102 Function Basics produced by another form of scaling. The following generates a pseudorandom floating- point value between 0.0 and 1.0: rand( )/static_cast<double>(RAND_MAX) The type cast is made so that we get floating-point division rather than integer division. Display 3.4 A Function Using a Random Number Generator (part 1 of 2) 1 #include <iostream> 2 #include <cstdlib> 3 using namespace std; 4 int main( ) 5 { 6 int month, day; 7 cout << "Welcome to your friendly weather program.\n" 8 << "Enter today’s date as two integers for the month and the day:\n"; 9 cin >> month; 10 cin >> day; 11 srand(month*day); 12 int prediction; 13 char ans; 14 cout << "Weather for today:\n"; 15 do 16 { 17 prediction = rand( ) % 3; 18 switch (prediction) 19 { 20 case 0: 21 cout << "The day will be sunny!!\n"; 22 break; 23 case 1: 24 cout << "The day will be cloudy.\n"; 25 break; 26 case 2: 27 cout << "The day will be stormy!\n"; 28 break; 29 default: 30 cout << "Weather program is not functioning properly.\n"; 31 } 32 cout << "Want the weather for the next day?(y/n): "; 33 cin >> ans; 34 } while (ans == 'y' || ans == 'Y'); 35 cout << "That's it from your 24-hour weather program.\n"; 36 return 0; 37 } Programmer-Defined Functions 103 Self-Test Exercises 5. Give an expression to produce a pseudorandom integer number in the range 5 to 10 (inclusive). 6. Write a complete program that asks the user for a seed and then outputs a list of ten ran- dom numbers based on that seed. The numbers should be floating-point numbers in the range 0.0 to 1.0 (inclusive). Programmer-Defined Functions A custom-tailored suit always fits better than one off the rack. My uncle, the tailor The previous section told you how to use predefined functions. This section tells you how to define your own functions. ■ DEFINING FUNCTIONS THAT RETURN A VALUE You can define your own functions, either in the same file as the main part of your pro- gram or in a separate file so that the functions can be used by several different programs. Display 3.4 A Function Using a Random Number Generator (part 2 of 2) S AMPLE D IALOGUE Welcome to your friendly weather program. Enter today’s date as two integers for the month and the day: 2 14 Weather for today: The day will be cloudy. Want the weather for the next day?(y/n): y The day will be cloudy. Want the weather for the next day?(y/n): y The day will be stormy! Want the weather for the next day?(y/n): y The day will be stormy! Want the weather for the next day?(y/n): y The day will be sunny!! Want the weather for the next day?(y/n): n That's it from your 24-hour weather program. 3.2 104 Function Basics The definition is the same in either case, but for now we will assume that the function definition will be in the same file as the main part of your program. This subsection dis- cusses only functions that return a value. A later subsection tells you how to define void functions. Display 3.5 contains a sample function definition in a complete program that demon- strates a call to the function. The function is called totalCost and takes two arguments— the price for one item and the number of items for a purchase. The function returns the total cost, including sales tax, for that many items at the specified price. The func- tion is called in the same way a predefined function is called. The definition of the function, which the programmer must write, is a bit more complicated. The description of the function is given in two parts. The first part is called the function declaration or function prototype. The following is the function declara- tion (function prototype) for the function defined in Display 3.5: double totalCost(int numberParameter, double priceParameter); The first word in a function declaration specifies the type of the value returned by the function. Thus, for the function totalCost, the type of the value returned is double. Next, the function declaration tells you the name of the function; in this case, total- Cost . The function declaration tells you (and the compiler) everything you need to know in order to write and use a call to the function. It tells you how many arguments the function needs and what type the arguments should be; in this case, the function totalCost takes two arguments, the first one of type int and the second one of type double. The identifiers numberParameter and priceParameter are called formal param- eters, or parameters for short. A formal parameter is used as a kind of blank, or place- holder, to stand in for the argument. When you write a function declaration, you do not know what the arguments will be, so you use the formal parameters in place of the arguments. Names of formal parameters can be any valid identifiers. Notice that a function declaration ends with a semicolon. Although the function declaration tells you all you need to know to write a function call, it does not tell you what value will be returned. The value returned is determined by the function