Trying to get index values of an ArrayList and add them to another ArrayList-CodePudding

Maybe somebody could help me. I don't understand what is wrong with my code.

I want loop through the ArrayList, get the index value of each element (index number itself) and add these index values to another array. Don't ask me why =))

Here's my code:

List<Integer> a = new ArrayList<>();
a.add(2);
a.add(5);
a.add(1);

List<Integer> b = new ArrayList<>();

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

    b.add(a.indexOf(i));

}

System.out.println("array b: "   b);

Output: array b: [-1, 2, 0]

But my expected output is: [0, 1, 2]

Maybe it's a very simple question but I'm really stuck and don't understand why the output is so strange.

CodePudding user response：

Because in your code you are checking what index of the element

like you define loop will iterate 3 time with i=0, i=1 and i =2

when i =0, there are no such element 0 in your array list it has only 2, 5, 1 so it is returning -1

when i = 1 you have an element, so it is returning the value of the index that's 2

when i = 2 again you have an element in array list, so it is returning an index of 2 that's 2

what you can do is

b.add(a.indexOf(i)); replace this line with b.add(a.indexOf(a.get(i)));

but that simply means adding i to new list

CodePudding user response：

The problem is that indexOf give you the index of the element. In the loop, the i has the following values - 0, 1, 2

On the first iteration, there is no 0, so you have -1
On the second iteration, there is 1 on the second index, so you have 2
On the third iteration, there is 2 on the index 0, so you have 0