I have problems making a function to handle a matrix using calloc

Question

I want to dynamicly construct a matrix in C from a text document using a function. I ran into problems when making the matrix using calloc and probably when giving the values to the matrice elements, and I couldn't find anything. I can handle a vector.

Code here:

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

void beolvas_ellista(int *, int *, int *, int *, int ***, char *);

int main()
{
    int ir, suly, csom, el, i, **ellista;

    //The following commented code works
    /*FILE * f;
    f=fopen("be.txt","r");

    fscanf(f,"%d",&ir);
    fscanf(f,"%d",&suly);
    fscanf(f,"%d",&csom);
    fscanf(f,"%d",&el);

    ellista=(int **)calloc(2,sizeof(int *));
    for(i=0;i<el;++i)
    {
        ellista[i]=(int *)calloc(el,sizeof(int));
    }
    i=0;
    while(!feof(f))
    {
        fscanf(f,"%d",&ellista[0][i]);
        fscanf(f,"%d",&ellista[1][i]);
        ++i;
    }

    for(i=0;i<el;++i)
        printf("%d %d\n",ellista[0][i],ellista[1][i]);

    fclose(f);*/

    beolvas_ellista(&ir, &suly, &csom, &el, &ellista, "be.txt");

    for(i=0;i<el;++i)
        printf("%d %d\n",ellista[0][i],ellista[1][i]);

    return 0;
}

void beolvas_ellista(int *ir, int *suly, int *csom, int *el, int ***ellista, char *allomany)
{
    int i;
    FILE * f;
    f=fopen(allomany,"r");

    fscanf(f,"%d",ir);
    fscanf(f,"%d",suly);
    fscanf(f,"%d",csom);
    fscanf(f,"%d",el);

    *ellista=(int **)calloc(2,sizeof(int *));
    for(i=0;i<*el;++i)
    {
        *ellista[i]=(int *)calloc(*el,sizeof(int));
    }

    i=0;
    while(!feof(f))
    {
        fscanf(f,"%d",ellista[0][i]);
        fscanf(f,"%d",ellista[1][i]);
        ++i;
    }

    fclose(f);
}

Here is the text file:

be.txt

Also here is the code that I used to gather information:

void beolvas(int*pn, int**pa, char*allomany)
{
int i;FILE*f;
f=fopen(allomany,"r");
fscanf(f,"%d",pn);
*pa=(int*)malloc((*pn)*sizeof(int));
for(i=0; i<*pn; i++)
fscanf(f,"%d",(*pa)+i);
fclose(f);
}
main()
{
int n, *a;
beolvas(&n, &a, "be.txt");
...
}

`int ***` is the sign of that you're doing something very, *very* wrong. — , Oct 28 '13 at 20:00
@H2CO3 if there is a better way of doing it can you give me an example please, i only know this type of transmitting 2D matrix as a poiter. — tibike_m, Oct 28 '13 at 22:11
I'd suggest that you use a true 2D array (or at least a pointer to its first element): `int (*arr)[COLS] = malloc(ROWS * sizeof *arr);`, etc. Also, please format your code (use indentation, whitespace) and use English function and variable names so that the code is easier to read. — , Oct 29 '13 at 05:16

user2927610 · Answer 1 · 2013-10-28T18:58:27.997

0

Your input file does not fit the code. You say in your input file *el = 8. In your code you take this as row count, but actually only spending 2 rows to the matrix in your first calloc call...

P.S. Try this instead:

Edit: Wrong analyse by me. Your actual problem is a predecence one. You need to dereference your * in the right way with brackets set, like so:

void beolvas_ellista(int *ir, int *suly, int *csom, int *el, int ***ellista, char *allomany)
{
    int i;
    FILE * f;
    f=fopen(allomany,"r");

    fscanf(f,"%d",ir);
    fscanf(f,"%d",suly);
    fscanf(f,"%d",csom);
    fscanf(f,"%d",el);


    *ellista=(int **)calloc(2,sizeof(int *));
    for(i=0;i<*el;++i)
    {
        (*ellista)[i]=(int *)calloc(*el,sizeof(int));

