periods, and the space character from the beginning and end of

a str In particular,", this.".strip('., ')and" this,,.".strip('., ')both return the new string object'this' The blank character in the argument string ishard to see (after all, it is a blank), but it is there!

5

6 3 • F L O W O F C O N T R O L W I T H F U N C T I O N S 269

P R O G R A M M I N G T I P

The strip method works only on the beginning and end of a string For example,'this string'.strip()returns the string'this string' Because no argumentsare provided, the default behavior is to return a new string where whitespace (tabs, carriage

returns, space characters, etc.) are removed from the ends, but the space in the middle of the string is unaffected Note that all the whitespace characters are removed from the beginning

and the end

For every line we fetch from the file, we strip that line to remove whitespace at thebeginning and end of the line In addition, word puzzles are often easier if all charactersare known to be only lowercase (or only uppercase) The string methodloweris usefulfor that Let’s create a function that strips out whitespace characters and makes all of theremaining characters lowercase

Code Listing 6.6

def clean word(word):

""" Return word in l o w e r c a s e s t r i p p e d o f whitespace """

Code Listing 6.7

# Find a word with a s i n g l e example o f the vowels a , e , i , o , u in that order

data file = open("dictionary.txt", "r")

def clean word(word):

""" Return word in l o w e r c a s e s t r i p p e d o f whitespace """

return word.strip().lower()

# main program

for word in data file: # f o r each word in the f i l e

word = clean word(word) # clean the word

Solving the Puzzle

To solve this puzzle, we want to determine the vowels and their order in a word Let’s write

a function that returns the vowels found in a word in the order in which they were found Afunction lets us consider each word in isolation For any word, we can check each character

to see whether it is a vowel and collect the vowels into a string in the order in which theywere found To begin, we create a variable that is associated with the empty string, with theintent that we will add discovered vowels to that string Our algorithm is:

rCreate an empty string; let’s call itvowels in word.

rFor each charactercharin the word

- ifcharis a vowel, add it to the stringvowels in word.How do we check whether a character is a vowel? We can create a string of vowelsvowels strand associate it with'aeiou'and check to see whether each character inthe dictionary entrywordisin(is a member of )vowels str

Here is our algorithm implemented as a function

Code Listing 6.8

def get vowels in word(word):

""" Return vowels in s t r i n g word −−include repeats """

vowel str = "aeiou"

vowels in word = ""

for char in word:

if char in vowel str:

vowels in word += char

return vowels in word

Let’s try the function out in a session to find the vowels in the word'create' Thevowels in'create'in order are'eae'

>>> word = "create"

>>> get vowels in word(word)

'eae'

r Open the file.

r For each word in the file

- strip the word

- if the word is too small, skip it

- get the vowels in the word

- if the vowels returned is exactly'aeiou', print the original word

Using that algorithm as the main part of our program, let’s put it all together

5 def clean word(word):

6 """ Return word in l o w e r c a s e s t r i p p e d o f whitespace """

7 return word.strip().lower()

8

9 def get vowels in word(word):

10 """ Return vowels in s t r i n g word −−include repeats """

11 vowel str = "aeiou"

12 vowels in word = ""

13 for char in word:

14 if char in vowel str:

15 vowels in word += char

16 return vowels in word

17

18 # main program

19 print("Find words containing vowels 'aeiou' in that order:")

20 for word in data file: # f o r each word in the f i l e

21 word = clean word(word) # clean the word

22 if len(word) <= 6: # i f word i s too small , s k i p i t

24 vowel str = get vowels in word(word) # g e t vowels in word

25 if vowel str == 'aeiou': # check i f you have e x a c t l y a l l

vowels in order

print(word)

Check Yourself: Function Practice with Strings

1. Give the output provided by the following program on the indicated input values.def func1 (str1, str2):

There are a couple of points that are worth noting about the example.

