I have to create a matrix with pointers in dynamic memory in C, fill it with random numbers then print it.

This is part of a larger assignment from college (I had to do a whole library of functions for matrixes) but for the purposes of the post, I think I managed to track down and isolate the problematic bits. Here’s the code:

```
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <stdbool.h>
int main()
{
int rows = 2;
int cols = 3;
int **matrix = malloc(sizeof(int*) * rows); //declaring and allocating dynamic memory for the matrix
for(int i=0;i<rows;i++)
*(matrix+i) = malloc(sizeof(int) * cols);
//generating random numbers for the matrix:
srand(time(NULL));
for(int i=0;i<rows;i++)
for(int j=0;j<cols;j++)
*(*(matrix+j)+i) = rand() % 20;
//printing matrix:
printf("{\t");
for(int i=0;i<rows;i++)
{
int j;
if(i>0)
printf("\t");
printf("{\t");
for(j=0;j<cols;j++)
printf("%d\t",*(*(matrix + j) + i));
printf("}");
if(i<rows-1)
printf("\n");
}
printf("\t}\n\n");
//destroying the matrix
for(int i=0;i<rows;i++)
free(matrix[i]);
free(matrix);
matrix = NULL;
return 0;
}
```

The debugger stops here:

`*(*(matrix+j)+i) = rand() % 20;`

Apparently, when trying to generate a random number for the first column of the second row (i.e. when it first changes rows) the debugger "cannot access memory" for that position, then the program crashes.

I tried changing the amount of rows and columns and I found it only crashes when row size is less than column size, which is weird because when it’s equal or more, it works just fine.

I’m trying with 2 rows by 3 columns. Expectation:

```
{ { 5 17 3 }
{ 1 8 11 } }
```

What actually happens:

`Process returned -1073741819 (0xC0000005)`

Values of the variables when the debugger stops (at generating random numbers):

```
i = 0
j = 2
**(matrix + 2) (aka matrix[0][2]) = Cannot access memory at address 0xabababababababab
```

At first I suspected I messed up when allocating memory for the matrix, but as I said when the matrix is square it does work fine, so I really don’t know what’s the issue here. I know the stackOverflow community doesn’t like when someone posts "I don’t know what I did wrong please help" kind of thing but I’ve really been a long time trying and couldn’t get to the root of the problem.

Can someone please shed some light on this for me?

### >Solution :

These nested for loops contain a bug

```
for(int i=0;i<rows;i++)
for(int j=0;j<cols;j++)
*(*(matrix+j)+i) = rand() % 20;
```

The pointer `matrix`

points to an array of pointers that in turn point to "rows". So the expression `matrix + i`

points to the `i-th`

"row". However instead you are using the index `j`

.

That is you need to write

```
*(*(matrix+i)+j) = rand() % 20;
```

instead of

```
*(*(matrix+j)+i) = rand() % 20;
```

Otherwise when the value of `cols`

is greater than the value of `rows`

the expression `matrix + j`

can access memory outside the allocated array of pointers (rows).

The same problem exists in this call of printf

```
printf("%d\t",*(*(matrix + j) + i));
```

where you have to write

```
printf("%d\t",*(*(matrix + i) + j));
```