我不明白为什么我的手工回答不同

Question

So here is the code:

import java.util.*;

public class HelloWorld{

     public static void main(String []args){
        ArrayList<Integer> what = new ArrayList<Integer>();
        what.add(3);
        what.add(7);
        what.add(2);
        for (int i=0; i<4;i++)
        {
            if(i%2==0)
            {
                what.add(0,i);
            }
            else
            {
                what.set(i,(what.get(i)*2));
            }
        }
        what.remove(3);
        System.out.println(what);
     }
}

When I solve this by hand, my answer comes out as: [2,0,2,2] However when plugging it in the correct answer is: [2,0,6,2] Can someone help me answer why I'm getting my answer and why the correct answer is correct?

Answer 1

Here I have debugged it using hand for you. See where you go wrong. add() function adds the provided integer to the position chosen by you while the set() function replaces the provided the integer with the integer at the place chosen by you.

Answer 2

I am not sure how you are doing it in your hand, but expected output by the program is correct. You can debug it to see where you are deviating and getting [2,0,2,2].

At no point, the array "what" has this value.

i = 0, [Start: 3,7,2 ]
i = 1, [Start:  0,3,7,2]
i = 2, [Start:  0,6,7,2]
i = 3, [Start:  2,0,6,7,2]
Loop End: [2, 0, 6, 14, 2]

Remove what[3]

Final Value: [2, 0, 6, 2]

Answer 3

initially your list has [3,7,2] elements,
you started a loop which will run from i=0 to 3
at i=0, condition i%2==0 is true,
it will add i at first position, list become [0,3,7,2]
i=1, condition false, element at i is 3, 3*2=6, put it at i, [0,6,7,2]
i=2, condition true, put i at first position [2,0,6,7,2]
i=3, condition false, element at i is 7, 7*2= 14, put it at i, [2,0,6,14,2]
loop ends.
remove element at position 3, i.e. 14, new list [2,0,6,2]