Question

The function is used to validate the input. It prompts the user for a numeric value (great or equal to 0 ) until it meets the coditions. if any character input precedes or follows the number, the input is to be treated as invalid.The required output is:

Enter a positive numeric number: -500
Error! Please enter a positive number:45abc
Error! Please enter a number:abc45
Error! Please enter a number:abc45abc
Error! Please enter a number:1800

Well, it seems easy:

#include <stdio.h>
main() {
    int ret=0;
    double num;
    printf("Enter a positive number:");
    ret = scanf("%.2lf",&num);

    while (num <0 ) {
        if (ret!=1){
            while(getchar()!= '\n');
            printf("Error!Please enter a number:");
        }
        else{
            printf("Error!Please enter a positive number:");
        }
        ret = scanf("%.2lf",&num);
    }
}

however, my code keeps put out Error!Please enter a number: regardless types of input. Any advice?

Answer 1

I think you’ll have problems doing the validation you want using just scanf(). You’ll do better to first scan in a string and then convert it to a numeric. But scanf() is dangerous for scanning in char strings, since its input length is not limited and you have to provide it a pointer to a finite-length input buffer. Better to use fgets(), which allows you to limit the input buffer length.

#include <stdio.h>
int main(int argc, char **argv)
{
    double num=-1;
    char input[80]; // arbitrary size buffer
    char* cp, badc; // badc is for detecting extraneous chars in the input
    int n;
    printf("Enter a positive number:");
    while (num < 0)
    {
        cp = fgets(input, sizeof(input), stdin);
        if (cp == input)
        {
            n = sscanf(input, "%lf %c", &num, &badc);
            if (n != 1) // if badc captured an extraneous char
            {
                printf("Error! Please enter a number:");
                num = -1;
            }
            else if (num < 0)
                printf("Error! Please enter a POSITIVE number:");
        }
    }

    printf("num = %f\n", num);

    return 0;
}