字符串的常用函數

1. 函數名: strlen 測量字符串的長度

2. 函數名: strcpy 複製字符串

3. 函數名: strncpy 複製字符串的前n個字符

4. 函數名:strcat 連接兩個字符串

5. 函數名: strncat 將一個字符串中的前n個字符連接到另一個字符串中

6. strcmp 比較兩個字符串的大小

   其實我們也可以自己編寫出實現上述功能的函數。

#include <stdio.h>
#define MAX_SIZE 100
int my_strlen(char *str)
{
    int count = 0;

    while(*str != '\0')
    {
        count++;
str++;
    }
    return count;
}

char *my_strcpy(char *str,char *dest)
{
    char *temp = str;
    while(*dest != '\0')
    {
        *temp = *dest;
temp++;
dest++;
    }
    *temp = '\0';
    return str;
}

char *my_strncpy(char *str,char *dest,int len)
{
    int i;
    char *temp = str;
    for(i = 0; i < len; i++)
    {
    *(temp + i) = *(dest + i);
    }
    return str;
}

char *my_strcat(char *str,char *dest)
{ 
    char *temp = str;
    while(*temp != '\0')
    {
        temp++;
    }
    while(*dest != '\0')
    {
        *temp = *dest;
temp++;
dest++;
    }
    *temp = '\0';
    return str;
}

char *my_strncat(char *str,char *dest,int len)
{ 
    int i;
    char *temp = str;
    while(*temp != '\0')
    {
        temp++;
    }
    for(i = 0; i < len; i++)
    {
        *(temp + i) = *(dest + i);
    }
    *(temp + i) = '\0';
    return str;
}

int my_strcmp(char *str, char *dest)
{
    while(*str != '\0')
    {
        if(*(str) > *(dest))
        {
            return 1;
        }
        else if(*(str) < *(dest))
        { 
            return -1; 
        } 
        dest++; 
str++;
    }
    if(*str == '\0'&& *dest == '\0')
    {
        return 0;
    }
    else if(*str != '\0' && *dest == '\0')
    {
        return 1;
    }
    else if(*str == '\0' && *dest != '\0')
    {
        return -1;
    }
    return 0;
}

int main()
{
    int count;
    int ret;

    char str[MAX_SIZE];
    char dest[MAX_SIZE];
    char *temp1;
    char *temp2;
    char *temp3;
    char *temp4;

    printf("please input str:\n");
    gets(str);
    printf("please input dest:\n");
    gets(dest);

    count = my_strlen(str);
    printf("count = %d\n",count);

    temp1= my_strcpy(str,dest);
    printf("temp1 = %s\n",temp1);

    temp2= my_strncpy(str,dest,5);
    printf("temp2 = %s\n",temp2);

    temp3 = my_strcat(str,dest);
    printf("temp3 = %s\n",temp3);

    temp4 = my_strncat(str,dest,5);
    printf("temp4 = %s\n",temp4);

    ret = my_strcmp(str,dest);
    printf("%d\n",ret);

    return 0;
}