strcat strcmp strlen strcpy 的实现

    在实习的时候，老师说字符串函数在面试的时候很容易考到，所以自己就把平时的练习总结了一下。
   代码比较简单，只是在面试的时候要熟练的写出来就比较好。

1.strcat()函数的实现

#include <stdio.h>

char *mystrcat(char *dest,char *src)
{
    char * temp = dest;
    while( '\0' != *dest)
    {
        dest++;
    }
    while('\0' != (*dest++ = *src++))
    {

    }
    return temp;
}

int main(int argc, const char *argv[])
{
    char dest[1024] = "wuhan";
    char src[] = "hrllo!";

    mystrcat(dest,src);

    printf("%s\n",dest);

    return 0;
}

2.strcmp()的实现

#include <stdio.h>
int mystrcmp(char *s1,char *s2)
{
    while(*s1 == *s2 && '\0' != *s1)
    {
        s1++;
        s2++;
    }
    return (*s1 - *s2);
}

int main(int argc, const char *argv[])
{
    char buf1[1024] = {0};
    char buf2[1024] = {0};
    int ret = 0;

    puts("please input string buf1 :");
    gets(buf1);
    puts("please input string buf2 :");
    gets(buf2);

    ret = mystrcmp(buf1,buf2);

    if(ret > 0)
        puts("buf1 > buf2");
    else if(ret ==0)
        puts("buf1 == buf2");
    else
        puts("buf1 < buf2");

    return 0;
}

3.strcpy()的实现

#include <stdio.h>
#include <string.h>

char *mystrcpy(char *p ,const char *q)
{
    char *temp = p;
    while( '\0' != (*p++ = *q++))
    {
        NULL;
    }
    return temp;

}

int main()
{
  char src[20] = "wuhan";
  char dest[20] = "beijing";

  mystrcpy(src,dest);

    printf(" after strcpy src[20] = %s\n",src);

   return 0;
}

4.strlen()的实现

#include <stdio.h>

int mystrlen(char *s)
{
    int length = 0;
    while('\0' != *s++)
    {
        length++;
    }
    return length;
}

int main(int argc, const char *argv[])
{
    char buf[512] = "hello world !";
    printf("strlen(buf) = %d\n",mystrlen(buf));

    return 0;
}

atoi()的实现

#include <stdio.h>
int my_atoi(char* pstr)
{
    int Ret_Integer = 0;
    int Integer_sign = 1;

    /*
    * 判断指针是否为空
    */
    if(pstr == NULL)
    {
        printf("Pointer is NULL\n");
        return 0;
    }

    /*
    * 跳过前面的空格字符
    */
    while(isspace(*pstr) == 0)
    {
        pstr++;
    }

    /*
    * 判断正负号
    * 如果是正号，指针指向下一个字符
    * 如果是符号，把符号标记为Integer_sign置-1，然后再把指针指向下一个字符
    */
    if(*pstr == '-')
    {
        Integer_sign = -1;
    }
    if(*pstr == '-' || *pstr == '+')
    {
        pstr++;
    }

    /*
    * 把数字字符串逐个转换成整数，并把最后转换好的整数赋给Ret_Integer
    */
    while(*pstr >= '0' && *pstr <= '9')
    {
        Ret_Integer = Ret_Integer * 10 + *pstr - '0';
        pstr++;
    }
    Ret_Integer = Integer_sign * Ret_Integer;

    return Ret_Integer;

}
int main()
{
    char a[] = "-100";
    char b[] = "456";
    int c = 0;

    int my_atoi(char*);

    c = atoi(a) + atoi(b);

    printf("atoi(a)=%d\n",atoi(a));
    printf("atoi(b)=%d\n",atoi(b));
    printf("c = %d\n",c);

    return 0;
}