c_cpp 在C语言中，返回指向子字符串中字符串开头的指针。

Posted 2021-05-14

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>


// Finds the pointer to a substring within a string.
static const char* find_substr(const char* string, const char* substring) {
  int on = 0;
  const char* index = NULL;
  const char* traverse = substring; // resettable
  while(*string)
  {
    if(on)
    {
      if(!*traverse) return index;
      else if(*traverse == *string)
      {
        traverse++;
      }
      else
      {
        traverse = substring;
        index = NULL;
        on = 0;
      }
    }
    else
    {
      if(*string == *substring)
      {
        on = 1;
        index = string;
        traverse++;
      }
    }
    string++;
  }
  if(!*traverse) return index;
  else return NULL;
}

int main(void) {
  const char * result = find_substr("Hello Apple!", "Apple!f");
  if(result == NULL) puts("NULL");
  else puts(result);
  return 0;
}