Replacing a substring with another

This is a discussion on Replacing a substring with another within the C Programming forums, part of the General Programming Boards category; I'm creating this program that is going to: 1. Read a file 2. Find occurances of a substring within the ...

  1. #1
    System-7
    Join Date
    Nov 2005
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    Replacing a substring with another

    I'm creating this program that is going to:
    1. Read a file
    2. Find occurances of a substring within the string read in
    3. Replace the occurances of this substring with another string
    4. Write to new file
    Code:
    #include <stdio.h>
    #include <string.h>
    #include <stdlib.h>
    
    #define MAX 512
    #define _GNU_SOURCE
    
    int main(int argc, char *argv[])
    {
      FILE *file, *file2;
      char line[MAX], *subLocation;
    
      if (argc != 4)
      {
        printf("You must enter: ./replace old-string new-string file-name\n");
        exit(1);
      }
    
      file = fopen(argv[3], "r");
      file2 = fopen("temp", "w");
    
      while (fgets(line,sizeof(line),file) != NULL)
      {
        /* Search for string 1 in string buffer */
        if (subLocation == NULL)
        {
          subLocation = strstr(line, argv[1]);
         /* position = subLocation-line */
    
        /* Write the line */
        fputs(line, file2);
        printf(line); /*Show me the line */
    
      }
      /* Close everything */
      fclose (file);
      fclose (file2);
      return 0;
    }
    I have 1 and 4 down lol. Now I'm pretty new to C...though I have done C++. It's fairly different and I was hoping you guys can shed some light onto some functions I can use for finding and replacing the substring.
    I've so far got strstr which will find the first occurance of the string (and i can loop to see if there is more than one). But I'm not sure what I should use to replace it. Any help would be greatly appreciated .

    Thanks,
    Dan

  2. #2
    and the hat of wrongness Salem's Avatar
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    > subLocation = strstr(line, argv[1]);
    Given a string of "LEFTMIDDLERIGHT", and you searched for MIDDLE

    then
    subLocation-line gives you 4 (the length of LEFT)
    subLocation+strlen(argv[1]) gives you a pointer to RIGHT

    - write out the left
    - write out the replacement
    - advance to the RIGHT
    - repeat until there's no more string
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  3. #3
    System-7
    Join Date
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    Thanks for clearing it up
    Dan

  4. #4
    Frequently Quite Prolix dwks's Avatar
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    Code:
    printf(line); /*Show me the line */
    You shouldn't do this, in case the line contains a %. Use
    Code:
    printf("%s", line);
    or better yet
    Code:
    puts(line);
    which is the same as
    Code:
    printf("%s\n", line);
    dwk

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