Problem in solving a character array problem from K&R book

This is a discussion on Problem in solving a character array problem from K&R book within the C Programming forums, part of the General Programming Boards category; Excercise: Write a program entab that replaces strings of blanks by the minimum number of tabs and blanks to achieve ...

  1. #1
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    Problem in solving a character array problem from K&R book

    Excercise:
    Write a program entab that replaces strings of blanks by the minimum number of tabs and blanks to achieve the same spacing. Use the same tab stops as for detab. When either a tab or a single blank would suffice to reach a tab stop, which should be given
    preference?
    My solution has not taken the final shape. Right now I am stuck in the last else-if. Program is working fine if l < 4 but not when l>4. I've been trying to debug for a while before giving up & looking for clues here.
    Please help.

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #define SPACE ' '
    #define IN 1
    #define OUT 0
    #define TAB '\t'
    
    int main(void) {
    	int buffer[50];
    	int i;
    	int l =0;
    	int m=0;
    	int c;
    	int STATE = OUT;
    for (i=0; i < 49 && (c = getchar()) != 'X' && c != '\n'; ++i){
    	buffer[i] = c; 
    }
    buffer [i+1] = '/0';
    while (buffer[m] != '/0') 
    	{
    	if (buffer[m] != SPACE){
    		STATE = IN; //In the word
    		l=0;
    	    putchar(buffer [m]); //print the input as output
    	    m++;
    	}
    	else if (buffer[m] == SPACE)
    	{
    	STATE = OUT; //Now outside the word
    	if (STATE == OUT)
    	{
    		l = l+1; //start counting the spaces
    
    if (l< 5){ //ignore if number of spaces are less than 5
    	putchar(buffer [m]); 
    	m++;
    }
    		else if (l >= 4) 
    		{
    			m = m - 4; //since 5 continuous spaces are detected push array index back
    			l= l-4; //push offset back
    			buffer[m] = TAB; //insert TAB at appropriate array field
    			putchar (buffer[m]); //stream the output with TAB
    			m++;
    		}
    
    
    		}
    
    	}
    		}
    
    		}

  2. #2
    and the hat of wrongness Salem's Avatar
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    The first step would be to learn how to indent code.
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #define SPACE ' '
    #define IN 1
    #define OUT 0
    #define TAB '\t'
    
    int main(void)
    {
      int buffer[50];
      int i;
      int l = 0;
      int m = 0;
      int c;
      int STATE = OUT;
      for (i = 0; i < 49 && (c = getchar()) != 'X' && c != '\n'; ++i) {
        buffer[i] = c;
      }
      buffer[i + 1] = '/0';
      while (buffer[m] != '/0') {
        if (buffer[m] != SPACE) {
          STATE = IN;               //In the word
          l = 0;
          putchar(buffer[m]);       //print the input as output
          m++;
        } else if (buffer[m] == SPACE) {
          STATE = OUT;              //Now outside the word
          if (STATE == OUT) {
            l = l + 1;              //start counting the spaces
            if (l < 5) {            //ignore if number of spaces are less than 5
              putchar(buffer[m]);
              m++;
            } else if (l >= 4) {
              m = m - 4;            //since 5 continuous spaces are detected push array index back
              l = l - 4;            //push offset back
              buffer[m] = TAB;      //insert TAB at appropriate array field
              putchar(buffer[m]);   //stream the output with TAB
              m++;
            }
          }
        }
      }
    }
    First point, /0 is not the same as \0

    Next, you need to count all the spaces before deciding what mix of spaces and tabs to output. So you only do this when you change state from OUT to IN.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  3. #3
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    Quote Originally Posted by alter.ego View Post
    Program is working fine if l < 4 but not when l>4. I've been trying to debug for a while before giving up & looking for clues here.
    One helpful tool is to run your program from the commandline with the following:

    entab | tr '\t' '$'

    Then you can see the actual tab characters in the output as '$' characters. On Windows, the program `tr' with mingw or you can download it from Coreutils Gnuwin32.

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