Developing Algorithms Using ArrayLists Practice

Sample questions

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1. Sample 1difficulty 2/5

   ArrayList<Integer> a = new ArrayList<>();
   a.add(1);
   a.add(2);
   a.add(3);
   ArrayList<Integer> b = new ArrayList<>();
   for (int x : a) b.add(x * 2);
   System.out.println(b);

   What is printed?

   • A

     [2, 4, 6]

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   • B

     [1, 2, 3]

   • C

     [1, 4, 9]

   • D

     []

   Why

   Each element of a is doubled and added to b: 1<em>2=2, 2</em>2=4, 3*2=6.

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2. Sample 2difficulty 2/5

   int[] arr = {3, 1, 4, 1, 5};
   ArrayList<Integer> list = new ArrayList<>();
   for (int x : arr) list.add(x);
   System.out.println(list.size());

   What is printed?

   • A

     0

   • B

     4

   • C

     3

   • D

     5

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   Why

   Each of the 5 elements is added to the list, so size is 5.

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3. Sample 3difficulty 2/5

   ArrayList<Integer> list = new ArrayList<>();
   list.add(5);
   list.add(2);
   list.add(5);
   list.add(7);
   list.add(5);
   list.add(2);
   int count = 0;
   for (int x : list) {
       if (x == 5) count++;
   }
   System.out.println(count);

   What is printed?

   • A

     2

   • B

     4

   • C

     0

   • D

     3

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   Why

   The value 5 appears at indices 0, 2, and 4. Count = 3.

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4. Sample 4difficulty 3/5

   ArrayList<Integer> list = new ArrayList<>();
   list.add(1); list.add(2); list.add(3); list.add(4);
   for (int i = 0; i < list.size(); i++) {
     if (list.get(i) % 2 == 0) {
       list.remove(i);
     }
   }

   What is list after the loop?

   • A

     [2, 4]

   • B

     [1, 3]

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   • C

     [1, 3, 4]

   • D

     [1, 2, 3, 4]

   Why

   Removing while incrementing skips elements; here both 2 and 4 happen to get removed correctly only with care, leaving [1,3].

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5. Sample 5difficulty 3/5

   ArrayList<Integer> list = new ArrayList<>();
   list.add(2); list.add(2); list.add(3); list.add(4);
   for (int i = 0; i < list.size(); i++) {
     if (list.get(i) == 2) {
       list.remove(i);
     }
   }

   What does list contain?

   • A

     []

   • B

     [3, 4]

   • C

     [2, 2, 3, 4]

   • D

     [2, 3, 4]

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   Why

   After removing index 0, the second 2 shifts to index 0 but i becomes 1, skipping it.

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