strcat()的编写

1、strcat()

#include <windows.h>
#include <assert.h>
#include <iostream>
//strcat()函数分4部分写
//1.定义4个char*
//2.变量检查不为空assert()
//3.指针指向第一个字符串的末尾
//4.逐个字符的赋值
char *strcat(char  *strDest, char *strSrc)
{
    char *address = strDest;
    assert( (strDest != NULL)&&(strSrc != NULL));
    while(*strDest)
    {
        strDest++;
    }
    while(*strDest++ = *strSrc++)
    {
        ;
    }
    return address;
}

void main()
{
    char l[10] ="li";    //注意此处需要时开辟空间的数组
    char *w = "wen";
    strcat(l,w);
    std::cout<<l;
}

 2、strcpy()

#define  NULL 0
#include <assert.h>
#include <stdlib.h>
#include <iostream>
char * strcpy(char *strDest, char *strSrc)
{
    char *address = strDest;
    assert(strDest!=NULL&&strSrc!=NULL);
    while (*strDest++ =  *strSrc++)
    {
        ;
    }
    return address;
}
void main()
{
    char a[10] = "li";
    char *b="lwn";
    strcpy(a,b);
    std::cout<<a;
    system("pause");
}

 3、strcmp()

#include <stdlib.h>
#include <iostream>
int strcmp(const char *strDest, const char *strSrc)
{
    while (*strDest == *strSrc)
    {
        if (*strDest =='\0')        //注意时单引号
            return 0;
        strDest++;
        strSrc++;
    }
    return *strDest - *strSrc;

}
void main()
{
    char a[10] = "li";
    char *b="li";
    strcmp(a,b);
    std::cout<<strcmp(a,b);
    system("pause");
}

 4.strlen()　　//由于strlen()函数不加头文件也能调用下文将名字改为strln()

#include <iostream>

int strln(const char *strDest)
{
    int i=0;
    while (*strDest !='\0')
    {
        strDest++;i++;
    }
    return i;
}

void main()
{
    char a[10] = "li4";
    std::cout<<strln(a);
    system("pause");
}