Matrix and Array Solidity

Question

I am trying to assign the value of a matriz to two arrays as follows:

pragma solidity ^0.4.24;
pragma experimental ABIEncoderV2;

contract ArrayExamples {

    int[][] public matriz;


    // Os dados são recebidos da seguinte forma: [0,0]
    function receive_data(int[] data) public {
        matriz.push(data);
    }


    function teste() public returns (int[] v) {

        int[] x;
        int[] y;

        uint t = matriz.length;

        for(uint i = 0; i < t; i++) {

            x[i] = matriz[i][0];
            y[i] = matriz[i][1];

        }

        return x;
    }

  }

The array is dynamic, but in the example above defaults to an array with two elements. [0.0]

When I assign the values ​​of x and y in for, only the values ​​of y are assigned correctly. x is responsible for taking only the first value from the dynamic array of the matrix and y takes the last value.

When I check the value of x, it appears that he took the last value from the dynamic array, and he should get the first value from the array.

If I enter the following array: [4,5] - both x and y get 5, where x should get 4.

Answer 1

You are using solidity v0.4 I'd suggest to use v0.5 instead

In solidity v0.4 the following generates a warning because x and y are not initialized.

function teste() public returns (int[] v) {
    int[] x;
    int[] y;

}

They don't have memory modifier so they refer to storage. They are not assigned a slot so by default they refer to slot 0 on storage which is used by matriz.

Your function in solidity 0.5 can be implemented like this:

function teste() public view returns (int[] memory v) {
    // allocate in memory
    int[] memory x = new int[](matriz.length);
    int[] memory y = new int[](matriz.length);

    uint t = matriz.length;
    for(uint i = 0; i < t; i++) {
        x[i] = matriz[i][0];
        y[i] = matriz[i][1];
    }
    return x;
}