1、原始函数，使用下标运算符

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

void upper(char x[])
{
    int tmp = 0;
    while(x[tmp])
    {
        x[tmp] = toupper(x[tmp]);
        tmp++;
    }
}

void lower(char x[])
{
    int tmp = 0;
    while(x[tmp])
    {
        x[tmp] = tolower(x[tmp]);
        tmp++;
    }
}

int main(void)
{
    char str1[128] = "ABcd";
    
    upper(str1);
    printf("upper result: %s\n", str1);
    
    lower(str1);
    printf("lower result: %s\n", str1);
    
    return 0;
}

 

 

 

 

2、使用指针

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

void upper(char *s1)
{
    while(*s1)
    {
        *s1 = toupper(*s1);
        s1++;
    }
}

void lower(char *s1)
{
    while(*s1)
    {
        *s1 = tolower(*s1);
        s1++;
    }
}

int main(void)
{
    char str1[128] = "ABcd";
    
    upper(str1);
    printf("upper result: %s\n", str1);
    
    lower(str1);
    printf("lower result: %s\n", str1);
    
    return 0;
}