Why we need getchar() although using scanf()?

Question

this code should read a positive number and if user enter a non-numeric value, it asking him again to enter a number and wait the input to check again till entering a number

 do
   {
        printf("Enter a positive number: ");
   }
  while ( (scanf("%d%c", &n,&c) != 2 || c!='\n' || n<0) ) ;

but actually when entering non-numeric it keeps doing the body of the while loop without reading the waiting to check the condition of the while loop while(scanf("%d%c", &n,&c) != 2 || c!='\n' || n<0)

when edit the condition to

int clean_stdin()
{
    while (getchar()!='\n');
    return 1;
}

 do
   {
        printf("Enter a positive number: ");
   }
  while ( (scanf("%d%c", &n,&c) != 2 || c!='\n' || n<0) && clean_stdin() ) ;

It executes in the right way, but I don't understand why we need to add getchar() although we already use scanf() in the condition

Answer 1

When scanf("%d%c", ... encounters non-numeric input, the "%d" causes the scanning to stop and the offending character to remain in stdin for the next input function. The "%c" does not get a chance to read that non-numeric character.

If code re-reads stdin with the same scanf("%d%c", ..., the same result. Some other way is needed to remove the non-numeric input. getchar(), getch(), etc. will read any 1 character.

Example code GetPositiveNumber()