AP Computer Science A : Logic Errors

Study concepts, example questions & explanations for AP Computer Science A

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Example Questions

Example Question #1 : Logic Errors

Consider the following code:

public static int[] del(int[] a,int delIndex) {

     if(delIndex < 0 || delIndex >= a.length) {

          return null;

     }

     int[] ret = new int[a.length - 1];

     for(int i = 0; i < a.length; i++) {

          if(i != delIndex) {

               ret[i] = a[i];

          }

     }

     return ret;

}

What is the error in the code above?

Possible Answers:

You need to implement a swap for the values.

There is a null pointer exception.

The intial conditional (i.e. the if statement) has incorrect logic.

The use of the array index i is incorrect.

The loop is infinite.

Correct answer:

The use of the array index i is incorrect.

Explanation:

The problematic line in the code above is the one that reads:

ret[i] = a[i];

This is going to work well until the end of the array.  The variable i is going to go for the length of the array a.  However, the array ret is one less in length.  This means that you will overrun your array, getting an ArrayIndexOutOfBoundsException at the very end of the looping.  You would need to implement two indices—one of which will not be incremented on that one time when you skip the element that is removed from the array.

Example Question #2 : Logic Errors

What is wrong with this loop?

int i = 0;

int count = 20;

int[] arr = [1, 3, 4, 6, 8, 768, 78, 9]

while (i < count) {

  if (i > 0) {

    arr[i] = count;

  }

  if (i < 15) {

      System.out.println("HEY");

  }

}

Possible Answers:

it prints Hey too many times

if statement is incorrect

The loop is infinite

count never gets decremented

Correct answer:

The loop is infinite

Explanation:

The loop is infinite because i never gets incremented to become greater than count. 

The loop in this case is,

while (i < count) {

  if (i > 0) {

    arr[i] = count;

  }

and the error occurs because there is no terminating factor in the while loop. A terminating factor would be,

while(i<count){

   if(i>0, i<10, i++){

    arr[i] = count;

  }

 

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