C语言学习记录——鹏哥字符分类函数、字符转换函数

#include <stdio.h>
#include <ctype.h>
int main()
{
    char ch1 = '&';
    int ret = isdigit(ch1);
    printf("%d\n\n", ret);//非数字字符，返回0
    char ch2 = '2';
    ret = isdigit(ch2);
    printf("%d\n", ret);//数字字符，返回非0的值
    return 0;
}

#include <stdio.h>
#include <ctype.h>
int main()
{
    char ch[20] = { 0 };
    scanf("%s", ch);
    int i = 0;
    while (ch[i])
    {
        if (isupper(ch[i]))
            ch[i] = tolower(ch[i]);
        printf("%c ", ch[i]);
        i++;
    }
    return 0;
}

#include <stdio.h>
#include <ctype.h>
int main()
{
    char ch[20] = { 0 };
    scanf("%s", ch);
    int i = 0;
    while (ch[i])
    {
        if (islower(ch[i]))
            ch[i] = toupper(ch[i]);
        printf("%c ", ch[i]);
        i++;
    }
    return 0;
}