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8.11. Hard Multiple Choice Questions

These problems are harder than most of those that you will usually see on the AP CS A exam.

7-11-1: Consider the following data field and incomplete method, partialSum, which is intended to return an integer array sum such that for all i, sum[i] is equal to arr[0] + arr[1] + ... + arr[i]. For instance, if arr contains the values {1, 4, 1, 3}, the array sum will contain the values {1, 5, 6, 9}. Which of the following is true about the two implementations of missing code on line 9 that are proposed?

private int[] arr;

public int[] partialSum() {
   int[] sum = new int[arr.length];

   for (int j = 0; j < sum.length; j++)
      sum[j] = 0;

   /* missing code */
   return sum;
}


Implementation 1

for (int j = 0; j < arr.length; j++)
    sum[j] = sum[j - 1] + arr[j];


Implementation 2

for (int j = 0; j < arr.length; j++)
   for (int k = 0; k <= j; k++)
      sum[j] = sum [j] + arr[k];

• Both implementations work as intended and are equally fast.
• Implementation 1 doesn't work and will cause an ArrayIndexOutOfBoundsException. If Implementation 1 was correct, it would be faster.
• Both implementations work as intended, but implementation 1 is faster than implementation 2.
• Implementation 1 doesn't work and will cause an ArrayIndexOutOfBoundsException.
• Both implementations work as intended, but implementation 2 is faster than implementation 1.
• Implementation 1 doesn't work and will cause an ArrayIndexOutOfBoundsException. If it did work, it would be faster than 2.
• Implementation 1 does not work as intended, because it will cause an ArrayIndexOutOfBoundsException.
• When j is 0, sum[j-1] will be sum[-1] which will cause an ArrayIndexOutOfBoundsException.
• Implementation 2 does not work as intended, because it will cause an ArrayIndexOutOfBoundsException.
• Implementation 1 doesn't work and will cause an ArrayIndexOutOfBoundsException.

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