r The use of functions made our problem-solving task easier The functions allowed us toconsider the problem in smaller pieces—the divide-and-conquer technique introduced

in Chapter 3

r The main program is very readable Even if we did not know the implementation ofthe functionsclean wordandget vowels in wordwe could guess what they(likely) did and understand what the program was trying to do

r Note the reuse of the identifier vowel strin both the main program and in thefunctionget vowels in word Because the function gets its own namespace, theseidentifiers are associated with different objects More on this in Section 8.1

6.3.5 Functions Calling Functions

There is no limitation to when a function can be called (except that it must be after its

def) It is often the case that a function will call another function This extension does notchange the process described previously, though it does make the flow of control slightlymore complicated for us to follow More on this in Chapter 8 In fact, a function can callitself—a complicated control flow to which we devote Chapter 16

6.3.6 When to Use a Function

There are no hard or fast rules about when to write something as a function or when to leavecode as part of a larger program However, here are some guidelines that may prove helpful

Only one purpose: A function should be the encapsulation of a single, identifiable

opera-tion A function should do one thing, and of course, do it well Functions that try to dotoo many things are candidates to be broken into multiple functions (i.e., refactored)

Readable: A function should be readable This is a reiteration of our venerable R ULE 2 Not too long: A function shouldn’t be too long What is “too long” depends on many

things, but the idea is that if a function does one thing, a person should be able to

274 C H A P T E R 6 • F U N C T I O N S - Q U I C K S T A R T

read it easily and understand its purpose If the function is too long, difficult to followsomehow, it might need to be broken into multiple functions

Reusable: A function should be reusable in contexts other than the program it was written

for originally If possible, the function should be self-contained and not dependent onsome nuance of the calling program If it has dependencies then those dependenciesshould be made clear so that others can use it more easily

Complete: A function should be complete, in that it works in all potential situations If you

write a function to perform one thing, you should make sure that all the cases where it

might be used are taken into account It is often the case that the core of the function isvery straightforward, but handling all the cases it might encounter requires supportingcode

Able to be refactored: We first mentioned the term refactoring in Section 2.2.9 Again,

refactoring is the process of taking existing code and modifying it such that its structure

is somehow improved but the functionality of the code remains the same Functions canplay a prominent role in this process, as they allow you to take long, difficult-to-followcode and break it into smaller, more manageable pieces If you need to “fix” some code,consider refactoring it into functions

The idea that a function should “do one thing” is important enough that we will make it

if there is a good reason to make a function longer than that

6.3.7 What If There Is No Return Statement?

Sometimes we write functions without a return statement Functions that do not return a

value are often called procedures.

In that case, what, if anything, is returned? The answer is that a special Python valueNoneis returned by default if the programmer does not provide a return statement.Noneis a kind of odd Python value—a value to represent nothing Kind of cosmic!There are some good reasons to use a procedure One that often comes up is speciallyformatted output Formatting output to look “pretty” can be laborious and potentiallycomplicated A print procedure is therefore a good place to do such output The procedureisolates all the printing format but does not return a value Other instances might require achange of state to the program, something like turning a graphics mode on or off or changing

6 3 • F L O W O F C O N T R O L W I T H F U N C T I O N S 275

the network connection to a different mode Each of these elements is best isolated as aprocess, but as a function they do not need to return a value

To see this process in action, let’s write a trivial function that simply prints its parameter

We will then assign the result of the function to a variable When we print the value of thevariable, we see thatNonewas printed

>>> def formatted output(my str, my int):

print('The result of the processing for',my str, 'was', my int)

# no return statement

>>> formatted output('Bill',100)

The result of the processing for Bill was 100

>>> result = formatted output('Fred',75) # capture the i m p l i c i t return

The result of the processing for Fred was 75

>>> print(result)

>>>

6.3.8 What if There Are Multiple Return Statements?

If there are multiple return statements, the first return encountered during the operation ofthe function stops the function at that point and returns that value Consider the followingexample function, which returns “positive,” “negative,” or “zero,” depending on the value

