C++ multiple random numbers adding up to equal a certain number

Question

Having a little trouble figuring this one out, I have a bunch of int variables which are generating random numbers from 1-9. I need them to generate the correct numbers to equal

Answer 1

The chance that you get '30' for a result is pretty small. You also don't get a random number from 1 to 9, you get them from 0 to 9.

Try this instead:

int numone,numtwo,numthree,numfour, finalsum;
do
{
   do
   {
     numone = 1+rand()%9;
     numtwo = 1+rand()%9;
   } while (numone+numtwo < 12);
   numthree = 1+rand()%9;
} while (numone+numtwo+numthree < 21);
numfour = 30-(numone+numtwo+numthree);
// for clarity
finalsum = numone + numtwo + numthree + numfour;

(Edit) After some lateral thinking: numthree needs to be between 30-1-(numone+numtwo) and 30-9-(numone+numtwo) -- perhaps there is room for a small further optimization there.

(Further edit) After the deliberations below I actually tested it, and indeed, this works per requirement:

#include 
#include 
#include 

int main (void)
{
  int numone,numtwo,numthree,numfour, finalsum;
  int i;

  srand(time(NULL));

  for (i=0; i<100; i++)
  {
    numone = 3+rand()%7;
    if (numone == 3)
      numtwo = 9;
    else
      numtwo = 12-numone+rand()%(7-(9-numone));
    if (numone + numtwo == 12)
      numthree = 9;
    else
      numthree = 21-(numone+numtwo)+rand()%(6-(18-(numone+numtwo)));
    numfour = 30 - (numone + numtwo + numthree);

    finalsum = numone + numtwo + numthree + numfour;
    printf ("%d + %d + %d + %d = %d\n", numone, numtwo, numthree, numfour, finalsum);
  }
}