Wrong values after using forEach in Arraylist-CodePudding

I`m doing the freecodecamp beginner tutorial and I have a problem with forEach function in Arraylist. Following Code


number1.forEach(number -> {
      number1.set(number1.indexOf(number), number * 2);
      
    });
    System.out.println(number1);

The values are [2, 4, 11, 12] so I would expect to get [4, 8, 22, 24] but instead I get [8, 4, 22, 24]

Any Ideas what`s wrong?

As stated above. I tried to check with indexOf if the values and indexes aren`t switched, because I used Sort function earlier, but they seem to be ok. Other than that, everything went smooth and easy. IDK

CodePudding user response：

You start with [2, 4, 11, 12]

During the first iteration, number = 2

You then call number1.set(number1.indexOf(2), 2 * 2)

We now have [4, 4, 11, 12]

During the second iteration, number = 4

Once again, we call number1.set(number1.indexOf(4), 4 * 2)

Here is the problem: indexOf returns the index of the first instance of that value!

So, number1.indexOf(4) = 0, so it places 8 into index 0 instead of index 1

As @Tibrogargan mentioned in the comments, this is a prime example of why you shouldn't modify an array while iterating across it

Your code for number1 will not work whenever there exists a number Y in number1 such that there exists a number X where Y = 2X