Note that the function works correctly whether the argument passed in is anintor

a float For that reason neither suffix is used in the name Multiple return values canmake following the flow of control in the function more difficult for the reader to follow

If possible, it is best to have as few returns as possible so the reader can more clearly followthe function

In this chapter, we introduced the concept of functions in a programming language andshowed how functions work in Python Of particular importance is that functions aid us inthe divide-and-conquer approach to problem solving After covering the list data structure

in the next chapter, we revisit functions to examine the complexities of passing mutableobjects to functions

E X E R C I S E S 277

r Caller’s arguments map to the function’s parameters in order left to right.

r A function has its own namespace.

r Parameters are in the function’s namespace.

r A function must be defined before it is called.

r Defining a function puts its name in the caller’s namespace.

r Syntax, function definition:

def function name(parameter list):

# statement suite

return# something

Rules

r R ULE 1: Think before you program!

r R ULE 2: A program is a human-readable essay on problem solving that also happens

to execute on a computer

r R ULE 3: The best way to improve your programming and problem skills is to practice!

r R ULE 4: A foolish consistency is the hobgoblin of little minds.

r R ULE 5: Test your code, often and thoroughly!

r R ULE 6: If it was hard to write, it is probably hard to read Add a comment.

r R ULE 7: All input is evil, until proven otherwise.

r R ULE 8: A function should do one thing.

Exercises

1. Draw the parts of a function and label the parts Write a brief definition for each part

2. What are three guidelines used to determine whether creating a separate function is thesensible thing to do?

3. What does this function do? What does it return for num = 5?

(b) Write a function that takes the choice of the user and makes calls to the otherfunctions that correspond to the operation to be performed (Give meaningful

names to the functions You don’t have to write the other function definitions.)

8. Write a function that takes in the final scores of two soccer teams as arguments andprints either who won the game or whether the game was tied Refer to the teams as

“Team1” and “Team2.” The function returns nothing

9. Write a function that takes as input an English sentence (a string) and prints the totalnumber of vowels and the total number of consonants in the sentence The functionreturns nothing Note that the sentence could have special characters like dots, dashes,and so on

E X E R C I S E S 279

10. The Fibonacci sequence is: 1, 1, 2, 3, 5, 8, 13 You can see that the first and secondnumbers are both 1 Thereafter, each number is the sum of the previous two numbers

(a) Write a function to print the first N numbers of the Fibonacci sequence.

(b) Write a function to print the Nth number of the sequence

11. Suppose you are purchasing something online on the Internet At the website, you get

a 10% discount if you are a member Additionally, you are also getting a discount of5% on the item because its Father’s Day

Write a function that takes as input the cost of the item that you are purchasing and

a Boolean variable indicating whether you are a member (or not), applies the discountsappropriately, and returns the final discounted value of the item

Note: The cost of the item need not be an integer

12. A leap year in the Gregorian calendar system is a year that’s divisible by 4 but not by

100, unless it is also divisible by 400 For example, 1896, 1904, and 2000 were leapyears but 1900 was not Write a function that takes in a year as input and prints whetherit’s a leap year (or not)

13. Error checking with meaningful error messages is an important part of programming.Consider the following scenario: A customer has to pay his monthly credit card bill.The credit limit on the card is $1000 The minimum payment due is always $20.Let the payment on the credit card be$P Write a function calledmake payment(P)that takes as an argument the total payment on the credit card ($P) and prints “Success”

or “Retry.” Try to think of all the errors that should be taken care of and implementthose in the function One example would be that if the payment is less than$20, theprogram should remind the user that it’s less than the minimum payment due

14. You buy an international calling card to India The calling card company has somespecial offers

(a) If you charge your card with$5 or$10, you don’t get anything extra

(b) For a$25 charge, you get$3 of extra phone time

(c) For a$50 charge, you get$8 of extra phone time

(d) For a$100 charge, you get$20 of extra phone time

Write a function that asks the user for the amount he/she wants on the card and returnsthe total charge that the user gets Note: Values other than those mentioned above arenot allowed

