How can I read different lines of a text file with fgets? - c

Question

For example, if a .txt has
Hello
There.
written in it, no matter how bigger N is in fgets(str, N, file), it will only store "Hello" in str, because it stops when it finds a '\n' character.
So, how could I read the whole file if, for example, I wanted to find a specific word in it?

Answer 1

So, how could I read the whole file
In order to read the whole file into a memory buffer, you could use the function fread. After turning the input into a string by appending a terminating null character, you could then use the function strstr to search the input for a certain word.
Here is a program which does this and searches the input for the word targetword:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main( void )
{
FILE *fp;
char buffer[1000];
size_t read;
//open input file
fp = fopen( "input.txt", "rt" );
if ( fp == NULL )
{
fprintf( stderr, "ERROR: Unable to open input file!\n" );
exit( EXIT_FAILURE );
}
//read entire file into buffer
read = fread( buffer, 1, sizeof buffer, fp );
//verify that buffer was not too small
if ( read == sizeof buffer )
{
fprintf( stderr, "ERROR: Memory buffer is too small to contain entire input!\n" );
exit( EXIT_FAILURE );
}
//add terminating null character to make input a valid
//null-terminated string
buffer[read] = '\0';
//search input for target word
if ( strstr( buffer, "targetword" ) != NULL )
printf( "Found word!\n" );
else
printf( "Did not find word!\n" );
fclose( fp );
}
However, instead of reading the entire file at once (which could require a very large memory buffer), it is more common to read one line at a time in a loop, and in every loop iteration, you check whether the current line contains the word you are looking for. That way, the memory buffer only has to be large enough to store one line of input at once, instead of the entire input.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
int main( void )
{
FILE *fp;
char line[100];
bool found = false;
//open input file
fp = fopen( "input.txt", "rt" );
if ( fp == NULL )
{
fprintf( stderr, "ERROR: Unable to open input file!\n" );
exit( EXIT_FAILURE );
}
//read one line per loop iteration
while ( fgets( line, sizeof line, fp ) != NULL )
{
//verify that line was not too long to fit into buffer
if ( strchr( line, '\n' ) == NULL )
{
fprintf( stderr, "line too long to fit buffer!\n" );
exit( EXIT_FAILURE );
}
//search for target word
if ( strstr( line, "targetword" ) != NULL )
{
found = true;
break;
}
}
if ( found )
printf( "Found word!\n" );
else
printf( "Did not find word!\n" );
fclose( fp );
}
However, both solutions have several possible issues:
If the target word targetword is part of another word, for example thetargetword, then it will state that it found the target word. I'm not sure if this is what you want or if you want the target word to appear by itself.
If the target word is syllabified so that, for example, target-\n appears in one line and word in the next line, then the program won't be able to find the word.
The search is case-sensitive, so it will only find targetword, but not Targetword or TARGETWORD.
All of these issues can be solved, if necessary, but would require additional work.