How to determine invalid inputs in c

Question

I want to take an float input but if the user gives a character input it will show invalid input, I didn't found a specific answer on the net. How is it done?

Answer 1

Consider the following code fragment:

int num;
float val;

num = scanf("%f", &val);
if(num != 1)
{
    printf("You didn't enter a float!\n");
}

scanf returns the number of items that it successfully scans. In your case, you're trying to scan one item, a float. If the user doesn't enter a float, scanf will return 0.

Note: If scanf fails, whatever garbage data the user entered will still be in the stdin stream. You'll have to manually flush it. Here's one such way:

while(((ch = getchar()) != '\n') && (ch != EOF))
    continue;

Warning: Don't use fflush on stdin.

Edit

Even if scanf doesn't fail, you may still have garbage in the stdin buffer. For example, if the user enters:

123xyz

123 will be assigned to the float, and everything after x will stay in the stdin stream.

Answer 2

While scanf is safe to parse a double, many compilers have deprecated its use (for good reason) becuase it is unsafe when parsing a string. Additionally, should the parse fail, you will be left with the remains in the input buffer and you will have to flush it yourself.

For these reasons, prefer a function like fgets, which checks the length of its supplied buffer, and then a function like strtod or sscanf to make the conversion.

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

int main(void)
{
    char buf[64];

    /* read */
    if (fgets(buf, sizeof buf, stdin)) {
        /* convert */
        char *err;
        double d = strtod(buf, &err);
        if (*err) {
            puts("entry invalid");
        } else {
            printf("you entered %lf", d);
        }
    }

    return 0;
}