how to implement matrix functions in C

Question

I am trying to make some simple functions that does some operations on matrixes. But i can`t undestand why the following code is not working!. Can you please help me to understand what is wrong here and what should i do???

void create(int*** p, const int n)
{
    *p = (int**)calloc(n, sizeof(int));
    if(*p == NULL){
        printf("Error1");
    }

    int i;
    for(i = 0; i < n; i++){
        (*(p[i]) = (int*)calloc(n, sizeof(int)));

        if(*(p[i]) == NULL){
            printf("Error2");
        }
    }
}
void initializeMatrix(int*** p, const int n)
{
    int i, j;

    for(i = 0; i < n; i++){
        for(j = 0; j < n; j++){
            scanf("%d", &((p[i][j])));
        }
    }
}
void show(const int** p, const int n)
{
    int i, j;
    for(i = 0; i < n; i++){
        for(j = 0; j < n; j++){
            printf("%d ", ((p)[i][j]));
        }
        printf("\n");
    }
}
int main()
{
    int** p = NULL;

    create(&p, 2);
initializeMatrix(&p, 2);

    show(p, 2);

    return 0;
}

you size is wrong here *p = (int**)calloc(n, sizeof(int)); should be *p = (int**)calloc(n, sizeof(int*)); — pm100, Apr 23 '18 at 18:19
Being a [three star programmer](http://wiki.c2.com/?ThreeStarProgrammer) is not a compliment. Instead of your `create` function emulating pass-by-reference let it *return* the created matrix. — Some programmer dude, Apr 23 '18 at 18:20
As for your problem, all allocation functions (including [`calloc`](http://en.cppreference.com/w/c/memory/calloc)) are measuring the size in *bytes*. The size the memory allocated by `calloc` is `n * sizeof(int)` bytes (in your case). — Some programmer dude, Apr 23 '18 at 18:22
By the way, in C you [don't need to cast the result of `calloc` (or other memory allocation functions)](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc). — Some programmer dude, Apr 23 '18 at 18:28
Another major problem, take a close look at your `initializeMatrix` function. Are you using the pointer `p` correctly there? Again, don't emulate pass-by-reference. — Some programmer dude, Apr 23 '18 at 18:29
See [this answer](https://stackoverflow.com/a/47235897/841108) to a duplicate question — Basile Starynkevitch, Apr 23 '18 at 18:50

R Sahu · Answer 1 · 2018-04-23T18:42:27.073

2

Problem 1

You are using *(p[i]) in create. it should be (*p)[i].

void create(int*** p, const int n)
{
    *p = (int**)calloc(n, sizeof(int));
    if(*p == NULL){
        printf("Error1");
    }

    int i;
    for(i = 0; i < n; i++){

        // FIXED 
        (*p)[i] = (int*)calloc(n, sizeof(int)));

        // FIXED
        if( (*p)[i] == NULL){
            printf("Error2");
        }
    }
}

Problem 2

You are passing the wrong size to first calloc in the function. You need to use sizeof(int*). After all, you want the returned value to be a pointer to n pointers to int, not a pointer to n objects of type int.

This is a critical error that leads to undefined behavior unless sizeof(int) is the same as sizeof(int*) on your platform.

Change the line to

    *p = (int**)calloc(n, sizeof(int*));

More importantly, you can make that function a bit easier by returning a pointer. It will be easier to write such a function.

int** create(int n)
    int **p = calloc(n, sizeof(*p));
    if(p == NULL){
        printf("Error1");
    }

    int i;
    for(i = 0; i < n; i++){

        p[i] = calloc(n, sizeof(*p[i])));

        if( p[i] == NULL){
            printf("Error2");
        }
    }

    return p;
}

and use it as:

int** p = create(2);

edited Apr 23 '18 at 18:42

answered Apr 23 '18 at 18:27

R Sahu

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Probably one [Calloc() call it will be cheaper](https://ideone.com/MhY6il) – Michi Apr 23 '18 at 18:55
@Michi, it definitely will. Then the OP has to map the two indices to one index appropriately. – R Sahu Apr 23 '18 at 19:03
@R Sahu You right but I'm sure that the OP needs to understand all of this. – Michi Apr 23 '18 at 19:05
@Michi Your `arr = calloc(sizeof( *arr ), sizeof *arr * row * col);` call uses incorrect size calculation. – chux - Reinstate Monica Apr 23 '18 at 19:05
1

UV for good use of `sizeof(* object_pointer)` rather than `sizeof( object_pointer_type)` – chux - Reinstate Monica Apr 23 '18 at 19:09

how to implement matrix functions in C

1 Answers1

Problem 1

Problem 2