C - how to open a txt file and then display the data

paulsm4 · 10-13-2010, 05:39 PM

Hi -

In C, you can only use "==" for primitive types (like an int, float or a single character). For a string, you must use a comparison function like "strcmp()".

Here's an example that might help:

Code:

$ cat history.txt
aaa
bbb
ccc
ddd

Code:

$ cat tmp.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAX_LINE 80
#define HISTORY_FILE "history.txt"

int
main(int argc, char *argv[])
{

  if (argc != 2)
  {
    printf ("USAGE: enter command (ex. \"history\")\n");
    return 1;
  }

  /* if user enters history has a string then display the history txt file. */
  if (strncmp (argv[1], "history", 7) == 0)
  {
    char input[MAX_LINE + 1];
    int line_no = 0;
    FILE *fp = fopen (HISTORY_FILE, "r");
    if (!fp)
    {
      printf ("ERROR: Unable to open (%s)!\n", HISTORY_FILE);
      return 1;
    }
    while (!feof (fp))
    {
      if (fgets (input, MAX_LINE, fp))
        printf ("%4d %s", line_no++, input);
    }
    fclose (fp);
  }
  else
  {
    printf ("unknown/unsupported command: %s\n", argv[1]);
    return 1;
  }
  return 0;
}

Code:

$ gcc -Wall -pedantic -o tmp tmp.c

$ ./tmp
USAGE: enter command (ex. "history")

$ ./tmp history
   0 aaa
   1 bbb
   2 ccc
   3 ddd
   4

rupertwh · 10-13-2010, 05:42 PM

Quote:

Originally Posted by UltimateDesi

I think i am making this harder then it should be,

For us, yes. All of your posts are extremely unclear as to what you are trying to achieve.

Quote:

Originally Posted by UltimateDesi

Code:

	if ( input == "history")  //if user enters history has a string }

What does input evaluate to?
Does that change when the contents of input changes?
What does "history" evaluate to?
Can they ever compare equal?

Hint: You are trying to comapare strings in C. *wink*

UltimateDesi · 10-14-2010, 10:28 PM

Quote:

Originally Posted by paulsm4

Hi -

In C, you can only use "==" for primitive types (like an int, float or a single character). For a string, you must use a comparison function like "strcmp()".

Here's an example that might help:

Code:

$ cat history.txt
aaa
bbb
ccc
ddd

Code:

$ cat tmp.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAX_LINE 80
#define HISTORY_FILE "history.txt"

int
main(int argc, char *argv[])
{

  if (argc != 2)
  {
    printf ("USAGE: enter command (ex. \"history\")\n");
    return 1;
  }

  /* if user enters history has a string then display the history txt file. */
  if (strncmp (argv[1], "history", 7) == 0)
  {
    char input[MAX_LINE + 1];
    int line_no = 0;
    FILE *fp = fopen (HISTORY_FILE, "r");
    if (!fp)
    {
      printf ("ERROR: Unable to open (%s)!\n", HISTORY_FILE);
      return 1;
    }
    while (!feof (fp))
    {
      if (fgets (input, MAX_LINE, fp))
        printf ("%4d %s", line_no++, input);
    }
    fclose (fp);
  }
  else
  {
    printf ("unknown/unsupported command: %s\n", argv[1]);
    return 1;
  }
  return 0;
}

Code:

$ gcc -Wall -pedantic -o tmp tmp.c

$ ./tmp
USAGE: enter command (ex. "history")

$ ./tmp history
   0 aaa
   1 bbb
   2 ccc
   3 ddd
   4

Thank you, this helped me a lot, I am also stuck on something like if user enters !12...it needs to go into my history text file and then go to line 12 and get the string and have it save to a buffer.

!number - will get the line in the history list that is preceded with that number
!! - get the last line from history text file
!string - will get the last command that begins with that string
!$ -

Aquarius_Girl · 10-14-2010, 11:28 PM

Quote:

Originally Posted by UltimateDesi

I am also stuck on something like if user enters !12...it needs to go into my history text file and then go to line 12 and get the string and have it save to a buffer.

If user enters !12, you just need to :

1. Reset your file pointer to the start of file.
2. Increment your file pointer by 12
3. Read the string character by character till you get a white space.
4. Display it.

Suggestion :
It is very important for you to get a hold on the file pointer manipulations.

UltimateDesi · 10-17-2010, 02:47 AM

Can somebody point me in the right direction. Lets say you want line 4 from text history file..how would i go about going into text file and getting that line and saving it to a variable or pointer.? I have read what anishakaul suggested but i still can't figure it out.

example of history.txt might contain

ls
pwd
cd ..
!!

Aquarius_Girl · 10-17-2010, 10:32 AM

Quote:

Originally Posted by UltimateDesi

I have read what anishakaul suggested but i still can't figure it out.

Look up on Google the following functions:

Code:

fseek
lseek
ftell
rewind