Determine if a character is a number in the C language.

In C language, the isdigit() function can be used to determine if a character is a digit. This function is included in the C standard library and its prototype is defined in the ctype.h header file.

The isdigit() function takes a character as an argument, and returns a nonzero value (true) if the character is a digit between 0 and 9, otherwise it returns 0 (false).

Here is an example code:

#include <stdio.h>
#include <ctype.h>

int main() {
    char ch = '5';  // 要判断的字符

    if (isdigit(ch)) {
        printf("%c是数字\n", ch);
    } else {
        printf("%c不是数字\n", ch);
    }

    return 0;
}

The output result is:

5是数字