Question about dynamic arrays

Question

I am currently playing around with Solidity and dynamic arrays. I have the following code:

contract Test{
    int256[] public test;

    function setTest(int256 b) public{
        test.push(b);
    }

    function setTest2(uint256 a, int256 b) public {
        test[a] = b;
    }
}

I executed the code in remix. When I call setTest(1) and call the getter test(0), the output is 1. If I do the same with setTest2(0, 1) and call the getter afterwards, the value does not seem to be saved. I don't understand why. Could someone explain what is going on here?

Thanks in advance.

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I realized that my question is not really clear and I will ask the question here and this time more clear. Thanks for the other answers already.

I call the function setTest2(0, 1) and call the getter afterwards, remix shows me that the value at position 0 is 0 and not 1. When I call the setTest(2) function and call the getter afterwards, the value 2 is at position 0 of the array and not at position 1. This means that the value of the first call has not been saved. Why?

Answer 1

When using Dynamic arrays, you can't use DynamicArray[a] = b, if position a is not yet filled by the push function. If it would be a Static array, then it would work.

Answer 2

When you call setTest(1), your array contains 1 at the position 0. After that, you call setTest2(0,1), which sets at the position 0 the value 1.

The value is the same, so you won't notice any difference. If you call setTest2(0,5), you will see it will be different.

Answer 3

Regarding your edit.
I just checked and it seems that calling setTest() on a position of the array, that hasn't been initialized previously doesn't work. So in otherwords, you can use that to change a value, but not add a new one.
That's why your setTest() always starts at the beginning. You first need to push a new element to a position in order for setTest2() to have any effect.