local var inside a function modifies another state variable

Question

I have this very simple contract:

    pragma solidity ^0.4.18;
    contract C{
    uint public a = 9;
    uint[] public data;

    function f(uint _a) public{
        a = _a;
        uint[] y;
        y.push(2900);
    }
}

I understand that local variables of type array (uint[] y) references storage, but I don't understand why y.push(2900) modifies storage variable named a, and no matter what value I push to y it always increments variable a with one unit. Thanks you

Answer 1

You are declaring y as a storage but you left unitialized so y points to the same slot than a.

The first 32 bytes of an array in storage is the array length, and after that follows the data.

So the first a = _a will set the array length. And then y.push(2900) will append a new value to the array incrementing its length.