Trying to implement a dynamic size matrix in C. Where is the mistake?

Question

I am having some troubles with implementing a dynamic matrix. Theoretically it seems clear what I have to do, and I have already read some similar question on this website (as well as on other resources). Although, the following very simple code doesn't work. Do you mind suggesting how to find the mistakes?

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

int main() {
    int dim1 = 10;  // #Rows
    int dim2 = 4;   // #Columns
    int i;
    int j;

    double **mtrx = (double **)malloc(sizeof(double **) * dim1);
    // mtrx is my dynamic matrix, of size dim1*dim2 //

    for (i = 0; i < dim1; ++i)
        mtrx[i]= (double *)malloc(sizeof(double *) * dim2);

    // (1) Inizialize//
    for (i = 0; i < dim1; ++i) {
        printf("(%d):\t", i);
        for (j = 0; j < dim2; ++j) {
            mtrx[i][j] = i;
            printf("mtrx[%d][%d] = %.f ", i, j, mtrx[i][j]); 
        }
        printf("\n");
    }

    // (2) Show the results//
    printf("\nShow the results:\n");

    for (i = 0; i < dim1; ++i) {
        printf("(%d):\t", i);
        for (j = 0; j < dim2; ++j)
            printf("mtrx[%d][%d] = %.f ", i, j, mtrx[i][j]); 
        printf("\n");
    }

    // (3) Free the memory
    printf("\n***free the memory***\n");

    for (i = 0; i < dim1; ++i )
        free(mtrx[i]);

    free(mtrx);

    return 0;
}

The purpose of my code is the following. First, declare a dynamic matrix of size dim1*dim2 (i.e. "dim1" rows and "dim2" columns). Than, phase (1), initialize every element following a simple pattern (mtrx[i][j] = i). Notice that here there is an "exceeding" printf instructions: I added it for "debugging" purposes, in oder to make sure of the correctness of the initialization procedure (that seems works correctly). Then, phase 2, I show the results, and finally I free the memory used.

The problems are:

phase 2 doesn't show the results "correctly";
at the free phase, I obtain an error like

***free (): invalid next size (fast) ...

My suspect: I may have wrongly declared the memory for my matrix, causing some overflow of data and an incompatibility with the "free" instruction, but unfortunately I don't manage to find the mistake.

I thank you in advance for any help!!!

In the first `malloc` you need to use `sizeof(double *)` and in the second `sizeof(double)`. Good luck. — Alex Lop., Dec 19 '15 at 10:07

score 2 · Answer 1 · answered Dec 19 '15 at 10:51

You are allocating an array of pointers to arrays. C99 allows you to allocate a 2D array with runtime sizes in one step this way:

double (*mtrx)[dim2] = malloc(sizeof(double) * dim1 * dim2);

If dim1 and dim2 are not too large, you may even define mtrx as a local 2D array with automatic storage:

double mtrx[dim1][dim2];

You need a modern C compiler for this, gcc or clang, Microsoft own C compiler is quite obsolete and does not support C99 features, standardized over 15 years ago. Luckily, gcc and clang are available in different ports for the Windows OS.

Damien · Accepted Answer · 2015-12-19T10:58:37.353

1

When you have a malloc, the argument is the size(byte) you want to allocate. So if you want only a table , you have to write

(sizeof(type of the table) * the_number_of_cells_you_want);

You should allocate sizeof(double*) * dim1 at first. Then in the loop:

for ( i = 0; i < dim1; ++i)
    mtrx[i]= malloc ( sizeof ( double ) * dim2);

edited Dec 19 '15 at 10:58

answered Dec 19 '15 at 10:11

Damien

160
3
14

score 0 · Answer 3 · edited May 23 '17 at 12:07

0

Need to change memory allocation in both place other code seem to be fine and don't cast the result of malloc.

double ** mtrx = malloc ( sizeof (double *) * dim1);

        mtrx[i]= malloc ( sizeof ( double ) * dim2);

edited May 23 '17 at 12:07

Community

1
1

answered Dec 19 '15 at 10:09

Mohan

1,871
21
34

Trying to implement a dynamic size matrix in C. Where is the mistake?

3 Answers3