"do while problem"

[Hardstyle]

int math[20][20],aux2,aux3;
     for(int i=0;i<40s;i++)
         {
             do{
              aux2=int( rand()%20);
              aux3=int( rand()%20);}
              while(math[aux2][aux3]==0);
                  math[aux2][aux3]=1;
          }

int math[20][20],aux2,aux3;
     for(int i=0;i<40s;i++)
         {
             do{
              aux2=int( rand()%20);
              aux3=int( rand()%20);}
              while(math[aux2][aux3]==0);
                  math[aux2][aux3]=1;
          }

To copy to clipboard, switch view to plain text mode 

this should make me 40 of math's element to be 1...and it give @ 30-38 numbers....not exactly 40 ....why ???
("rand()" is working good...)