15. Chat:

(a) In certain chat programs or messaging applications, there is a limit on the number

of characters that you can send in a message Write a function that takes as inputthe message (a string) and checks whether the number of characters is less than

160 (or not) If the length of the message is less than 160, the message should bereturned If the length of the message is greater than 160, a string consisting of onlythe first 160 characters should be returned

17. (Refactoring) In Chapters 1 and 2, there were exercises to calculate football quarterbackpass ratings and then to output the quality of the quarterback.

(a) Write the pass rating as a function Have the main program prompt for the fiveinputs and then use those inputs as arguments to the function

(b) Write the quality rating as a function and add it to the existing program

18. In an exercise in Chapter 2, we presented an odometer puzzle from the Car Talk radio

program that involved numerical palindromes Refactor your solution by writing apalindome function and using that in your solution

19. Write a function that takes as input a string that stores date and time (24-hour clock)

in the following format:

“MM/DD/YYYY HR:MIN:SEC” and prints the following:

rDD/MM/YYYY

rHR:MIN:SEC

rMM/YYYY

rWhether the time is a.m or p.m.

Validation of the input in the function is necessary For example, if the user gives

an input of “122/04/1990 13:12:12”, the given string is invalid, as there can be only

12 months in a year Think of all possible erroneous inputs and write code to handlethem The function doesn’t return anything

20. Write a function that takes as input a string that stores date and time in the format

“MM/DD/YYYY HR:MIN:SEC” and prints the number of seconds elapsed since

“01/01/YYYY 00:00:00”

21. Write a function that prints all numbers in the range A to B (inclusive) that haveall digits belonging to the set{1,3,4,8,9} Check whether A is less than or equal toB; otherwise, swap them before proceeding The function takes two integer arguments:

A and B

22. Given a string of length three representing a set (i.e., surrounded by curly braces) such as

"{ABC}", write a function that takes the string as an argument and returns a string of itspermutations in comma-separated form, such as"{ABC, ACB, BAC, BCA, CAB, CBA}" Hint: use multipleforloops

23. Implement a textual progress bar in Python In any progress bar, the space occupied

by it is finite Let’s say the textual progress bar could show only 10 Xs So you have to

Programming Projects

1 U.S Flag in Turtle Graphics:

Draw the United States flag using at least four functions The regular polygons in the

US flag are natural candidates for functions

(a) Draw the flag shown earlier in the chapter with 13 stars arranged in rows Addcolor for maximum effect

(b) Draw the flag with the 13 stars arranged in a circle (Hint: Is it really a circle, or is

it some other regular figure?)

(c) Rewrite your program so it takes as input one positive real number that specifies ascale factor That is, 0.5 will cause a half-size flag to be drawn and 2.8 will draw aflag that is 2.8 times larger

2 DNA Sequencing:

The term DNA sequencing refers to methods for determining the order of the nucleotide

bases, adenine, thymine, cytosine, and guanine in a molecule of DNA The standardrepresentation of the bases is to use their first letters, ATCG, so that DNA is represented

as a string using only those four characters However, DNA strings are millions of bases(characters) long

Substring matching is the process of determining whether a shorter string (the

substring) is contained within a longer string Substring matching plays importantroles in the reconstruction of an unknown DNA string from pieces and in searchingfor interesting substrings within a known DNA string

Python provides a find(substring, start, end) string method that

re-turns the lowest index (integer) where the substring is found in the index range

start≤index < end Thestartandendarguments are optional, but for thisexercise we will make them required (you will learn later how to handle optionalarguments) If the substring is not found, -1 is returned

(a) Without using thefindstring method, write a function that behaves exactly likethefind string method As your function is not a string method, the string to

