Swap rows from an array

Question

Below is my code. I need to exchange line 1 with line 3 and line 2 with line 4. The problem is that the third line is not changing and I don’t find the problem.

    int matriz[TAM][TAM], n = 0, x = 0, y = 0, aux = 0, aux2 = 0, aux3 = 0;

    printf("Informe um tamanho par para sua matriz:  \n");
    scanf("%d", &n);

    for(x = 0; x < n; x++){
        for(y = 0; y < n; y++){

            printf("Preencha a matriz: \n");
            scanf("%d", &aux2);

            matriz[x][y]=aux2;

       }
    }

    for(x = 0; x < n; x++){
        for(y = 0; y < n; y++){
                if (x<=(n/2)){

                aux = matriz[x][y];
                matriz[x][y] = matriz[x+2][y];
                matriz[x+2][y] = aux;
                printf("%d ", matriz[x][y]);
                }

                if ( x==3 ||(x>(n/2)))
                {
                aux3 = matriz[x][y];
                matriz[x][y] = matriz[x-2][y];
                matriz[x-2][y] = aux3;
                printf("%d ", matriz[x-2][y]);
                }

        }

    printf("\n");
    }
   }

Answer 1

Instead of x + 2 and x - 2 which will only work if you are informed 4 to n, use x + n / 2 and x - n / 2.

What is repeated within the two ifs can be moved to before or after them.

Remember that the first index is zero. This means that your if (x<=(n/2)), in a four-line matrix, will go to the third line, and not to the second. What you wanted was to use the < instead of <=.

Instead of using if ( x==3 ||(x>(n/2))), just use else. Also, due to the previous item, it should be >= instead of >, which would eliminate the need for this x==3.

In the printf of the first if, you use matriz[x][y], but the second one uses matriz[x-2][y].

You also don’t need a lot of variables aux, only one is needed.

Here’s what your code looks like with these fixes:

int matriz[TAM][TAM], n = 0, x = 0, y = 0;

printf("Informe um tamanho par para sua matriz:  \n");
scanf("%d", &n);

for (x = 0; x < n; x++) {
    for (y = 0; y < n; y++) {
        printf("Preencha a matriz: \n");
        scanf("%d", &matriz[x][y]);
   }
}

for (x = 0; x < n; x++) {
    for (y = 0; y < n; y++) {
        int aux = matriz[x][y];
        if (x < n / 2) {
            matriz[x][y] = matriz[x + n / 2][y];
            matriz[x + n / 2][y] = aux;
        } else {
            matriz[x][y] = matriz[x - n / 2][y];
            matriz[x - n / 2][y] = aux;
        }
        printf("%d ", matriz[x][y]);
    }

    printf("\n");
}

It is possible to simplify your code a little more and unify the if and the else when checking the position of the line you want to exchange (let’s call nx) can be calculated from x thus:

int nx = (x + n / 2) % n;

To see how this works, see that being n equal to 4, we will have to:

• If x for 0, nx will be 2.
• If x for 1, nx will be 3.
• If x for 2, nx will be 0.
• If x for 3, nx will be 1.

This will work for any other even value of n.

This way, your code looks like this:

int matriz[TAM][TAM], n = 0, x = 0, y = 0;

printf("Informe um tamanho par para sua matriz:  \n");
scanf("%d", &n);

for (x = 0; x < n; x++) {
    for (y = 0; y < n; y++) {
        printf("Preencha a matriz: \n");
        scanf("%d", &matriz[x][y]);
   }
}

for (x = 0; x < n; x++) {
    for (y = 0; y < n; y++) {
        int nx = (x + n / 2) % n;
        int aux = matriz[x][y];
        matriz[x][y] = matriz[nx][y];
        matriz[nx][y] = aux;
        printf("%d ", matriz[x][y]);
    }

    printf("\n");
}