【C语言】标准库ctype.h：判断和映射字符。

#include <ctype.h>

标准库ctype.h：

函数中的参数：都是无符号字符转换为int值进行传递。

判断函数中的返回：若满足判断条件，返回非零值。若不满足判断条件，返回0。

转换函数中的返回：若有对应的大写/小写字母，返回转换后的字符(转换为int值)。若没有，返回原字符(转换为int值)。

// 部分函数举例
#include <ctype.h>
#include <stdio.h>

// 函数声明
char trans(char c);

int main(void)
{
    char s[] = "year:2024.\tGOOD LUCK!\n";
    int n = sizeof(s);              // 获取字符串s的内存大小
    int c;                          // 将要被判断的字符，转为int值作为参数传递给函数
    char t[n];                      // 字符串t，此处存储大小写转换后的字符串
    for(int i = 0; i < n; i++)
    {
        c = s[i];
        t[i] = c; 
        if(isdigit(c))              // 判断字符是否是数字   
            printf("index %d element:\"%c\" is digit\n", i, c);
        else if(isalpha(c))         // 判断字符是否是字母
        {
            printf("index %d element:\"%c\" is ", i, c);
            t[i] = trans(c);        // 函数,大小写字母转换
        }
        else if(isspace(c))         // 判断字符是否是空白字符
            printf("index %d element:\"%c\" is space\n", i, c);
        else if(ispunct(c))         // 判断字符是否是标点符号
            printf("index %d element:\"%c\" is punctuation\n", i, c);            
    }
    printf("End: t = %s", t);
    return 0;
}

char trans(char c)
{
    if(islower(c))                 // 判断字符是否是小写字母
    {
        printf("lowercase letter, convert to uppercase letter\n", c);
        return toupper(c);         // 小写字母转为大写字母
     }
    else if(isupper(c))            // 判断字符是否是大写字母
    {
        printf("uppercase letter, convert to lowercase letter\n", c);
        return tolower(c);         // 大写字母转为小写字母
    }
}

// 结果：
index 0 element:"y" is lowercase letter, convert to uppercase letter
index 1 element:"e" is lowercase letter, convert to uppercase letter
index 2 element:"a" is lowercase letter, convert to uppercase letter
index 3 element:"r" is lowercase letter, convert to uppercase letter
index 4 element:":" is punctuation
index 5 element:"2" is digit
index 6 element:"0" is digit
index 7 element:"2" is digit
index 8 element:"4" is digit
index 9 element:"." is punctuation
index 10 element:"      " is space
index 11 element:"G" is uppercase letter, convert to lowercase letter
index 12 element:"O" is uppercase letter, convert to lowercase letter
index 13 element:"O" is uppercase letter, convert to lowercase letter
index 14 element:"D" is uppercase letter, convert to lowercase letter
index 15 element:" " is space
index 16 element:"L" is uppercase letter, convert to lowercase letter
index 17 element:"U" is uppercase letter, convert to lowercase letter
index 18 element:"C" is uppercase letter, convert to lowercase letter
index 19 element:"K" is uppercase letter, convert to lowercase letter
index 20 element:"!" is punctuation
index 21 element:"
" is space
End: t = YEAR:2024.     good luck!

补充：

if...else if...else...：若执行语句只有单行，可以省略大括号{ }；若执行语句超过一行，必须使用大括号{ }。