Need help using the flush function in my code?

Hi,
I'm currently working on a project for my programming class, I'm nearly done except for one flaw that I do not know how to fix.

I do not really know how the flush works but I plugged in to multiple parts of my functions and it has fixed a couple errors.

This code draws a diamond out of stars
__*
_***
*****
_***
__*

Here's my current problem, the first run of the program works fine but if the program asks me to run the program again, if I was to put a character instead of a number, the program will draw the last diamond that was made.

Here is the code, I feel like I am so close to finishing, if someone can tell me what I'm doing wrong that would be great thanks >.<

Code:

---

/*Creates a specifically sized Diamond*/
#include <stdio.h>
void flush(void){
  while (getchar() != '\n');
}
int main(void){
  char reply;
  int a,b,d,x,acc,counter;
do {
  int num;
  printf("enter ODD size: ");
        scanf("%i",&num);
        flush();
        while (num<1 || num%2==0){
          printf("Error, input must be an odd integer greater than 0, input another number: ");
          scanf("%i",&num);
          flush();
        }
          b=-1,d=1,x=0;
          counter = num/2;
          for (x;x<((num/2)+1);x++){
            for (a=0;a<counter;a++){
              printf(" ");
            }
            for (acc=0;acc<num-((counter*2));acc++){
              printf("*");
                }
              printf("\n");


          counter-=1;
          }
          for (x=0;x<((num/2));x++){
            for (a=0;a>counter;a--){
              printf(" ");
            }
            for (acc=0;acc<num+(counter*2);acc++){
              printf("*");
            }
            printf("\n");
            counter -=1;
        }
        printf("more? ");
        scanf("%c",&reply);
        flush();
 }while (reply=='y' || reply=='Y');
}

---

scanf has a return value that indicates the number of items it was able to read and convert. you should always check it. the second time you scanf("%i",&num) you aren't getting a value, so scanf fails and returns 0 and does not change the value of 'num'. this lets it fall through your validity test and reexecute the code innards of your loop.

Code:

---

status = scanf("%i",&num);
if (status != 1) {
    // erroneous input do something (ask again or whatever)
}

---

Thank you, that helped, much appreciated.