C - Wrong printed values on a matrix

Question

So I created a program that generates a random 4x4 matrix, with values ranging from 0 to 17. Afterwards, it would calculate and print the sum of each rows and columns. The problems are, (1) values of row sums are incorrect, (2) the last element, which is the m[4][4], seems to double in value, giving an incorrect value for the sum of Column 4 as well. Here is the code:

srand(time(NULL));

printf("Generating matrix... ");
getch();
printf("\n\n");

for (i = 1; i <= 4; i++) {
    for (j = 1; j <= 4; j++) {

        m[i][j] = rand() % 17;
        printf("%d\t", m[i][j]);

        if (i == 1) {                          //  Calculation of Row Sums
            rSum[1] += m[i][j];
        } else if (i == 2) {
            rSum[2] += m[i][j];
        } else if (i == 3) {
            rSum[3] += m[i][j];
        } else if (i == 4) {
            rSum[4] += m[i][j];
        }

        if (j == 1) {                        //  Calculation of Column Sums
            cSum[1] += m[i][j];
        } else if (j == 2) {
            cSum[2] += m[i][j];
        } else if (j == 3) {
            cSum[3] += m[i][j];
        } else if (j == 4) {
            printf("\n");
            cSum[4] += m[i][j];
        }
    }
}

getch();
printf("\n\n");

for (i = 1; i <= 4; i++) {                  //  Printing of Values

    printf("Sum of Row %d: %d\n", i, rSum[i]);
    printf("Sum of Column %d: %d\n", i, cSum[i]);
    printf("-----------------------------------------------\n");
}

return 0;

Answer 1

Arrays are 0 based in C. You should change all your loops to iterate this way:

for (i = 0; i < 4; i++) {

And also change other parts of your code where you use explicit index values. This part can be simplified drastically.

The arrays rSum and cSum must be initialized to 0. Failure to do it might explain the incorrect values computed by your program.

Here is an improved version:

#include <stdio.h>
#include <stdlib.h>

int main(void) {
    int m[4][4];
    int rSum[4] = { 0 };
    int cSum[4] = { 0 };
    int i, j;

    srand(time(NULL));

    printf("Generating matrix... ");
    getch();
    printf("\n\n");

    for (i = 0; i < 4; i++) {
        for (j = 0; j < 4; j++) {
            m[i][j] = rand() % 17;
            printf("%d\t", m[i][j]);

            rSum[i] += m[i][j];    //  Calculation of Row Sums
            cSum[j] += m[i][j];    //  Calculation of Column Sums
        }
        printf("\n");
    }

    getch();
    printf("\n\n");

    for (i = 0; i < 4; i++) {         //  Printing of Values
        printf("Sum of Row %d: %d\n", i, rSum[i]);
        printf("Sum of Column %d: %d\n", i, cSum[i]);
        printf("-----------------------------------------------\n");
    }

    return 0;
}