file use

This is a discussion on file use within the C Programming forums, part of the General Programming Boards category; Ive been following the examples and tutorials in K&R the c programming language yet i have no idea how to ...

  1. #1
    Registered User redruby147's Avatar
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    Question 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;
    }

  2. #2
    Registered User
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    That program counts the tabs you enter from the keyboard. To use it with a file, do this:

    ./program < input.txt

  3. #3
    spurious conceit MK27's Avatar
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    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...
    Last edited by MK27; 09-14-2008 at 08:24 AM.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  4. #4
    CSharpener vart's Avatar
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    ((getc(fstRO)) != EOF) if (c == '\t')
    what magical connection have you made between changing c and calling getc?
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  5. #5
    Registered User redruby147's Avatar
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    Question

    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.

  6. #6
    Registered User C_ntua's Avatar
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    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')

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