Problem with rand() (trying to create minesweeper)

Hello is there any problem on how i use rand()? It's strange it doesn't work with q and w (q=n+1. w=m_1) but if in their position i put n and m it works. What's wrong? If you need the function tamplo to check tell me...thnx

Code:

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

void tamplo(int n, int m, int **pa);

int main(int argc, char *argv[])
{
   int i,j,n,m,o,p,a,b,r1,r2,q,w;
   int **pa=NULL;
   printf("Dwste tis diastaseis tou paixnidiou n x m\n");
   printf("Prosoxh, to tamplo mporei na exei megethos to poly 24x30\n");
   scanf("%d%d", &n,&m);
   printf("Epilxte to epipedo duskolias tou paixnidiou\n");
   printf("Pathste 1 gia eukolo, 2 gia metro, 3 gia duskolo kai 4 gia akatorthoto\n");
   scanf("%d", &o);
   if(o==1)
   {
      p=m*n/8;
   }
   if(o==2)
   {
      p=m*n/7;
   }
   if(o==3)
   {
      p=m*n/6;
   }
   if(o==4)
   {
      p=m*n/5;
   }    
   pa=(int**)calloc(n, sizeof(int*)); //desmeush mnhmhs gia dimiourgia 2d pinaka
   for(i=0;i<n;i++)
   {
         pa[i]=(int*)calloc(m, sizeof(int));
   }
   tamplo(n,m,pa);
   printf("dwste tis syntetagmenes tou tetragwnou pou thelete na anoixete\n");
   scanf("%d%d", &a,&B);
   pa[a-1][b-1]=9;
   q=n+1;
   w=m+1;
   for(i=0;i<p;i++)
   {
       do
       {                     
           r1=rand()%q;
           r2=rand()%w;
       }while(r1==a && r2==B);
       pa[r1][r2]=1;//to 10 antiprosopevei tis narkes
   }
   tamplo(n,m,pa);    
   for(i=0;i<n;i++)
   {
       free(pa[i]);
   }
   free(pa);
   system("PAUSE");	
   return 0;
}

> It's strange it doesn't work with q and w (q=n+1. w=m_1)
That's because such values would be out of range for accessing your array.

If you have
int array[N];

Then valid subscripts are from 0 to N-1

And the result of rand()%N is also in the range 0 to N-1 as well (convenient huh).

> int i,j,n,m,o,p,a,b,r1,r2,q,w;
There is a very old disease called SLID, which is "Single Letter Identifier Disease".
Pick some more meaningful variable names.
i and j are fine for loop indices, but pretty much everything else should be meaningful.

I would variously suggest numberOfMines, difficultyLevel etc.

You're right. Thank you so much i was searching for it a couple of hours! :/ Now only the hard part for minesweeper remains...minefinder :S