Summary of the implementation method of C++ function strstr

Summary of the implementation method of strstr function in C++

Function description:

Include file: string.h

Function name: strstr

Function prototype: extern char *strstr(char *str1, char *str2);

Function: find the string str2 from str1, if so, return a pointer to str1 from str2 in str1, if not, null.

Return value: returns a pointer to the location. If it cannot be found, returns a null pointer.

Method 1:

#include <iostream> 
#include <assert.h> 
using namespace std; 
 
char* My_strstr(char *src,char *substr) 
{ 
  assert(src != NULL && substr != NULL); 
 
  unsigned int size = strlen(src); 
  for(int i = 0; i < size; ++i,++src) 
  { 
    char *p = src; 
    for(char *q = substr;;p++,q++) 
    { 
      if(*q == '\0')  // in src Find the continuous in substr The substring stops and returns  
      { 
        return src; 
      } 
      if(*q != *p) 
      { 
        break; 
      } 
    } 
  } 
 
  return NULL; 
} 
 
int main() 
{ 
  char *res = My_strstr("abcdefg","cde"); 
  if(res != NULL) 
  { 
    cout<<"exist:"<<res<<endl; 
  } 
  else 
  { 
    cout<<"no exist!"<<endl; 
  } 
  return 0; 
} 

Method 2:

#include <iostream> 
#include <assert.h> 
using namespace std; 
char* My_strstr(const char* s1,const char* s2) 
{ 
  int n; 
  if(*s2) 
  { 
    while(*s1) 
    { 
      for(n=0;*(s1+n)==*(s2+n);n++) 
      { 
        if(!*(s2+n+1)) 
          return (char*)s1; 
      } 
      s1++; 
    } 
    return NULL; 
  } 
  else 
    return (char*)s1; 
} 
 
int main() 
{ 
  char *res = My_strstr("abcdefg","cde"); 
  if(res != NULL) 
  { 
    cout<<"exist:"<<res<<endl; 
  } 
  else 
  { 
    cout<<"no exist!"<<endl; 
  } 
  return 0; 
} 

Method 3:

#include <iostream> 
#include <assert.h> 
using namespace std; 
char* My_strstr(const char* s1,const char* s2) 
{ 
  const char *p=s1; 
  const size_t len=strlen(s2); 
  for(;(p=strchr(p,*s2))!=0;p++)// strchr Find string s The first character to appear in c The location of the  
  { 
    if(strncmp(p,s2,len)==0) 
    { 
      return(char*)p; 
    } 
  } 
  return(0); 
} 
int main() 
{ 
  char *res = My_strstr("abcdefg","cde"); 
  if(res != NULL) 
  { 
    cout<<"exist:"<<res<<endl; 
  } 
  else 
  { 
    cout<<"no exist!"<<endl; 
  } 
  return 0; 
} 

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