I want to add a line of text after the fifth line in a text file.This is my code:

#include <stdio.h> 

int main(){
  FILE *f;
  f = fopen("file1.txt", "r");

  int nCount = 0;
  char c = fgetc(f);
  while(nCount != 5){
    if(c == '\n'){
      nCount  ;
    }
    // c = fgetc(f);
    fseek(f, 1, SEEK_SET);
  }

  c = fgetc(f);

  fprintf(f, "I have appended a line");
  
 
  return 0;
}

Line is not being appended.What is wrong?

CodePudding user response：

What is wrong?

You can't insert data in the middle of a file without shifting all the later lines.

To add a line, consider forming a new file.

// Pseudo code
Open input file
Open output file
nth_line_found = false
while ((len = read_a_chunk(input, buffer)) > 0) 
  if (!nth_line_found) {
    Count lines in buffer
    while counting, 
      if (count == nth) {
        write first part of buffer
        write "I have appended a line\n"
        write last part of buffer
        nth_line_found = true
    if (nth_line_found still false) write buffer
  } else {
    write buffer
  }

  If desired, delete input file and rename output to input.

This approach implies reading with fread() rather than fgets() to avoid length (and null character) limitations of fgets().

CodePudding user response：

THIS IS JUST A SKETCH!

Read the shole file into memory

fopen();
int lineno = 0;
while (fgets()) {
    addlinetomemory();
    lineno  ;
    if (lineno == 42) addextraline();
}
fclose();

now reopen in write mode

fopen();
for (int line = 0; line < lineno; line  ) {
    fputs(); // write line
}
fclose();

Voila!

CodePudding user response：

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

int main(int argc, char *argv[])
{
FILE *f_in,*f_out;

f_in=fopen("file1.txt", "rt");
if(!f_in) return -1;
f_out=fopen("file2.txt", "wt");
if(!f_out) return -1;

char *oneLine = (char*) malloc(LINE_MAX);
if(oneLine){

    int lineCounter=0;

    while(fgets(oneLine,LINE_MAX,f_in)){
        lineCounter  ;
        fputs(oneLine,f_out);
        if(lineCounter == 5)
            fputs("a new line\n",f_out);
    }

    fclose(f_in);
    fclose(f_out);
    free(oneLine);
    unlink("file1.txt");
    rename("file2.txt","file1.txt");
    return 0;
} else
    return -1;
}

CodePudding user response：

If your text file is of a reasonable size (to fit into memory) the following works with one open file... I leave it as an exercise to deal with the Windows/DOS convention of CRLF where LF works just as well. (This is just a rough cut, but it may lead you to the results you want.)

It could do with more testing of return values, especially the fwrite() calls.

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

int my_main() {
    char *fname = "log.txt";

    FILE *fp = fopen( fname, "rb " );
    if( fp == NULL ) {
        fprintf( stderr, "Cannot open %s r/w\n", fname );
        return 1;
    }

    // measure space needed
    fseek( fp, 0, SEEK_END );
    size_t size = ftell( fp );
    fseek( fp, 0, SEEK_SET );

    // get space
    char *iobuf = (char*)malloc( size );
    if( iobuf == NULL ) {
        fprintf( stderr, "Cannot allocate %ld byte buffer\n", size );
        fclose( fp );
        return 1;
    }

    // load entire file
    size_t nread = fread( iobuf, sizeof(char), size, fp );
    if( nread != size ) {
        fprintf( stderr, "Expecting %ld - Read %ld\n", size, nread );
        return 1;
    }

    int lnCnt = 5; // skip past 5 lines
    for( char *cp = iobuf; cp < iobuf   size; cp   )
        if( *cp == '\n' && --lnCnt == 0 )
            break;
    cp  = 1;
    // Up to you to handle situation where #lines < 5

    // position after 5th '\n'
    fseek( fp, cp - iobuf, SEEK_SET );

    // write the new string
    char *newText = "I wish I was a fish\n"; // Written after whatever line
    fwrite( newText, sizeof newText[0], strlen( newText ), fp );

    // then write back the rest
    fwrite( cp, sizeof *cp, size - (cp - iobuf), fp );

    fclose( fp );

    return 0;
}

Output:

This is line 1
This is line 2
This is line 3
This is line 4
This is line 5
I wish I was a fish
This is line 6
This is line 7