C program that reads from file or stdin and removes blank lines

Question

I am new to C and here I have C program that will take in an X amount of files from the command line and output the files to stdout but deletes the blank lines. If the user doesn't input any files it will read directly from stdin.

Everything functions smoothly except for the removing of the lines.

Here is my program

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#define NUMCHARS 1024


int main (int argc, char** argv)

{
    int good_file = 0;

    if(argc <= 1){
        good_file++;
        test(stdin);
    }

    FILE* files[argc - 1];
    int i;
    for(i = 0; i < argc - 1; i++){
        if ((files[i]=fopen(argv[i+1], "r"))==NULL){
            continue;
            }
                else{
                    good_file++;
                    test(files[i]);
            }

    }

        if(!good_file){
            fprintf(stderr, "ERROR!\n");
        }

}

int test (FILE *file)
    {
        char buffer[NUMCHARS];

        while (fgets(buffer, NUMCHARS, file) != NULL)
                part2(buffer);
            fclose(file);

    }

int part2 (char *buffer)
{
    if(*buffer != '\n' || *buffer != '\t' || *buffer != ' '){           //if(*buffer != '\n') successfully deletes (skips) plain newlines
    fputs(buffer, stdout);
    }
}

In function part 2, as you can see by my comment the program will successfully delete (skips) just newlines if I remove the || to the '\t' and the ' ' in that if statement. But a lot of blank lines are not necessarily "blank". i.e they will have tabs or white space on them. Using the logic I had with removing newlines I applied it to tabs and white space as well.

But with this new implementation it won't even skip the newlines. And it doesn't work what so ever.

Appreciate the feedback!

possible duplicate of [Execute block if a variable is not one of some specific values](http://stackoverflow.com/questions/26337003/execute-block-if-a-variable-is-not-one-of-some-specific-values) — Barmar, Apr 28 '15 at 21:33
Add `while (*buffer)` directly above `part2(buffer);` You are not iterating over each character in buffer... — David C. Rankin, Apr 28 '15 at 21:38
Thank you. That fixes my problem. But I just realized that this will skip lines that aren't blank but are preceded by a tab or space. So I got some work to do — collinskewl2, Apr 28 '15 at 21:40
David C Rankin, just realized that and am going to work on a solution — collinskewl2, Apr 28 '15 at 21:40
Why do you need a program for something that is trivial with `awk`, `sed` or `grep`? — Dmitry Grigoryev, Apr 28 '15 at 21:52
I realize that but that would make this program easier to solve. I am trying to get familiar with C programs so I just take practice programs from my C textbook — collinskewl2, Apr 28 '15 at 21:54

score 2 · Answer 1 · edited Apr 28 '15 at 21:44

2

Your condition here is always true, so does precisely nothing:

if (*buffer != '\n' || *buffer != '\t' || *buffer != ' ') {
    fputs(buffer, stdout);
}

You're asking "Is the first character not a newline or not a tab or not a space?" Every character can say that: a space is not a newline, and a newline is not a tab. You need to check all the characters in the line:

int is_all_white(char *s) {
    while (*s) {
        if (!('\n' == *s || '\t' == *s || ' ' == *s)) return 0;
        s += 1;
    }
    return 1;
}

... 

if (! is_all_white(buffer)) {
    fputs(buffer, stdout);
}

edited Apr 28 '15 at 21:44

David Ranieri

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answered Apr 28 '15 at 21:41

Lee Daniel Crocker

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Also, feel free to use `isspace()` if you include `ctypes.h`. – Lee Daniel Crocker Apr 28 '15 at 21:43
Thank you for the reply! what exactly does the `while (*s)` mean? And does the `s += 1;` increment the pointer s by one after each iteration? – collinskewl2 Apr 28 '15 at 21:50

R Sahu · Answer 2 · 2015-04-28T21:44:58.417

0

The conditional of the following if statement

if(*buffer != '\n' || *buffer != '\t' || *buffer != ' ')

is true for every line.

You should change the logic to use:

int isBlankLine(char const* line)
{
   for ( ; *line != '\0'; ++line )
   {
      if ( !isspace(*line) )
      {
         return 0;
      }
   }
   return 1;
}

int part2 (char *buffer)
{
   if (!isBlankLine(buffer) )
   {
      fputs(buffer, stdout);
   }
}

edited Apr 28 '15 at 21:44

answered Apr 28 '15 at 21:38

R Sahu

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@Teyras, indeed. Fixed the answer. – R Sahu Apr 28 '15 at 21:45

score 0 · Answer 3 · answered Apr 28 '15 at 21:41

Your if condition is always satisfied - you check if the first character of buffer is not a newline, and if it is a newline, you check that if it's not a tab.

You could replace all the || in the condition with && - that would mean that every line whose first character is not a newline, space or tab gets printed.

What you really want to do is check every character of the line with the [isspace]1 function. If there is a non-whitespace character, you can print it.

score -2 · Answer 4 · answered Apr 28 '15 at 21:41

-2

if(*buffer != '\n' || *buffer != '\t' || *buffer != ' '){
    fputs(buffer, stdout);
}

replace to

char c;
if(1==sscanf(buffer, " %c", &c)){
    fputs(buffer, stdout);
}

answered Apr 28 '15 at 21:41

BLUEPIXY

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How is sscanf supposed to check if the line is empty? – Teyras Apr 28 '15 at 21:44
@Teyras Of course, Return value if the only white space character does not become `1`. – BLUEPIXY Apr 28 '15 at 21:46
So? What if the line contains two spaces? – Teyras Apr 28 '15 at 21:47
2

Oh, indeed, `sscanf` skips whitespace characters. But the code is still hard to understand for a beginner... – Teyras Apr 28 '15 at 21:59

C program that reads from file or stdin and removes blank lines

4 Answers4