file use

Ive been following the examples and tutorials in K&R the c programming language yet i have no idea how to use files, for example The code below shows an attempt at counting the amount of tabs in a text file however, once compiled $./program text.txt doesn't do anything. Any help is appreciated.

Code:

#include <stdio.h>

int main (int argc, char *argv[])
{
int c, nt;
	nt = 0;
	while ((c = getchar()) !=EOF)
		   if (c == '\t')
		   nt++;

   printf("%d tabs", nt);
return 0;
}

That program counts the tabs you enter from the keyboard. To use it with a file, do this:

./program < input.txt

You haven't opened any files. Open a file for reading like this:

Code:

FILE *fstRO = fopen(argv[1], "r");
if (fstRO == NULL) {perror("Couldn't open"); exit -1;}

and remember to fclose the stream (fstRO) later. In this case, the file will be whatever you use as the first argument to the program when you start it. The second line is to make sure the file has actually been opened; it requires stdlib.h . Then, you want to read character-by-character thru the file and add up the tabs -- except getchar won't do this (unless you pipe fstRO into stdin) because getchar reads from stdin. So instead use getc:

Code:

while ((getc(fstRO)) != EOF) if (c == '\t') nt++;

So here's your code with corrections:

Code:

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

int main (int argc, char *argv[]) {
        int c, nt=0;
        FILE *fstRO = fopen(argv[1], "r");
        if (fstRO == NULL) {perror("Couldn't open"); exit -1;} 

        while ((getc(fstRO)) != EOF) if (c == '\t') nt++;
        fclose(fstRO);
        printf("&#37;d tabs\n", nt);
        return 0;
}

Unfortunately, there is one last problem that you'll have to test yourself: Most likely, "tabs" as a single character don't exist in your text files at all (even if they got there because you pressed "tab" somewhere sometime). They're just a number of inserted spaces. So all the output is ever likely to be is:

0 tabs

Oh well...

((getc(fstRO)) != EOF) if (c == '\t')

what magical connection have you made between changing c and calling getc?

Great! thanks! But there is something odd If we look at one method which i think used the original code gives

Code:

vivi@debian:~/c$ ./robwhit < test.c
7 tabsvivi@debian:~/c$

whereas another method brings up no tabs on exactly the same file.

Code:

vivi@debian:~/c$ ./mk27 test.c
0 tabs

Not sure why this is.

Why won't tabs exist at the file? Tab is one character. Every 'X' is one charcter (1 byte). The program displaying the information of a text "translates" a tab character into spaces.

It is because MK27 had a mistake as pointed out by Vart. Obviously he wanted to write:

Code:

((c = getc(fstRO)) != EOF) if (c == '\t')