how to read specific lines of a text document and write them to another text | C

Question

I have created a function that takes as a parameter the name of a source file, the name of a destination file and the beginning and end lines of the source file lines that will be copied to the destination file, like the example below. All I want to do is to input the lines that I want to copy to the other text file like the example below:

The code I show you just "reads" the content of the one text file and "writes" another one. I want to "write" specific lines that the user gives, not the whole text file

Inputs by the user:
Source_file.txt //the file that the destination file will read from
destination_file.txt //the new file that the program has written
2 3 // the lines that it will print to the destination file: 2-3

Source_file.txt:

1
2
3
4
5
6

destination_file.txt

2
3

code:

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

void cp(char source_file[], char destination_file[], int lines_copy) {
    char ch;
    FILE *source, *destination;

    source = fopen(source_file, "r");
    if (source == NULL) {
        printf("File name not found, make sure the source file exists and is ending at .txt\n");
        exit(EXIT_FAILURE);
    }

    destination = fopen(destination_file, "w");
    if (destination == NULL) {
        fclose(source);
        printf("Press any key to exit...\n");
        exit(EXIT_FAILURE);
    }

    while ((ch = fgetc(source)) != EOF)
        fputc(ch, destination);

    printf("Copied lines %d  from %s to %s \n",
           lines_copy, source_file, destination_file, ".txt");

    fclose(source);
    fclose(destination);
}

int main() {
    char s[20];
    char d[20];
    int lines;

    printf("-Enter the name of the source file ending in .txt\n"
           "-Enter the name of the destination file ending in .txt\n"
           "-Enter the number of lines you want to copy\n\n");

    printf(">subcopy.o ");
    gets(s);
    printf("destination file-> ");
    gets(d);
    printf("Lines: ");
    scanf("%d", &lines);

    cp(s, d, lines);

    return 0;
}

Answer 1

In cp(), in order to select the lines to keep, you have to know their position in the input-file. Thus, you need to count lines.

Using fgets instead of fgetc will allow you to count the lines.

---

On the other hand, if I wanted to select lines 3 and 7 to 12 in a file, I'd use:

sed -n -e "3p;7,12p" < input.txt > output.txt

Answer 2

this is a very simple solution, let's say you know that the maximun length of a line will be 100 characters for simplicity (if a line is longer than 100 characters only the first 100 will be taken)

at the top (outside main) you can write

#ifndef MAX_LINE_SIZE
#define MAX_LINE_SIZE 100
#endif

i know many people don't like this but i think in this case it makes the code more elegant and easier to change if you need to modify the maximum line size.

to print only the wanted lines you can do something like this

char line[MAX_LINE_SIZE];

int count = 0;

while (fgets(line, MAX_LINE_SIZE, source)){
    count++;
    if (3 <= count && count <= 5){
        fputs(line, destination);
    }
}

The while loop will end when EOF is reched because fgets returns NULL.

P.S. there could be some slight errors here and there since i wrote it pretty fast and going by memory but in general it should work.

Answer 3

There are some problems in your program:

• Do not use gets(), it may cause buffer overflows.

• Always use type int to store the return value of fgetc() in order to distinguish EOF from regular byte values.

• You pass an extra argument ".txt" to printf(). It will be ignored but should be removed nonetheless.

To copy a range of lines from source to destination, you can just modify your function this way:

#include <stdio.h>
#include <string.h>
#include <errno.h>

void cp(char source_file[], char destination_file[], int start_line, int end_line) {
    int ch;
    int line = 1, lines_copied;
    FILE *source, *destination;

    source = fopen(source_file, "r");
    if (source == NULL) {
        printf("Cannot open input file %s: %s\n",
               source_file, strerror(errno));
        exit(EXIT_FAILURE);
    }

    destination = fopen(destination_file, "w");
    if (destination == NULL) {
        printf("Cannot open output file %s: %s\n",
               destination_file, strerror(errno));
        fclose(source);
        exit(EXIT_FAILURE);
    }

    while ((ch = fgetc(source)) != EOF) {
        if (line >= start_line && line <= end_line) {
            fputc(ch, destination);
        }
        if (ch == '\n') {
            line++;
        }
    }
    lines_copied = 0;
    if (line > start_line) {
        if (line >= end_line) {
            lines_copied = end_line - start_line + 1;
        } else {
            lines_copied = line - start_line + 1;
        }
    }
    printf("Copied lines %d from %s to %s\n",
           lines_copy, source_file, destination_file);

    fclose(source);
    fclose(destination);
}

int main() {
    char source_file[80];
    char destination_file[80];
    int start_line, end_line;

    printf("-Enter the name of the source file ending in .txt\n"
           "-Enter the name of the destination file ending in .txt\n"
           "-Enter the start and end line\n\n");

    printf(">subcopy.o ");
    if (scanf("%79s", source_file) != 1) {
        return 1;
    }
    printf("destination file-> ");
    if (scanf("%79s", destination_file) != 1) {
        return 1;
    }
    printf("Start and end lines: ");
    if (scanf("%d %d", &start_line, &end_line) != 2) {
        return 1;
    }

    cp(source_file, destination_file, start_line, end_line);

    return 0;
}