如何编写确定输入是素数还是数值的C程序

Question

I'm trying to write a programme with C that asks for a number to be input and then determines whether or not it is a prime number. If the input is a non-numeric character or string, it outputs the message INVALID INPUT. I have managed to create code that can determine if a number is prime and also if it is non-numeric but when I try and combine them, it doesn't work. Eg if I input a non-numeric value it will output the correct message but when I input a numeric value, nothing happens and I'm not sure why. Can anybody help?

#include <stdio.h>
#include <string.h>
#define FALSE 0
#define TRUE 1

int numeric( char *string )
{
    int i, valid;
    valid = TRUE;

    if (string[i] < '0' || string[i] >'9')
        valid = FALSE;
    return valid;
}

void main()
{
    char number[5];
    printf("Please enter a decimal number>");
    gets( number );

    if (numeric(number) == TRUE)
    {
        int n, i, a = 0;
        scanf_s("%d", &n);

        for (i = 2; i <= n / 2; ++i)
        {
            if (n%i == 0)
            {
                a = 1;
                break;
            }
        }

        if (n == 1)
        {
            printf("1 isn't a prime number.");
        }
        else
        {
            if (a == 0)
                printf("number entered is a prime number");
            else
                printf("number entered is not a prime number");
        }
    }
    else 
        printf("INVALID INPUT \n"); 
} 

Answer 1

In addition to the bug mentioned in @Michail's comment (failing to initialise i to zero in numeric :

Your problem is that in the truthy branch of if (numeric(number)) , you call scanf_s , which will try to read new input from STDIN . You've already read the number from STDIN in your call to gets . The behaviour you observe ("nothing happens") is to be expected: your program is waiting for you to enter more input.

You leave yourself open to buffer overflow but that's a separate issue.