282 C H A P T E R 6 • F U N C T I O N S - Q U I C K S T A R T

search must be an argument—let’s make it the first one The resulting format ofyour function will be:

find(some string, substring, start, end)

(b) Biology researchers frequently want to find all the locations where a substring

is found, not simply the first one Write a function named multi find(some string, substring, start, end) that, instead of returning oneinteger index, returns a string that contains zero or more indices separated bycommas In this case, the string will contain digits representing the integer indices

If the substring is not found, an empty string is returned You may use thefindmethod that you wrote earlier

(c) A nice feature of ourmulti findfunction is that if the substring is not found, anempty string is returned In particular, if the substring is not found, the returnedempty string resolves to be Falsein a Boolean expression The returned valuewill be True otherwise That feature allows one to usemulti find in an if

statement, such as:if multi find(S,substring,0,20) The Pythonfindstring method does not share that characteristic (why?) Write a program thatexercises both yourfindand yourmulti findfunctions including their use inBoolean expressions Create some strings using only the base letters, ATCG, andsearch for substrings within them

3 Heap of Beans:

We are going to play a game called the heap of beans You start with a heap of beans

(we will start with 16 beans) and two players Each player can remove 1, 2, or 3 beansfrom the pile of 16 Each player is required to take some number of beans from the pileduring each turn The players take turns removing beans, and the player who takes thelast bean from the pile is the loser You can try this game with a partner using 16 pieces

of paper as beans

Each player is to be represented by a function, so we will have two functions.Each function takes a single argument, representing the present number of beans, andreturns the number of beans remaining after the player function takes its turn Duringthe operation of the player function, the function reports (prints) how many beanswere removed Note that the function decides how many beans to remove It takes noinput from the user

You also need a main program The main program initializes the heap of beans to

16 and then alternately calls the first player function and the second player functionuntil one of them gets the last bean The main then reports who the loser is and exits.(a) Write a simple player function such as one that always takes exactly one bean Itisn’t very interesting, but it lets you test your program

(b) Now for the fun part Write a “smart player” function and test it against otherstudents’ functions in your class A class tournament would be best—who has thebest strategy?

• 7

C H A P T E R

Lists and Tuples

List, a Scrowl of the Names of several Persons of the same Quality with whom

we have Business

E Phillips, New World of Words, 1696

7.1 W H A T I S A L I S T ?

PYTHON’S BUILT-IN LIST TYPE IS ALSO A COLLECTION TYPE IN FACT, LIKE STRINGS,

lists are a sequence type and are an iterable type They therefore share some of the teristics of strings However, a list differs from a string in two fundamental ways:

charac-r A list can contain elements other than characters In fact, a list can contain a sequence

of elements of any type, even different typed elements mixed together in the same list.

r A list is a mutable type This means that, unlike a string object, a list object can bechanged after it is initially created

If we keep these differences in mind, we can use our knowledge of the string type towork with lists

Making Python Lists

Lists can be created with thelistconstructor or with square brackets ([ ]) This capabilitycan be a little confusing, as the same square brackets are also used as the index operator.One way to tell the difference is the presence of the comma character: each element of a list

is separated by a comma Thus, if you see square brackets with multiple elements separated

by commas, you know it is a list Otherwise, you will have to use the context to determinewhat the brackets mean

283

284 C H A P T E R 7 • L I S T S A N D T U P L E S

Here are a few examples of making a list in a Python session:

>>> a list = [1,2,'a',3.14159]

>>> week days list = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday']

>>> list of lists = [ [1,2,3], ['a','b','c']]

>>> list from collection = list('Hello')

>>> a list

[1, 2, 'a', 3.1415899999999999]

>>> week days list

['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday']

Here are a few things to notice about these lists

ra listis a sequence of four elements (two integers, a character, and a floating-pointnumber) Any Python type can be a list element, and different types can be in the same

list Again, this differs from a string, which requires its elements to all be characters.

