rand() not produce random [c] - c

Question

I was asked to write a program that produces a 2d array of random numbers. My code produces strange results. Regardless of the size of the matrix it produces a "reverse diagonal matrix" (not sure what else to call it). I was to understand that rand() produces a pseudorandom number between 0 and RAND_MAX but somehow the results are dependent on the size of my matrix, I am unsure how this behavior could happen.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define MAT_SIZE 5
void main (void)
{
srand(time(0));
unsigned char * ptr = malloc(MAT_SIZE*MAT_SIZE);
unsigned char i, j;
for(i = 0; i < MAT_SIZE; i++)
{
for(j = 0; j < MAT_SIZE ; j ++)
{
*(ptr + i + j ) = rand();
}
}
for(i = 0; i < MAT_SIZE ; i++)
{
for(j = 0; j < MAT_SIZE ; j ++)
{
printf("%5d", *(ptr + i + j));
}
printf("\n");
}
free(ptr);
}
Output for 3*3
142 141 11
141 11 230
11 230 28
Output for 5*5
232 157 62 131 245
157 62 131 245 54
62 131 245 54 138
131 245 54 138 246
245 54 138 246 108
Even a pseudorandom number shouldn't behave differently based on how it is used. Is there something I'm not understanding about the program that forces these results?

Answer 1

The problem is not with the random numbers that are generated but with how you are saving them in your matrix. To access element (j, i) you do not want
*(ptr + i + j )
but
*(ptr + MAT_SIZE * i + j )

Answer 2

You are only accessing the first i+j members of the array, and looping over the same indices repeatedly. Instead of:
*(ptr + i + j )
Try
*(ptr + i * MAT_SIZE + j )

Answer 3

Note that you can dynamically allocate a 2D array as follows:
unsigned char (*ptr)[MAT_SIZE] = malloc( MAT_SIZE * sizeof *ptr );
...
ptr[i][j] = rand();
...
free( ptr );
Makes life a little simpler. This way you can use normal 2D array indexing instead of having to map i and j to a single dimension.
Nits:
Unless your compiler documentation explicitly lists void main() as a valid signature for main, use int main( void ) instead. I know you've seen thousands of examples of void main() in books and on line, but just because the compiler doesn't complain about it doesn't mean it isn't wrong.
rand() returns int, which will not fit into an unsigned char. Overflow on unsigned types is well-defined, but even so, you may want to explicitly map your rand result onto the range [0..255].