    }



    i=0;

    while(!feof(f))
    {
        fscanf(f,"%d",&((*ellista)[0][i]));
        fscanf(f,"%d",&((*ellista)[1][i]));
        ++i;
    }

    fclose(f);
}

edited Oct 28 '13 at 18:58

answered Oct 28 '13 at 18:49

user2927610

1
1

the matrice is 2*8, i tested the code and it works, i have some poiter issues inside the function at allocating the space or when giving the values and i can't figure that out – tibike_m Oct 28 '13 at 18:53
@Miklosflame "i tested the code and it works" No it doesn't. Your `feof` check in your while-loop is wrong. `feof` will not be established on the last line read; it will be established on the first read-attempt *after* that. And the read-attempts you're using, `fscanf()` are completely unchecked for validity. In other words, assuming the sizing is done correctly, you're overrunning your allocated matrix size and walking straight into the land of **undefined behavior**. [See this question](http://stackoverflow.com/questions/5431941/while-feof-file-is-always-wrong). – WhozCraig Oct 28 '13 at 19:02
Please have a look at my edit, especially for the location in calloc and fscanf calls how to set brackets when dereferencing "ellista". My first analyse was wrong, sorry for that. – user2927610 Oct 28 '13 at 19:05
@user2927610 i've corrected that mistake but still get errors, so it is now because of the file reading. – tibike_m Oct 28 '13 at 19:16
@WhozCraig in the main function i had no problems with the feof, but now i'm trying to fix it – tibike_m Oct 28 '13 at 19:22
@Miklosflame You did, you just didn't know it. – WhozCraig Oct 28 '13 at 19:30
@WhozCraig is it only the while loop error beside the correction of user2927610 ? – tibike_m Oct 28 '13 at 19:50
@Miklosflame No, thats not the only issue. But I see you were looking for a 2xN matrix, so I'll delete the second implementation in my answer. – WhozCraig Oct 28 '13 at 19:58

score 0 · Accepted Answer · edited May 23 '17 at 12:05

The following bullet-list the items wrong in your function.

You're incorrectly using feof() as the break condition of your while loop. See this question for more information.
You ignore the return result of fscanf() and with that thereby have no assurance at all the parameter parsing succeeded or not. See the documentation of fscanf().
Your code does not fit the model of the file content. The file, according to your code, should set ir, suly, csom, and el to values 0, 0, 7, and 8 respectively. You then allocate space for exactly two pointers-to-int, saving the result in ellista, then proceed to index into *ellista up to el items, which is clearly not 2. It is neither clear nor evident that you want a 2xN matrix or an Nx2 matrix when this is finished, and the code as-written does neither correctly.
Stylistic, but helpful: You should be setting your out-parameters on success of your function, not on initial entrance or parse. Eg: Your ellista by-address parameter should be set as the last operation, not the first, based on the success of the function. Declare a local int** local; temp var, run your algorithm populating that, and upon success set the out-parameter.

All of that said, I think you want a 2xN matrix, and if so, the code below will do that. Note this does not check the results of the malloc and calloc calls, which i leave to you. This function will return zero (0) on success, non-zero on failure :

int beolvas_ellista(int *ir, int *suly, int *csom, int *el, int ***ellista, const char *allomany)
{
    FILE * f = fopen(allomany,"r");
    int ** local =  NULL;
    int i, res = -1;

    if (f == NULL)
        return res;

    if (fscanf(f,"%d",ir) == 1 &&
        fscanf(f, "%d",suly) == 1 &&
        fscanf(f,"%d",csom) == 1 &&
        fscanf(f,"%d",el) == 1)
    {
        // allocate two pointers, then in those two pointers, allocate
        //  space for *el integers.
        local = malloc(2 * sizeof(*local));
        local[0] = calloc(*el, sizeof(*(local[0])));
        local[1] = calloc(*el, sizeof(*(local[0])));

        for (i=0; i<*el; ++i)
        {
            if (fscanf(f, "%d", local[0]+i) != 1 ||
                fscanf(f, "%d", local[1]+i) != 1)
                break;
        }

        // only if i == *el did we finish the above
        if (i == *el)
        {
            *ellista = local;
            res = 0;
        }
        else
        {   // failed to read file content. free up memory
            // and return error state.
            free(local[0]);
            free(local[1]);
            free(local);
        }
    }
    fclose(f);
    return res;
}

I have problems making a function to handle a matrix using calloc

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