Adding Matrices Code in C HELP! I don't know what's wrong

Question

The assignment asks us to finish a code that adds matrices up and I'm not too sure what I'm supposed to be returning. It tells me that there's a runtime error but I don't know how to fix it. It will be due pretty soon so someone please help me!

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

int** getMatrix(int n, int m);
int** allocateMatrix(int n, int m);
int** addMatrices(int** A, int** B, int n, int m);
void printMatrix(int** A, int n, int m);
void deallocateMatrix(int** A, int n);


// This program reads in two n by m matrices A and B and
// prints their sum C = A + B
//
// This function is complete, you do not need to modify it
// for your homework
int main() {
    int n = 0, m = 0;
    printf("Enter the number of rows and columns: ");
    scanf("%d %d", &n, &m);
    assert(n > 0 && m > 0);
    printf("Enter matrix A:\n");
    int** A = getMatrix(n, m);
    printf("Enter matrix B:\n");
    int** B = getMatrix(n, m);
    int** C = addMatrices(A, B, n, m);
    printf("A + B = \n");
    printMatrix(C, n, m);
    deallocateMatrix(A, n);
    deallocateMatrix(B, n);
    deallocateMatrix(C, n);
}

// Creates a new n by m matrix whose elements are read from stdin
//
// This function is complete, you do not need to modify it
// for your homework
int** getMatrix(int n, int m) {
int** M = allocateMatrix(n, m);
int i, j;

for ( i = 0; i < n; i++) {
    printf("Input row %d elements, separated by spaces: ", i);
    for ( j = 0; j < m; j++) {
        scanf("%d", &M[i][j]);
    }
}
return M;
}

// Allocates space for an n by m matrix of ints
// and returns the result
int** allocateMatrix(int n, int m) {
// Homework TODO: Implement this function
int i, j;
int** L;

L = (int**)malloc(sizeof(int) * n);
for(i = 0; i < n; i++) {
    for(j = 0; j < m; j++){
        L[i] = (int*) malloc(sizeof(int) * m);
    }
}
}

// Adds two matrices together and returns the result
int** addMatrices(int** A, int** B, int n, int m) {
// Homework TODO: Implement this function
int j, i;
int** C;

for(i = 0; i < n; i++) {
    for(j = 0; j < m; j++) {
        C[i][j] = A[i][j] + B[i][j];
    }
}

}

// Prints out the entries of the matrix
void printMatrix(int** A, int n, int m) {
// Homework TODO: Implement this function
int i, j;
int** C;

for (i = 0; i < n; i++) {
  for (j = 0 ; j < m; j++) {
     printf("%d\t", C[i][j]);
  }
  printf("\n");
  }
}

// Deallocates space used by the matrix
void deallocateMatrix(int** A, int n) {
int i;

for ( i = 0; i < n; i++) {
    free(A[i]);
}
free(A);
}

Unrelated to your problems but please [read this discussion](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) about casting the result of `malloc`. — Some programmer dude, Feb 22 '17 at 04:38

score 1 · Answer 1 · answered Feb 22 '17 at 04:36

1

A few things:

In allocateMatrix you allocate space for n integers, not pointers to integers.
In many function you use an uninitialized pointer C.
In many function declared to return something, you don't return anything at all.

These things, and probably others, leads to undefined behavior.

answered Feb 22 '17 at 04:36

Some programmer dude

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I just recently learned how to allocate spaces, I don't really understand what I'm supposed to do/use malloc. Also aren't I initializing them with "int** C;"? – Sara Feb 22 '17 at 04:43
Should I return like: "return C**;"? and "return L**;"? – Sara Feb 22 '17 at 04:43
@Sara A pointer must point somewhere for it to be valid. You must initialize it, or assign to it. And yes you should do e.g. `return L;` in your `allocateMatrix` function. – Some programmer dude Feb 22 '17 at 04:55
Where would I point it to and how do I do that? int **C;? I'm sorry for so many questions. I'm not that good at this :/ – Sara Feb 22 '17 at 05:03
I know that pointers point to things but I don't know when to use them and why we would need anything to point to anything. – Sara Feb 22 '17 at 05:05
@Sara Allocate memory for `C` as well, just like you did for `A` and `B` – Spikatrix Feb 22 '17 at 05:24
Now my code is running but it's giving me only 0's. I added in int** C = allocateMatrix(n, m); to all the int** C I had, is the problem in my addMatrices? @CoolGuy – Sara Feb 22 '17 at 05:29

score 0 · Answer 2 · edited May 23 '17 at 12:09

Expanding a bit on @Someprogrammerdude 's answer

You should be allocating room for n integer pointers (int*s) and then in each row, allocate memory for m integers (ints). So, use

// Allocates space for an n by m matrix of ints
// and returns the result
int** allocateMatrix(int n, int m) {
    int i, j;
    int** L;

    L = malloc(sizeof(int*) * n);       /* Make `n` rows */
    for(i = 0; i < n; i++) {            /* For each row, */
        L[i] = malloc(sizeof(int) * m); /* Make room for `m` ints */
    }

    return L; /* Return the allocated variable. See 3rd point ↓ */
}

I've removed the casts because they aren't recommended. I leave error checking malloc upto you.

You use C uninitialized: int** C;. Allocate memory for it just as you did for A and B:
```
int** C = allocateMatrix(n, m);
```
You forgot to return variables from functions. Add return L; and return C; at the end of the functions allocateMatrix and addMatrices respectively.

Adding Matrices Code in C HELP! I don't know what's wrong

2 Answers2