Returning New Array and getting wrong output - arrays

Question

#define N 3
int subMatrix(int a[][N]) {
int i, j;
int sum = 0;
int arr[N];
for (i = 0; i < N; i++) {
for (j = 0; j < N; j++) {
sum += a[i][j];
sum -= a[j][i];
}
arr[i] = sum;
sum = 0;
}
return *arr;
}
void main() {
int a[N][N] = {
{9,2,4},
{3,7,11},
{3,1,2}
};
for (int i = 0; i < N; i++) {
printf("%5d", subMatrix(a[i]));
}
}
The function works fine, the problem is when I'm returning the new array and loop over it in the main function I get the first element of the array and the other elements are addresses.
i did it before with another array with size of doubles and it worked.
There is something i miss?
double avgMatrix(int a[][C]) {
int i, j, sum=0;
double M[R];
for (i = 0; i < R; i++) {
for (j = 0; j < C; j++) {
sum += a[i][j];
}
M[i] = (double)sum / C;
sum = 0;
}
return *M;
}
void main() {
int a[R][C] = {
{9,2,4},
{3,8,11},
{3,1,2}
};
for (int i = 0; i < R; i++)
printf("%5.2lf", avgMatrix(a[i]));
}
this code works. what can be the difference?

Answer 1

I do not really understand what your function subMatrix does.
Your code needs a few modifications to be able to compile.
First, include the necessary header #include <stdio.h>, because your code needs printf.
Second, make sure the passed parameter and the attribute be the same type.
Third, if you would like to return an array from a function, you should use dynamic allocation function to help you do that. malloc
/* At least, make sure to include necessary head files
* #include <stdio.h>
*/
#define N 3
int subMatrix(int a[][N]) {
int i, j;
int sum = 0;
int arr[N];
for (i = 0; i < N; i++) {
for (j = 0; j < N; j++) {
/* Because the passed parameter is one dimensional
* so the following code does not make sense.
*/
sum += a[i][j];
sum -= a[j][i];
}
arr[i] = sum;
sum = 0;
}
/* arr is a local variable. It is actually a pointer
* It should never be returned.
* In fact, *arr is only the first element of the array of arr.
* At least, you should return the address of the first element.
* Considering your purpose, to use dynamic allocation is proper.
*/
return *arr;
}
/* 'void main()' is not right.
* 'int main(void)' is the right way.
*/
void main() {
int a[N][N] = {
{9,2,4},
{3,7,11},
{3,1,2}
};
for (int i = 0; i < N; i++) {
/* a[i] is a one-dimensional array,
* but subMatrix needs a two dimensional one.
*/
printf("%5d", subMatrix(a[i]));
}
}
A possible working code:
#include <stdio.h>
#include <stdlib.h>
#define N 3
int *subMatrix(int a[N][N]) {
int sum = 0;
int *arr = (int *)malloc(N*sizeof(int));
for (int i = 0; i < N; i++) {
for (int j = 0; j < N; j++) {
sum += a[i][j];
sum -= a[j][i];
}
arr[i] = sum;
sum = 0;
}
return arr;
}
int main(void) {
int a[N][N] = {
{9,2,4},
{3,7,11},
{3,1,2}
};
int *arr = subMatrix(a);
for (int i = 0; i < N; i++)
printf("arr[%d] : %d\n", i, arr[i]);
free(arr);
}
Is this what you want? Try it.