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  1. #include <stdio.h>
  2. #include <stdlib.h>
  3. #include <errno.h>
  4. #include <string.h>
  5.  
  6. #define MAX_LENGTH 100
  7.  
  8. char* StrStr(const char *str, const char *target);
  9.  
  10. int main(void) {
  11.   char *result, *pointer;
  12.   char inputstring[MAX_LENGTH]; // here we define a new char array (string) which will be used for the input.
  13.  
  14.   /**
  15.    * fgets is used to read a string until a newline (i.e press return key)
  16.    * from stdin and into inputstring with a limit of MAX_LENGTH characters.
  17.    * It can be thought of as a way to get input from the user. Instead you
  18.    * could set inputstring to a predefined array of characters (in the code).
  19.    */
  20.   fgets(inputstring, MAX_LENGTH, stdin);
  21.   pointer = inputstring; // set pointer to the address of inputstring
  22.   while((result = StrStr(pointer, "a!c")) != NULL) { // while "a!c" can be found in the remainder of inputstring
  23.     int position = result - inputstring; // get the position (i.e index of the target)
  24.     pointer += position+1; // to avoid finding the same target indefinitely add position+1 to the pointer
  25.     printf("%d\n", position); // print the index to stdout (i.e the console)
  26.   }
  27. }
  28.  
  29. char* StrStr(const char *str, const char *target) {
  30.   if (!*target) return NULL;
  31.   char *p1 = (char*)str;
  32.   while (*p1) { // is the pointer pointing at a NUL terminator? If isn't do what is inside the while loop
  33.     char *p1Begin = p1, *p2 = (char*)target;
  34.     /**
  35.      * is the pointers p1 and p2 not pointing to a NUL terminator?
  36.      * is the pointers also pointing at the equivalent data?
  37.      * or is p2 pointing at an exclamation mark?
  38.      * If these conditions are true then point to the next character in str and target.
  39.      */
  40.     while ((*p1 && *p2 && *p1 == *p2) || *p2 == '!') {
  41.       p1++;
  42.       p2++;
  43.     }
  44.     if (!*p2)
  45.       return p1Begin;
  46.     p1 = p1Begin + 1;
  47.   }
  48.   return NULL;
  49. }
  50.  
Success #stdin #stdout 0s 2160KB
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stdin
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abcdefghijklmopqrstuvwxyzaxc
stdout
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0
25
https://ideone.com/aM9b9F
language:
C (gcc 8.3)
created:
9 years ago
visibility:
public

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