Chapter 1 - A Tutorial Introduction 20

Answer to Exercise 1-20, page 34
Solution by Richard Heathfield

Thanks to Rick Dearman for pointing out that my solution used fgets() which has not been introduced by page 34. I've fixed the solution to use K&R's getline() function instead. Further thanks to Roman Yablonovsky who, in Oct 2000, pointed out that the solution was buggy, and hinted at a fix. Basically, the problem he spotted was that my solution failed to keep track of the cumulative effect of multiple tabs in a single line. I've adopted his fix (which was in fact also slightly buggy, but I've fixed that too).

Write a program detab that replaces tabs in the input with the proper number of blanks to space to the next tab stop. Assume a fixed set of tab stops, say every n columns. Should n be a variable or a symbolic parameter?

 

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

#define MAX_BUFFER   1024
#define SPACE        ' '
#define TAB          '\t'

int CalculateNumberOfSpaces(int Offset, int TabSize)
{
   return TabSize - (Offset % TabSize);
}

/* K&R's getline() function from p29 */
int getline(char s[], int lim)
{
  int c, i;

  for(i = 0; i < lim - 1 && (c = getchar()) != EOF && c != '\n'; ++i)
    s[i] = c;
  if(c == '\n')
  {
    s[i] = c;
    ++i;
  }
  s[i] = '\0';

  return i;
}

int main(void)
{
  char  Buffer[MAX_BUFFER];
  int TabSize = 5; /* A good test value */

  int i, j, k, l;

  while(getline(Buffer, MAX_BUFFER) > 0)
  {
    for(i = 0, l = 0; Buffer[i] != '\0'; i++)
    {
      if(Buffer[i] == TAB)
      {
        j = CalculateNumberOfSpaces(l, TabSize);
        for(k = 0; k < j; k++)
        {
          putchar(SPACE);
          l++;
        }
      }
      else
      {
        putchar(Buffer[i]);
        l++;
      }
    }
  }

  return 0;
}

 

In answer to the question about whether n should be variable or symbolic, I'm tempted to offer the answer 'yes'. :-) Of course, it should be variable, to allow for modification of the value at runtime, for example via a command line argument, without requiring recompilation.