How does the C language determine the type of a character?

In the C language, you can use the character classification functions in the ctype.h header file to determine the type of characters. These functions mainly include the following:

1. isalnum(): Determines if a character is alphanumeric.
2. isalpha() is used to check if a character is an alphabet.
3. isdigit(): determines if a character is a digit.
4. islower(): determines whether a character is a lowercase letter.
5. isupper(): Check if the character is an uppercase letter.
6. isspace(): Determine whether a character is a white-space character, such as a space, tab, or newline.

The methods of using these functions are as follows:

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

int main() {
    char ch = 'A';
    
    if (isalnum(ch)) {
        printf("字符是字母或数字\n");
    }
    
    if (isalpha(ch)) {
        printf("字符是字母\n");
    }
    
    if (isdigit(ch)) {
        printf("字符是数字\n");
    }
    
    if (islower(ch)) {
        printf("字符是小写字母\n");
    }
    
    if (isupper(ch)) {
        printf("字符是大写字母\n");
    }
    
    if (isspace(ch)) {
        printf("字符是空白字符\n");
    }
    
    return 0;
}

The output result is assumed to be ‘A’ if the value of character ‘ch’ in the above code is ‘A’.

字符是字母或数字
字符是字母
字符是大写字母