While loop to validate input is a number in C?

Question

So today I am trying to find a way to check if this input is a number. I cant find a way to make this code work so far. If I enter a number, the while loops ends, which is my intention. But, if I enter anything else, the code SHOULD print the printf once and then reprompt me for input (via the scanf statement back at the top of loop). But it instead prints the printf statement infinitely. How can I fix this?

float num1;
while (scanf("%f",&num1)==0)
    {
        printf("Invalid input. Please enter a number: ");
    }

[Also, check this one](http://stackoverflow.com/q/27616286/2173917) — Sourav Ghosh, Feb 26 '17 at 06:48

score 1 · Accepted Answer · answered Feb 26 '17 at 06:41

If scanf fails to preform the requested conversion, it will leave the input stream unchanged. So while your check is correct, you need to clean the input stream of the erroneous input before re-attempting to read a number again.

You can do it with scanf itself and and the input suppression modifier:

float num1;
while (scanf("%f",&num1)==0)
{
  printf("Invalid input. Please enter a number: ");
  scanf("%*s");
}

%*s will instruct scanf to parse the input as though it's attempting to convert a string of characters (removing characters in the process from the stream), but thanks to the asterisk, it won't attempt to write it anywhere.

Turns out that fixed my code! Thank you very much for the help! — King Sutter, Feb 27 '17 at 15:15

While loop to validate input is a number in C?

1 Answers1