库函数strstr的实现

函数strstr的原型是char *strstr(char *str1, char *str2); 其功能是在str1中返回指定字符串str2的第一次出现的位置。 

#include <stdio.h>#include <string.h>int main(void){char *str1 = "Borland International", *str2 = "nation", *ptr;ptr = strstr(str1, str2);printf("The substring is: %s/n", ptr);return 0;} /*output:The substring is: national */

下面是实现strstr功能的一种方法。(注：有错误！)

/* Modification date: 2010-5-28 test: src: abcd efghcd abcdef dst: cd return: cd efghcd abcdef*/#include <cstdio> #include <cstring>// strlen, strstr#include <cassert>// assertchar* my_strstr(char* src,char* dst){// checkassert(src);assert(dst);// calculate the each sizechar *szSrc = src,*szDst = dst;int nSrcSize = strlen(szSrc), nDstSize = strlen(szDst);// checkif (nSrcSize < nDstSize){return NULL;// error}// compareint nCompareCount = nSrcSize-nDstSize+1;int i,j;for (i = j = 0; i<nSrcSize && j<nDstSize; ++i){if (src[i] != dst[j]){if (i == nCompareCount-1)// this check can save time{return NULL;// no substring}j = 0;}else// to find the first same value{++j;}}if (j == nDstSize)// absolute equal{return &src[i-nDstSize];// have substring}return NULL;// no substring}int main(){ //char *szSrc = "wcdj is a guy", *szDst = "is";char szSrc[128] = {0}, szDst[128] = {0};printf("input src: ");//scanf("%s",szSrc);gets(szSrc);printf("input dst: ");//scanf("%s",szDst);gets(szDst);// call my functionchar* ptr = my_strstr(szSrc,szDst);if (ptr){printf("The substring is: %s/n", ptr);} else{printf("has no substring/n");}getchar();return 0; }

 

上述方法只考虑了目的字符串dst的回溯，但是没有考虑源字符串src的回溯，纠正如下：使用nNext记录回溯的位置。

/* Modification date: 2011-4-13 test: src: xxy dst: xy return: xy*/#include <cstdio> #include <cstring>// strlen, strstr#include <cassert>// assertchar* my_strstr(char* src, char* dst){// checkassert(src);assert(dst);// calculate the each sizechar *szSrc = src,*szDst = dst;int nSrcSize = strlen(szSrc), nDstSize = strlen(szDst);// checkif (nSrcSize < nDstSize){return NULL;// error}// compare int nCompareCount = nSrcSize-nDstSize+1; int nNext = 0;bool bFirst = false;int i = 0, j = 0;while (i < nSrcSize && j < nDstSize){if (src[i] != dst[j]){if (--nCompareCount == 0)// save time { return NULL;// no substring } // backtracej = 0;if (bFirst){i = nNext;bFirst = false;continue;}}else{++j;if (bFirst == false){nNext = i+1;bFirst = true;}}++i;}if (j == nDstSize)// absolute equal{return &src[i-nDstSize];// have substring}return NULL;// no substring}int main(){ char szSrc[128] = {0}, szDst[128] = {0};printf("input src: ");fgets(szSrc, sizeof(szSrc), stdin);printf("input dst: ");fgets(szDst, sizeof(szDst), stdin);// clear '/n' in the end of stringszSrc[strlen(szSrc)-1] = '/0';szDst[strlen(szDst)-1] = '/0';char* ptr = my_strstr(szSrc,szDst);if (ptr){printf("The substring is: %s/n", ptr);} else{printf("has no substring/n");}getchar();return 0; }