为什么二维数组不加上z[0][0]的值

Question

#include<stdio.h>
int n,sum=0;
int z[3][3]={{3,6,8},{10,2,3},{7,1,9}};
void niuma();
int main()
{
    niuma();
    return 0;
}
void niuma()
{
    int x, y;
    for (x = 0; x < 3; x++)
    {
        for (y = 0; y < 3; y++)
        {
            if (x = y) { sum += z[x][y]; }
        }
    }


    printf("%d", sum);
}

Answer 1

第１７行：if (x = y) { sum += z[x][y]; }　， x == y　，少了一个‘＝’号