实现strcmp()函数

本文主要是介绍实现strcmp()函数，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

一）strcmp()源代码

/***
*strcmp - compare two strings, returning less than, equal to, or greater than
*
*Purpose:
*       STRCMP compares two strings and returns an integer
*       to indicate whether the first is less than the second, the two are
*       equal, or whether the first is greater than the second.
*
*       Comparison is done byte by byte on an UNSIGNED basis, which is to
*       say that Null (0) is less than any other character (1-255).
*
*Entry:
*       const char * src - string for left-hand side of comparison
*       const char * dst - string for right-hand side of comparison
*
*Exit:
*       returns -1 if src <  dst
*       returns  0 if src == dst
*       returns +1 if src >  dst
*
*Exceptions:
*
*******************************************************************************/int __cdecl strcmp (const char * src,const char * dst)
{int ret = 0 ;//直到src和dst当前数值不相等且dst不为'\0'时,退出whilewhile( ! (ret = *(unsigned char *)src - *(unsigned char *)dst) && *dst) ++src, ++dst;if ( ret < 0 )ret = -1 ;else if ( ret > 0 )ret = 1 ;return( ret );
}

说明：我们要看的是while循环这个语句， ! (ret = *(unsigned char *)src - *(unsigned char *)dst)意思是拿指针变量src所指向的字符值（即*src）减去指针变量dst所指向的字符值（即*dst），差值赋给ret，再取非运算，最后与*dst进行与运算；
拿abc<abcd举例，第一次因为a=a，则执行++src, ++dst；2次自加后，ret值为负，跳出while语句，执行if语句的判断，输出为-1；
这里要注意的是：
1.unsigned char*是强制转换类型。
2.若src和dst都为空串，返回值是多少呢？因为空串不是指地址为空，还是有地址的，这样就很明确了。

二）实现的strcmp()函数

#include "stdio.h"int strcmp(const char *pStr1, const char *pStr2)
{if(NULL == pStr1 && NULL == pStr2){return 0;}while(('\0' != *pStr1) && ('\0' != *pStr2)){if(*pStr1 < *pStr2){return -1;}else if(*pStr1 > *pStr2){return 1;}else{++ pStr1, ++ pStr2;}}if(('\0' == *pStr1) && ('\0' == *pStr2)){return 0;}else if(('\0' == *pStr1) && ('\0' != *pStr2)){return -1;}else {return 1;}
}
int main()
{char str1[] = "abcdeftghhlllg";char str2[] = "abcdeftghhtthhh";printf("the Result is: %d\n",strcmp(str1, str2));return 0;
}

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