rAs previously noted, each element of the list is separated from the next by a comma.The comma indicates elements of a collection, and the square brackets indicate thatcollection is a list

rThelist of listsis a list of only two elements The first element is a list, as is thesecond Again, a list can be a sequence of any typed element—even another list!

rThelist from collectionis a list built using the constructorlist The ing list is['H', 'e', 'l', 'l', 'o'], containing five elements, each a single-character string This constructor takes a single argument and that argument must be

result-an iterable Thelistconstructor takes each element of the argument iterable and addsthat element to the new list Non-iterable types (integers, floats, Booleans, etc.) cannot

be used as an argument to thelistconstructor since they do not contain multipleelements (are not iterable)

rThe special list with no elements, designated as[], is called the empty list Like other

“empty” elements, it is equivalent to aFalsevalue in Python

Note the naming convention for lists As with other collections, we append the collectiontype to the end of the name, “list” for lists This protocol is similar to the naming convention

we have used for other types As per R ULE 4, we will violate this convention when a more

meaningful name is appropriate, such as thelist of lists

>>> spreadsheet list = [ ['Name','Age','GPA'], ['Bill', 25, 3.55], ['Rich', 26 , 4 00]]

>>> row = spreadsheet list[1]

This concept generalizes to any level of nesting of lists For example, to represent the x y z

3-D plane, how would we approach it? This would be a list of lists of lists The outermostlist would represent a 2-D plane, the list within that list would represent the value of all the

values in a particular row (say x ), and then a particular value would be the index into that row (say y ).

7.2 W H A T Y O U A L R E A D Y K N O W H O W T O D O

W I T H L I S T S

Because lists are sequences and iterable, many things that you learned to do with strings alsowork for lists Let’s take a look and refresh your memory

286 C H A P T E R 7 • L I S T S A N D T U P L E S

7.2.1 Iteration

As with a string, you can iterate through each element of a list using aforloop Because

a list is also a sequence, the elements are iterated through in the order set by the list Thefollowing session reminds you of how it works

>>> for element in ['abc', 12, 3.14159, True]:

print ("{:<7} which is type {}".format(element, type(element)))

abc which is type <class 'str'>

12 which is type <class 'int'>

3.14159 which is type <class 'float'>

1 which is type <class 'bool'>

>>>

This session emphasizes that a list is iterable and that the elements can be of any type

We also print out the results in a slightly more elegant way using string formatting

7.2.2 Indexing and Slicing

Indexing and slicing work exactly the same with lists and strings To remind yourself, look

rAccessing an element at an index that does not exist in the list is an error.

rThe slice operation is the same as with strings Within square brackets, you may haveone or two colons (:) The number before the first colon is the start index, the number

myList = [1, 'a', 3.14159, True]

myList

2 1

1

myList[1] → 'a' myList[:3] → [1, 'a', 3.14159]

Index forward Index backward

FIGURE 7.1 The structure of a list.

7 2 • W H A T Y O U A L R E A D Y K N O W H O W T O D O W I T H L I S T S 287

after the first colon is the end index, and the number after the second colon is the step.The defaults for all three are (in order): the beginning of the list, the end of the list,and a step of 1

r A slice uses a half-open range, meaning that the end index is not included in the slice.The following session shows these operations:

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

IndexError: list index out of range

7.2.3 Operators

You can use both the addition (+) and multiplication (*) operators with lists with resultssimilar to those of strings The addition operator takes two lists as operands and concatenatesthem together, making a new list whose contents are the first list joined at its end to thebeginning of the second list The multiplication operator takes a list and an integer asoperands The integer indicates how many times the list is replicated

Comparison works as it did with strings, so you can use the >,<,==,<=,>=, and != signs

as operators between two list operands As with strings, comparison begins by ing the first element of each list If the first elements are equal, the comparison processmoves to the second element of each list The process continues in this way, comparingcorresponding elements, until the process finds that two elements are different At thatpoint, the comparison between the two different elements determines the result of theoperation If one list is equal to but shorter than the other list, the longer list is consideredgreater

compar-Finally, theinoperator for membership testing works as it did in strings The expression'a' in ['a','b','c']is a query, testing whether the first operand exists in the secondoperand In this case it does, returningTrue