definition. In Display 3.3 the function definition is in lines 24 to 30 of the program. A function definition describes how the function computes the value it returns. A function definition consists of a function header followed by a function body. The function header is written similar to the function declaration, except that the header does not have a semicolon at the end. The value returned is determined by the statements in the function body. The function body follows the function header and completes the function defini- tion. The function body consists of declarations and executable statements enclosed within a pair of braces. Thus, the function body is just like the body of the main part of a program. When the function is called, the argument values are plugged in for the for- mal parameters, and then the statements in the body are executed. The value returned by the function is determined when the function executes a return statement. (The details of this “plugging in” will be discussed in Chapter 4.) function declaration or function prototype type for value returned formal parameter function definition function header function body Programmer-Defined Functions 105 Display 3.5 A Function Using a Random Number Generator (part 1 of 2) 1 #include <iostream> 2 using namespace std; 3 double totalCost(int numberParameter, double priceParameter); 4 //Computes the total cost, including 5% sales tax, 5 //on numberParameter items at a cost of priceParameter each. 6 int main( ) 7 { 8 double price, bill; 9 int number; 10 cout << "Enter the number of items purchased: "; 11 cin >> number; 12 cout << "Enter the price per item $"; 13 cin >> price; 14 bill = totalCost(number, price); 15 cout.setf(ios::fixed); 16 cout.setf(ios::showpoint); 17 cout.precision(2); 18 cout << number << " items at " 19 << "$" << price << " each.\n" 20 << "Final bill, including tax, is $" << bill 21 << endl; 22 return 0; 23 } 24 double totalCost(int numberParameter, double priceParameter) 25 { 26 const double TAXRATE = 0.05; //5% sales tax 27 double subtotal; 28 subtotal = priceParameter * numberParameter; 29 return (subtotal + subtotal*TAXRATE); 30 } S AMPLE D IALOGUE Enter the number of items purchased: 2 Enter the price per item: $10.10 2 items at $10.10 each. Final bill, including tax, is $21.21 Function declaration; also called the function prototype Function call Function body Function definition Function head 106 Function Basics A return statement consists of the keyword return followed by an expression. The function definition in Display 3.5 contains the following return statement: return (subtotal + subtotal*TAXRATE); When this return statement is executed, the value of the following expression is returned as the value of the function call: (subtotal + subtotal*TAXRATE) The parentheses are not needed. The program will run the same if the parentheses are omitted. However, with longer expressions, the parentheses make the return statement easier to read. For consistency, some programmers advocate using these parentheses even with simple expressions. In the function definition in Display 3.3 there are no statements after the return statement, but if there were, they would not be executed. When a return statement is executed, the function call ends. Note that the function body can contain any C++ statements and that the state- ments will be executed when the function is called. Thus, a function that returns a value may do any other action as well as return a value. In most cases, however, the main purpose of a function that returns a value is to return that value. Either the complete function definition or the function declaration (function proto- type) must appear in the code before the function is called. The most typical arrange- ment is for the function declaration and the main part of the program to appear in one or more files, with the function declaration before the main part of the program, and for the function definition to appear in another file. We have not yet discussed dividing a program across more than one file, and so we will place the function definitions after the main part of the program. If the full function definition is placed before the main part of the program, the function declaration can be omitted. ■ ALTERNATE FORM FOR FUNCTION DECLARATIONS You are not required to list formal parameter names in a function declaration (function prototype). The following two function declarations are equivalent: double totalCost(int numberParameter, double priceParameter); and double totalCost(int, double); We will usually use the first form so that we can refer to the formal parameters in the comment that accompanies the function declaration. However, you will often see the second form in manuals. This alternate form applies only to function declarations. A function definition must always list the formal parameter names. return statement Programmer-Defined Functions 107 Example Pitfall Pitfall A RGUMENTS IN THE W RONG O