Dynamically Allocating Multidimensional Arrays

Question

I've a problem of execution when i trey to run this program, it's simply to allocate dynamically a multidimensional array with chosen values and just zero it. it compiles correctly but it doesn't execute.

 #include<stdio.h>
 #include<stdlib.h>
 int main(void) {
 int **tab;
 int ligne;
 int col;
 printf("saisir le nbre de lignes volous\n");
 scanf("%d", &ligne);
 printf("saisir le nbre de colonnes volous\n");
 scanf("%d", &col);
 tab = (int**)malloc(ligne*sizeof(int*));
 int i ,j;
 for (i=0 ; i < ligne; i++) {
 *(tab+i) = (int*)malloc(col*sizeof(int));
 }
 for (i = 0; i < ligne; i++) {
 for (j = 0; j < col; j++) {
 **(tab + i+ j) = 0;
 }
 }
 for (i = 0; i < ligne; i++) {
 for (j = 0; j < col; j++) {
 printf("%d\t", **(tab + i +j));
 }
 printf("\n");
 }
 free(tab);
 return 0;
 }

thank you.

• kotasha: yeah actually it is :) thank you
• AndersK: use fgets to read from keyboard, then sscanf to readout the value or simply use atoi to convert.
• M.M: **(tab+i+j) is the wrong way to access element. You could use tab[i][j].
• Luis Colorado: What do you mean with it doesn't execute?
• phuclv: Don't cast the result of malloc in C

Answer 1

Use simple [row][col] access to your double pointer. It is more readable and you can avoid errors, as you coded.

#include<stdio.h>
#include<stdlib.h>
int main(void) {
 int **tab;
 int ligne;
 int col;
 printf("saisir le nbre de lignes volous\n");
 scanf("%d", &ligne);
 printf("saisir le nbre de colonnes volous\n");
 scanf("%d", &col);
 tab = malloc(ligne*sizeof(int*));
 if (tab != NULL)
 {
 int i ,j;
 for (i=0 ; i < ligne; i++)
 {
 tab[i] = malloc(col*sizeof(int));
 if (tab[i] == NULL)
 {
 fprintf(stderr, "Malloc failed\n");
 return 1;
 }
 }
 int k=0;
 for (i = 0; i < ligne; i++) {
 for (j = 0; j < col; j++) {
 tab[i][j] = k++;
 }
 }
 for (i = 0; i < ligne; i++) {
 for (j = 0; j < col; j++) {
 printf("%d\t", tab[i][j]);
 }
 free(tab[i]);
 printf("\n");
 }
 }
 free(tab);
 return 0;
}

Answer 2

int main(void) {
 int ligne;
 int col;
 printf("saisir le nbre de lignes volous\n");
 scanf("%d", &ligne);
 printf("saisir le nbre de colonnes volous\n");
 scanf("%d", &col);
 int tableSize = ligne * (col*sizeof(int));
 int * table = (int*) malloc(tableSize);
 int i,j;
 for (i=0 ; i < ligne; i++) {
 for (j = 0; j < col; j++) {
 *(table + i+ j) = 0;
 }
 }
 for (i = 0; i < ligne; i++) {
 for (j = 0; j < col; j++) {
 printf("%d\t", *(table + i +j));
 }
 printf("\n");
 }
 free(table);
 return 0;
}

Answer 3

Here, I did some changes and added some comments to the changes

#include<stdio.h>
#include<stdlib.h>
int main(void) {
 int **tab = NULL;
 int ligne = 0;
 int col = 0;
 char buffer[128] = {0};
 printf("saisir le nbre de lignes volous\n");
 // to avoid leaving \n in buffer after you enter the first value
 // you should also check also return value of fgets 
 // and quit program if it returns NULL
 // in general it is good practice to check return values
 // of all run-time functions.
 if (fgets(buffer,sizeof(buffer),stdin)==NULL) {
 return 1;
 }
 ligne = atoi(buffer);
 printf("saisir le nbre de colonnes volous\n");
 if (fgets(buffer,sizeof(buffer),stdin) == NULL) {
 return 1;
 }
 col = atoi(buffer);
 tab = malloc(ligne*sizeof(int*)); // do not cast malloc
 int i ,j;
 // use tab[i] and tab[i][j] syntax, it is easier to read
 for (i=0 ; i < ligne; i++) {
 tab[i] = malloc(col*sizeof(int)); 
 }
 for (i = 0; i < ligne; i++) {
 for (j = 0; j < col; j++) {
 tab[i][j] = 0;
 }
 }
 for (i = 0; i < ligne; i++) {
 for (j = 0; j < col; j++) {
 printf("%d\t", tab[i][j]);
 }
 printf("\n");
 }
 // before you free tab, you need to free all lines
 for (i=0 ; i < ligne; i++) {
 free(tab[i]);
 }
 free(tab);
 return 0;
}

Answer 4

As you have allocated your arrays, (the one dimensional parts) your array can be addressed as table[i][j], and never as you do in

for (i = 0; i < ligne; i++) {
 for (j = 0; j < col; j++) {
 **(tab + i+ j) = 0; /* <--- this is an error */
 }
}

as you see tab + i + j is a pointer to which you offset i (the ligne number) plus j (the col number) and both aren't actually the same size (columns are one cell size and rows are one line size) You'd better to write tab[i][j] as tab[i] is a pointer (allocated with malloc(3)) that points to a single dimensional array (and the different pointers tab[0], tab[1],... tab[n] don't have to be correlated between them, as they come from distinct malloc() calls) If you don't like the brackets notation, then you should write the equivalent

*(*(mat + i) + j) /* equivalent to mat[i][j] */

and never the notation you use in your code.

**(tab + i + j) /* equivalent to *mat[i + j] */