The following session demonstrates these operations on lists:

>>> my list = [1,2,3]

>>> your list = ['a','b','c']

>>> concat list = my list + your list # concat l i s t s , operand unchanged

7 2 • W H A T Y O U A L R E A D Y K N O W H O W T O D O W I T H L I S T S 289

Traceback (most recent call last):

File "<pyshell#13>", line 1, in <module>

compar-Finally, comparisons underly other operations that depend on order, such as the sorting,maximizing, and minimizing operations discussed later in this chapter

7.2.4 Functions

There are a number of functions that work with any collection In particular, you haveseen thelenfunction With strings, thelenfunction returns the number of characters

in a string With lists, thelenfunction returns the number of comma-separated elements

in the list Again, remember that a list can have as an element another list and that liststill counts as one element Thus, the result oflen([1,[1,2,3],3])is 3 The secondelement, the element at index 1, is another list,[1,2,3] Here are some functions thatwork on collections, including lists:

len(C)Return the length of collection C, i.e., the number of elements

min(C) Return the minimum element in collection C If the argument is a list of lists,only the first element in each list is considered for the purposes of comparison

max(C) Return the maximum element in collection C If the argument is a list of lists,only the first element in each list is considered for the purposes of comparison

sum(L)Return the sum of elements in list L The particular function requires that the list

>>> str list = ['a', 'b', 'c', 'd', 'e']

>>> nested list = [int list, float list, str list]

>>> len(int list)

5

>>> sum(str list) # elements must be numbers

Traceback (most recent call last):

File "<pyshell#25>", line 1, in <module>

sum(str list)

TypeError: unsupported operand type(s) for +: 'int' and 'str'

>>> min(nested list) # d i f f e r e n t t y p e s : 1 and ' a '

Traceback (most recent call last):

File "<pyshell#26>", line 1, in <module>

min(nested list) TypeError: unorderable types: str() < int()

>>> for element in my list: # i t e r a t e through l i s t elements

print(element ,end=' ') # p r i n t s on one l i n e

1 3 4 8

>>>

In this example, the variableelementis assigned the values 1, 3, 4, and 8, one at a time,and then the statement(s) in the suite are executed using that value forelement In thistrivial example, the print statement will execute with each value ofelementand print it

7.3 L I S T S A R E D I F F E R E N T T H A N S T R I N G S

We’ve seen what’s the same about lists and strings, so let’s look a bit more at what’s different

7.3.1 Lists Are Mutable

When we covered strings, we emphasized that they are immutable In particular, we notedthat you could not have a string index operator on the left-hand side of the assignment

7 3 • L I S T S A R E D I F F E R E N T T H A N S T R I N G S 291

statement In other words, you could not change a character of a string at some index within

an existing string to a different character Once created, the string cannot be changed

Lists, on the other hand, are mutable; the element at some index in a list can be changed This is called index assignment You can even change an entire slice of the list to be different! This is called slice assignment These two operations show both the power and one of the

dangers of mutable objects They are very flexible and very useful However, you must takecare in using them because you can accidentally change a list in ways you did not intend.Let’s see how mutability works in a session:

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

TypeError: can only assign an iterable

Let’s note a few things about this session:

r A list in combination with the index operator can occur on the left side of the assignmentstatement This means that, after the assignment occurs, the value at that index ischanged to be the value of the right-hand side of the assignment statement This isindex assignment

r As a result of every operation in that session, the list is changed This means that everytime we perform one of these operations, the changed list is carried forward to the nextline in the session In other words, the list elements associated withmy listat theend of the session are the cumulative result of all the operations that came before, inthe order they were performed

r Because the assignment operator does not return a value, we have to type the name ofthe variable associated with the list to see what was changed as a result This is not new,

being assigned must be a collection Each individual element of an assigned collection

is added to the list at the slice indicies You cannot slice assign an individual value to

a list, though the session shows that assigning a list with a single value is acceptable.Only assigning a collection makes some sense if you consider it for a moment A slice

is in fact a collection (a subsequence of the original), so to replace it implies that weneed another collection The collection replacing the slice can be bigger, smaller, orthe same size