RDER When a function is called, the computer substitutes the first argument for the first formal param- eter, the second argument for the second formal parameter, and so forth. Although the computer checks the type of each argument, it does not check for reasonableness. If you confuse the order of the arguments, the program will not do what you want it to do. If there is a type violation due to an argument of the wrong type, then you will get an error message. If there is no type viola- tion, your program will probably run normally but produce an incorrect value for the value returned by the function. U SE OF THE T ERMS P ARAMETER AND A RGUMENT The use of the terms formal parameter and argument that we follow in this book is consistent with common usage, but people also often use the terms parameter and argument interchangeably. When you see the terms parameter and argument, you must determine their exact meaning from context. Many people use the term parameter for both what we call formal parameters and what we call arguments . Other people use the term argument both for what we call formal parameters and what we call arguments . Do not expect consistency in how people use these two terms. (In this book we sometimes use the term parameter to mean formal parameter , but this is more of an abbreviation than a true inconsistency.) ■ FUNCTIONS CALLING FUNCTIONS A function body may contain a call to another function. The situation for these sorts of function calls is the same as if the function call had occurred in the main part of the program; the only restriction is that the function declaration (or function definition) must appear before the function is used. If you set up your programs as we have been doing, this will happen automatically, since all function declarations come before the main part of the program and all function definitions come after the main part of the program. Although you may include a function call within the definition of another function, you cannot place the definition of one function within the body of another function definition. A R OUNDING F UNCTION The table of predefined functions (Display 3.2) does not include any function for rounding a number. The functions ceil and floor are almost, but not quite, rounding functions. The func- tion ceil always returns the next-highest whole number (or its argument if it happens to be a whole number). So, ceil(2.1) returns 3.0, not 2.0. The function floor always returns the nearest whole number less than (or equal to) the argument. So, floor(2.9) returns 2.0, not 108 Function Basics 3.0. Fortunately, it is easy to define a function that does true rounding. The function is defined in Display 3.6. The function round rounds its argument to the nearest integer. For example, round(2.3) returns 2, and round(2.6) returns 3. To see that round works correctly, let’s look at some examples. Consider round(2.4). The value returned is the following (converted to an int value): floor(2.4 + 0.5) which is floor(2.9), or 2.0. In fact, for any number that is greater than or equal to 2.0 and strictly less than 2.5, that number plus 0.5 will be less than 3.0, and so floor applied to that number plus 0.5 will return 2.0. Thus, round applied to any number that is greater than or equal to 2.0 and strictly less than 2.5 will return 2. (Since the function declaration for round specifies that the type for the value returned is int, we have type cast the computed value to the type int.) Now consider numbers greater than or equal to 2.5; for example, 2.6. The value returned by the call round(2.6) is the following (converted to an int value): floor(2.6 + 0.5) which is floor(3.1), or 3.0. In fact, for any number that is greater than 2.5 and less than or equal to 3.0, that number plus 0.5 will be greater than 3.0. Thus, round called with any num- ber that is greater than 2.5 and less than or equal to 3.0 will return 3. Thus, round works correctly for all arguments between 2.0 and 3.0. Clearly, there is nothing special about arguments between 2.0 and 3.0. A similar argument applies to all nonnegative numbers. So, round works correctly for all nonnegative arguments. round Display 3.6 The Function round (part 1 of 2) 1 #include <iostream> 2 #include <cmath> 3 using namespace std; 4 int round(double number); 5 //Assumes number >= 0. 6 //Returns number rounded to the nearest integer. 7 int main( ) 8 { 9 double doubleValue; 10 char ans; 11 do 12 { 13 cout << "Enter a double value: "; 14 cin >> doubleValue; Testing program for the function round Programmer-Defined Functions 109 Self-Test Exercises 7. What is the output produced by the following program? #include <iostream> using namespace std; char mystery(int firstParameter, int secondParameter); int main( ) { cout << mystery(10, 9) << "ow\n"; return 0; } char mystery(int firstParameter, int secondParameter) { if (firstParameter >= secondParameter) return ’W’; else return ’H’; } Display 3.6 The Function round (part 2 of 2) 15 cout << "Rounded that number is " << round(doubleValue) << endl; 16 cout << "Again? (y/n): "; 17 cin >> ans; 18 }while (ans == ’y’ || ans == ’Y’); 19 cout << "End of testing.