first scanf gets other chrachters for the next scanfs

Question

I'm making a simple calculator in C and I want the program to show the message "Enter an operator!" when the user enters something that isn't an operator, enter, space or tab. but if the user enters a number like 123 the message is shown 3 times instead of once. how can I fix this?

char op;
printf("please choose an operator (+,-,*,/): ");
while (op!='+' && op!='*' &&op!='-' &&op!='/' )
    {
    if (op!='\n'&& op!= '\t'&& op!=' ')
    printf ("Enter an operator!\n");
    scanf ("%c",&op);
    }

Answer 1

A scan set could be used %1[+-/*] will scan one character in the set of operators.
If the character is not in the set, scanf will return 0. Clean the input stream and try again.

#include <stdio.h>
#include <stdlib.h>

int main ( void) {
    char op[2] = "";
    int result = 1;

    printf ("Enter an operator!\n");
    do {
        if ( result != 1) {
            printf ("Enter an operator!\n");
        }
        result = scanf ( "%1[+-/*]", op);
        if ( result == EOF) {
            fprintf ( stderr, "EOF\n");
            return 1;
        }
        int clean = 0;
        while ( ( clean = fgetc ( stdin)) != '\n') { // clean input stream
            if ( clean == EOF) {
                fprintf ( stderr, "EOF\n");
                return 1;
            }
        }
    } while ( result != 1);

    printf ( "operator %s\n", op);

    return 0;
}