7.3.2 List Methods

As we mentioned previously, a method is a function that works with a particular type ofPython object The string type has a large number of methods that can be applied to strings,

but those methods can be applied only to strings Lists have their own associated methods

that work only with lists The number of list methods is quite a bit smaller—only nine.They come in two different categories:

rThose that change the list

rThose that do not change the list

It is important that we keep those two categories straight, so we know whether our list

is changed as a result of using the method

Non-modifying methods

These methods do not change the list, but they return a value as a result of their processing.

index(x) Return the index of the first element in the list whose value is equal to x.

Python throws an error if there is no such value in the list

count(x)Return the number of times x appears in the list Returns 0 if x does not appear

in the list

Methods that Modify the List

There are two things to note about these methods First, the list that calls the method will

be modified as a result of the operation of the method Second, none of the methods exceptthepopmethod returns a value This fact may seem odd, but taking a closer look mayhelp In our previous experience, every string method returned a value: a new string object

Remember, though, that strings are immutable They had to return a value, typically a

string, so that the change could be captured These list methods do not need to return a listobject for the change, as the list itself is changed (it is mutable)

7 3 • L I S T S A R E D I F F E R E N T T H A N S T R I N G S 293

append(x)Append an element to the end of the list The length of the list is increased

by one The element is appended exactly, so a collection is added as a single element

pop() Remove the element at the end of the list and return that element The list isshortened by one element If an index is specified,a.pop(an index)removes theelement at that position and returns that item

extend(C)Requires a collection C as an argument The list is extended by adding each

individual element of the argument collection C to the end of the list.

insert(i, x)Insert an element at a given position The first argument is the index

before which to insert in the list Thus,my list.insert(1, 'a')inserts the'a'into position 1 of the list, sliding all the rest of the list elements down one (the element

at index 1 moves to index 2, the element at index 2 moves to index 3, and so on)

remove(x)Remove the first element from the list whose value is x An error results if

there is no such item The length of the list is decreased by one if successful

sort()Sort the elements of the list, in place If sorting a list of lists, only the first element

in each list is considered in the comparison operations Only the order of the listelements is modified (unless already sorted).1

reverse()Reverse the elements of the list, in place Only the order of the list elements

is modified

Let’s open a Python shell and demonstrate each method Remember the dot format for

methods, in which you provide a list object (an object of typelist), followed by a dot (.),followed by the method name:

rIf you want to add the contents of another collection to the end, use extend Theextend method adds each of the individual elements of the argument collection tothe list, one at a time The addition begins a the end of the list The argument must be

a collection

rTheremovemethod searches for and removes a particular element, but it only removesthe first occurrence of that element If that element is not in the list, Python throws anerror Because of this behavior, its use is usually preceded by a check for that element’sexistence using theinoperator Theindexmethod, which returns the index of thefirst occurrence of an element, throws a similar error if the index is not found It is alsooften used in conjunction with anincheck

rThepopmethod gets its name from its association with a classic data structure called a

stack We will talk more about stacks in Chapter 16 Python will throw an error if you

try topopan empty list

rThesortmethod must be used with some care It works best with homogeneous lists—those containing all elements of the same type—due to difficulties in the comparison

of different types that we have discussed

P R O G R A M M I N G T I P

Ordering: when dealing with functions, methods, or operations that depend on ordering

(such assort, sorted, reverse, min, max, <, >, ==) use only homogeneouselements, e.g., all numbers or all strings Mixed types do not have a natural ordering andwill generate an error in Python