I am making a task management system and I the users should be able to view tasks only with the status TO DO.
I have a file with entries looking like this:
8291 Math Homework Pg|1-2 2022/04/03 TD
8292 English Assignment Pg|1-2 2022/04/03 IP
8293 Science Homework Pg|1-2 2022/04/03 TD
if the user enter TD then I want the program to read only these lines from the file:
8291 Math Homework Pg|1-2 2022/04/03 TD
8291 Science Homework Pg|1-2 2022/04/03 TD
How can I do this in C?
Your comment brings your question to the point "a line meets my criteria and how I can read only those lines".
In this clear but strict phrasing the answers is that you can't.
You can only read all the lines and then ignore those which do not match your requirements.
I understand that the details of doing that you do not need help with because you state "I am able to read all tasks" and there are hints in other user comments on how to qualify a read line (in contrast to not reading a line which does not qualify) and you only focus on not reading unqualified lines. So I assume that once you change your goal to reading and ignoring instead of not reading you are set. If you do need help with reading, testing, ignoring I propose to ask that in a separate focused question.
You will have to read all lines in the file. There is no way to read only the lines that meet a certain criteria.
Therefore, you should read all lines in the file in a loop, using the function fgets, so that in every loop iteration, exactly one line is processed. In every loop iteration, you can then check whether the current line meets the criteria using the function strstr, and if it does, you can then print that line. Otherwise, you will do nothing in that loop iteration (i.e. not print that line).
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main( void )
{
FILE *fp;
char line[500];
//open input file
fp = fopen( "input.txt", "r" );
if ( fp == NULL )
{
fprintf( stderr, "Error opening file!\n" );
exit( EXIT_FAILURE );
}
//read exactly one line per loop iteration
while ( fgets( line, sizeof line, fp ) != NULL )
{
//print the line, if it meets the criteria
if ( strstr( line, "TD\n" ) != NULL )
{
printf( "%s", line );
}
}
//cleanup
fclose( fp );
}
For the input
8291 Math Homework Pg|1-2 2022/04/03 TD
8292 English Assignment Pg|1-2 2022/04/03 IP
8293 Science Homework Pg|1-2 2022/04/03 TD
this program has the following output:
8291 Math Homework Pg|1-2 2022/04/03 TD
8293 Science Homework Pg|1-2 2022/04/03 TD
However, this program will only work if the input file has POSIX-compliant file endings, i.e. if the last character in the file is a newline character. If the last character in the file is not a newline character, then strstr( line, "TD\n" ) will never report a match on the last line, because it will be unable to find the newline character.
One way to solve this would be to use
strstr( line, "TD" )
instead of:
strstr( line, "TD\n" )
However, this is not an ideal solution, because if the first part of the line happens to contain the substring "TD", then you will get a false positive. You only want "TD" at the end of the line to be a match.
Therefore, the best solution would probably be to not use strstr on the whole line, but to use strcmp on the last 2 characters in the line:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main( void )
{
FILE *fp;
char line[500], *p;
const char *search_string = "TD";
const int search_string_len = strlen( search_string );
//open input file
fp = fopen( "input.txt", "r" );
if ( fp == NULL )
{
fprintf( stderr, "Error opening file!\n" );
exit( EXIT_FAILURE );
}
//read exactly one line per loop iteration
while ( fgets( line, sizeof line, fp ) != NULL )
{
if ( ( p = strchr( line, '\n' ) ) != NULL )
{
//remove newline character
*p = '\0';
}
else
{
//make p point to null terminating character
p = line + strlen(line);
}
//if the line has less characters then the search
//string, then a match is not possible
if ( p - line < search_string_len )
continue;
//compare the last characters on the line with the
//search string
if ( strcmp( p - search_string_len, search_string ) != 0 )
continue;
//all tests were passed, so print the line
printf( "%s\n", line );
}
//cleanup
fclose( fp );
}