AP Computer Science A Chief Reader Report from the 2018 Administration © 2018 The College Board Visit the College Board on the Web www collegeboard org Chief Reader Report on Student Responses 2018 AP[.]
Chief Reader Report on Student Responses: 2018 AP® Computer Science A Free-Response Questions • Number of Students Scored • Number of Readers • Score Distribution • Global Mean 65,133 317 Exam Score 3.18 N 16,105 13,802 14,222 7,738 13,266 %At 24.7 21.2 21.8 11.9 20.4 The following comments on the 2018 free-response questions for AP® Computer Science A were written by the Chief Reader, John Cigas, Professor, Park University They give an overview of each free-response question and of how students performed on the question, including typical student errors General comments regarding the skills and content that students frequently have the most problems with are included Some suggestions for improving student preparation in these areas are also provided Teachers are encouraged to attend a College Board workshop to learn strategies for improving student performance in specific areas © 2018 The College Board Visit the College Board on the Web: www.collegeboard.org Question #1 Task: Methods and Control Max Points: Topic: Frog Simulation Mean Score: 5.34 What were the responses to this question expected to demonstrate? This question tested the student's ability to: • Write program code to create objects of a class and call methods; and • Write program code to satisfy methods using expressions, conditional statements, and iterative statements Students were provided with the specifications of the FrogSimulation class The FrogSimulation class encapsulates a simulation of a frog hopping in a straight line It contains two private integer instance variables, goalDistance and maxHops, which represent the distance in inches from the starting point to the goal and the maximum number of hops allowed to reach the goal It also contains a private method, hopDistance, which returns an integer representing the distance in inches to be moved when the frog hops Implementation for this method was not shown In part (a), students were asked to write the FrogSimulation method simulate, which determines whether a frog is successful in reaching goalDistance Students were required to use the private method hopDistance within the context of a loop to update an initialized variable representing the frog's position The loop iterates until one of the following conditions becomes true • • • The frog has reached or passed the goal, in which case a value of true is immediately returned The frog has reached a negative position, in which case a value of false is immediately returned A frog has taken maxHops hops without reaching the goal, in which case a value of false is returned In part (b), students were asked to write the FrogSimulation method runSimulations(int num), which uses a loop to call the simulate method num times Each time simulate returns true, a previously initialized variable is incremented The method returns a decimal value representing the proportion of simulations in which the frog successfully reached or passed the goal How well did the response address the course content related to this question? How well did the responses integrate the skills required on this question? Write program code to create objects of a class and call methods Both parts of this question involved calling a method within the context of a loop and then using the returned result Most responses were successful in calling the methods hopDistance and simulate within their respective loops and then using the returned result appropriately Write program code to satisfy methods using expressions, conditional statements, and iterative statements Both parts of this question involved the use of a loop with a specific upper bound The majority of responses demonstrated this concept However, in part (a), the question included additional conditions to trigger an early termination of the loop Responses were less successful with this concept, either using an incorrect comparison operator, placing the required conditional statements outside of the loop, or omitting at least one of the required conditions completely In part (b), most responses calculated a proportion by finding the quotient of two values However, a significant number of responses failed to return a correctly calculated decimal value © 2018 The College Board Visit the College Board on the Web: www.collegeboard.org What common student misconceptions or gaps in knowledge were seen in the responses to this question? Common Misconceptions/Knowledge Gaps of: Write program code to create objects of a class and call methods Responses that Demonstrate Understanding Responses failed to call the FrogSimulation instance method hopDistance from within the FrogSimulation class correctly position = hopDistance(); position = this.hopDistance(); position = sim.hopDistance(); position = FrogSimulation.hopDistance(); position = obj.hopDistance(); Common Misconceptions/Knowledge Gaps Write program code to satisfy methods using expressions, conditional statements, and iterative statements Responses that Demonstrate Understanding of: Responses failed to check for a negative position int position = 0; for (int x = 0; x < maxHops; x++) { postion += hopDistance(); if (position >= goalDistance) { return true; } } return false; or int position = 0; for (int x = 0; x < maxHops; x++) { position += hopDistance(); if (position >= goalDistance) { return true; } if (position < 0) { return false; } } return false; or © 2018 The College Board Visit the College Board on the Web: www.collegeboard.org int position = 0; for (int x = 0; x < maxHops; x++) { position += hopDistance(); } if (position >= goalDistance) { return true; } else { return false; } int position = 0; int numHops = 0; while (position < goalDistance && position >= && numHops < maxHops) { position += hopDistance(); numHops++; } return position >= goalDistance; or int position = 0; int numHops = 0; while (position < goalDistance && numHops < maxHops) { position += hopDistance(); numHops++; } return position >= goalDistance; Responses used integer arithmetic instead of double arithmetic to calculate a decimal value int count = 0; for (int x = 0; x < num; x++) { if (simulate()) { count++; } } return count / num; Declare a variable as double for use in the calculation double count = 0.0; for (int x = 0; x < num; x++) { if (simulate()) { count++; } } return count / num; © 2018 The College Board Visit the College Board on the Web: www.collegeboard.org Responses used improper casting to produce a double quotient from two integer values int count = 0; for (int x = 0; x < num; x++) { if (simulate()) { count++; } } return (double)(count / num); Cast an integer variable as a double within the calculation int count = 0; for (int x = 0; x < num; x++) { if (simulate()) { count++; } } return (double)count / num; Responses looped an incorrect number of times for (int x = 0; x = minLength && str.length() max || str.indexOf(notAllowed) != -1) { return true; } else { return false; } } Responses omitted the public access modifier on the isValid method header Ocassionally, students omitted the boolean return type or the parameter public boolean isValid(String str) boolean isValid(String str) public isValid(String str) public boolean isValid() Some responses added additional parameters to the method heading public boolean isValid(int min, int max, String str) © 2018 The College Board Visit the College Board on the Web: www.collegeboard.org Common Misconceptions/Knowledge Gaps of: Write program code to create objects of a class and call methods Responses that Demonstrate Understanding Many responses used an incorrect comparison when searching for the unwanted string str.indexOf(notAllowed) == -1 str.indexOf(notAllowed) minLength && str.length() < maxLength Responses frequently would compare the string length to the default values of and 20 str.length() >= && str.length() = minLength && str.length() maxLength or str.length() < || str.length() > 20 © 2018 The College Board Visit the College Board on the Web: www.collegeboard.org Responses only checked one condition when returning a value if (str.length() < minLength || str.length() > maxLength) { return false; } else { return true; } or if (str.indexOf(notAllowed) == -1) { return true; } else { return false; } Students who used nested conditionals often did not return a value under all circumstances if (str.length() < || str.length() > max) { return false; } if (str.indexOf(notAllowed) != -1) { return false; } return true; or if (str.length() < minLength || str.length() > maxLength) { return false; } else if (str.indexOf(notAllowed) != -1) { return false; } else { return true; } if (str.length() < minLength || str.length() > maxLength) { return false; } else if (str.indexOf(notAllowed) != -1) { return false; } © 2018 The College Board Visit the College Board on the Web: www.collegeboard.org ... = new ArrayList(); Some responses constructed and assigned a new ArrayList to a local variable named allPairs instead of the instance variable allPairs ArrayList allPairs =... more than just assigning values to the instance variables, such as using an array or list parameter to construct and populate an instance variable Reinforce that if an instance variable is an object,... Failure to declare any instance variables meant that students could not earn points for the declarations, the initialization in the 3-parameter constructor, the initialization in the 1-parameter constructor,