\n"; 20 return 0; 21 } 22 //Uses cmath: 23 int round(double number) 24 { 25 return static_cast<int>(floor(number + 0.5)); 26 } S AMPLE D IALOGUE Enter a double value: 9.6 Rounded, that number is 10 Again? (y/n): y Enter a double value: 2.49 Rounded, that number is 2 Again? (y/n): n End of testing. 110 Function Basics Self-Test Exercises 8. Write a function declaration (function prototype) and a function definition for a function that takes three arguments, all of type int, and that returns the sum of its three arguments. 9. Write a function declaration and a function definition for a function that takes one argu- ment of type double. The function returns the character value ’P’ if its argument is posi- tive and returns ’N’ if its argument is zero or negative. 10. Can a function definition appear inside the body of another function definition? 11. List the similarities and differences between how you invoke (call) a predefined (that is, library) function and a user-defined function. ■ FUNCTIONS THAT RETURN A BOOLEAN VALUE The returned type for a function can be the type bool. A call to such a function returns one of the values true or false and can be used anywhere that a Boolean expression is allowed. For example, it can be used in a Boolean expression to control an if-else statement or to control a loop statement. This can often make a program easier to read. By means of a function declaration, you can associate a complex Boolean expression with a meaningful name. For example, the statement if (((rate >= 10) && (rate < 20)) || (rate == 0)) { } can be made to read if (appropriate(rate)) { } provided that the following function has been defined: bool appropriate(int rate) { return (((rate >= 10) && (rate < 20)) || (rate == 0)); } 12. Write a function definition for a function called inOrder that takes three arguments of type int. The function returns true if the three arguments are in ascending order; other- wise, it returns false. For example, inOrder(1, 2, 3) and inOrder(1, 2, 2) both return true, whereas inOrder(1, 3, 2) returns false. Programmer-Defined Functions 111 13. Write a function definition for a function called even that takes one argument of type int and returns a bool value. The function returns true if its one argument is an even num- ber; otherwise, it returns false. 14. Write a function definition for a function isDigit that takes one argument of type char and returns a bool value. The function returns true if the argument is a decimal digit; oth- erwise, it returns false. ■ DEFINING void FUNCTIONS In C++ a void function is defined in a way similar to that of functions that return a value. For example, the following is a void function that outputs the result of a calcula- tion that converts a temperature expressed in degrees Fahrenheit to a temperature expressed in degrees Celsius. The actual calculation would be done elsewhere in the program. This void function implements only the subtask for outputting the results of the calculation. void showResults(double fDegrees, double cDegrees) { cout.setf(ios::fixed); cout.setf(ios::showpoint); cout.precision(1); cout << fDegrees << " degrees Fahrenheit is equivalent to\n" << cDegrees << " degrees Celsius.\n"; } As the above function definition illustrates, there are only two differences between a function definition for a void function and for a function that returns a value. One difference is that we use the keyword void where we would normally specify the type of the value to be returned. This tells the compiler that this function will not return any value. The name void is used as a way of saying “no value is returned by this func- tion.” The second difference is that a void function definition does not require a return statement. The function execution ends when the last statement in the func- tion body is executed. A void function call is an executable statement. For example, the above function showResults might be called as follows: showResults(32.5, 0.3); If the above statement were executed in a program, it would cause the following to appear on the screen: 32.5 degrees Fahrenheit is equivalent to 0.3 degrees Celsius. void function definition void function call . as the main part of your pro- gram or in a separate file so that the functions can be used by several different programs. Display 3.4 A Function Using a Random Number Generator (part 2 of 2) S AMPLE . must write, is a bit more complicated. The description of the function is given in two parts. The first part is called the function declaration or function prototype. The following is the function. is for the function declaration and the main part of the program to appear in one or more files, with the function declaration before the main part of the program, and for the function definition