Thread: Print only specific lines in the file

I understand some perl and a bit of C++. I am just learning C and I been trying many examples that use strChr, strstr, rewind, scanf, Index, strtok, fseek and fsetpos for over a week. Now I am trying to figure which combination of these functions should I use, and how. What I want to do is open my file and copy only specific lines to another file. I hope my explanation is clear. This is a very large file with many lines to skip. They may be anywhere in the list before and after the keyword, PROLIST. I'm hoping for some examples.

First, I need to print each line after every occurrences of the word PROLIST:

(1.txt)

Code:

 Skip this line.
  Skip this line.
  PROLIST
 Print this line
 Skip this line.
 PROLIST
 Print this line
 Skip this line.
 Skip this line.

Also in a second example, could it show how to print each line before and after every occurrences of the word PROLIST:

(A.txt)

Code:

  Skip.
  Print this line
  PROLIST
  Print this line
  Skip.
  Skip.
  Print this line
  PROLIST
  Print this line

I use Linux and BSD’s. I can post examples of what I have done so far with the listed C functions above; however, it seems that each instance worked perfectly for what it does but I hit a dead-end at what I’m trying to do with them. My first list wrapped is more important then the complicated one below it.

Note to the Administrator:: Could you please correct the misalignment of the first block of text wrapped in the code tag. Nothing I tried fixed it.


I appreciate any feedback.

I might do it like this.

Code:

#include <stdio.h>
#include <stdbool.h>
#include <string.h>
 
int main() {
    FILE *fin = fopen("1.txt", "r");
    FILE *fout = stdout; // for testing
 
    char line[1000];
    bool print_line = false;
 
    while (fgets(line, sizeof line, fin) != NULL) {
        if (print_line) {
            fputs(line, fout);
            print_line = false;
        }
        if (strstr(line, "PROLIST") != NULL)
            print_line = true;
    }
 
    fclose(fin);
    return 0;
}

Using strstr like I did means that it counts a line with "PROLIST" anywhere in it.
We can be more specific if you want

For the second one, here's a possibility.

Code:

#include <stdio.h>
#include <stdbool.h>
#include <string.h>
 
int main() {
    FILE *fin = fopen("A.txt", "r");
    FILE *fout = stdout; // for testing
 
    char line[1000], last_line[1000] = {0};
    bool print_line = false;
 
    while (fgets(line, sizeof line, fin) != NULL) {
        if (print_line) {
            fputs(line, fout);
            print_line = false;
        }
        if (strstr(line, "PROLIST") != NULL) {
            fputs(last_line, fout);
            print_line = true;
        }
        strcpy(last_line, line);
    }
 
    fclose(fin);
    return 0;
}