用C语言编写函数计算子字符串substr在主字符串mainstr中的索引值

/**

Author: snowdream <yanghui1986527@gmail.com>

Data: 2012.03.05

Description:
假设一个主要字符串“Hello World!”,和一个子字符串"World".
在大小写敏感的前提下，如果主字符串包含子字符串，请写出一个函数
计算出该子字符串在主字符串中的索引index。否则返回 -1 作为索引值。

*/

#include <stdio.h>

int findstr(char* substr,char* mainstr)
{
int index = -1;

for(int i = 0; *(mainstr+i)!='\0';i++)
{
for(int j = 0; *(substr+j)!='\0';j++)
{
if(*(substr+j) != *(mainstr+i+j))
break;

if(*(substr+j+1) =='\0' )
index = i;
}

if(index != -1)
break;
}

return index;
}

int main()
{
int index = -1;
int index1 = -1;

char* mainstr = "Hello World!";
char* substr = "cctv";
char* substr1 = "World";

index = findstr(substr,mainstr);
index1 = findstr(substr1,mainstr);

printf("The index of %s in %s is: %d\n",substr,mainstr,index);
printf("The index of %s in %s is: %d\n",substr1,mainstr,index1);

return 0;
}

snowdream@snowdream:~$ gcc findstr.c -std=gnu99 -o findstr -g
snowdream@snowdream:~$ ./findstr
The index of cctv in Hello World! is: -1
The index of World in Hello World! is: 6