"The C Programming Language", 2nd edition, Kernighan and Ritchie

Answer to Exercise 1-14, page 24

Solution by Richard Heathfield

Write a program to print a histogram of the frequencies of different characters in its input.

Naturally, I've gone for a vertical orientation to match exercise 13. I had some difficulty ensuring that the printing of the X-axis didn't involve cheating. I wanted to display each character if possible, but that would have meant using isprint(), which we haven't yet met. So I decided to display the value of the character instead. (The results below show the output on an ASCII system - naturally, a run on an EBCDIC machine would give different numbers.) I had to jump through a few hoops to avoid using the % operator which, again, we haven't yet met at this point in the text. 


#include <stdio.h>

/* NUM_CHARS should really be CHAR_MAX but K&R haven't covered that at this stage in the book */
#define NUM_CHARS 256

int main(void)
{
  int c;
  long freqarr[NUM_CHARS + 1];

  long thisval = 0;
  long maxval = 0;
  int thisidx = 0;

  for(thisidx = 0; thisidx <= NUM_CHARS; thisidx++)
  {
    freqarr[thisidx] = 0;
  }

  while((c = getchar()) != EOF)
  {
    if(c < NUM_CHARS)
    {
      thisval = ++freqarr[c];
      if(thisval > maxval)
      {
        maxval = thisval;
      }
    }
    else
    {
      thisval = ++freqarr[NUM_CHARS];
      if(thisval > maxval)
      {
        maxval = thisval;
      }
    }
  }

  for(thisval = maxval; thisval > 0; thisval--)
  {
    printf("%4d  |", thisval);
    for(thisidx = 0; thisidx <= NUM_CHARS; thisidx++)
    {
      if(freqarr[thisidx] >= thisval)
      {
        printf("*");
      }
      else if(freqarr[thisidx] > 0)
      {
        printf(" ");
      }
    }
    printf("\n");
  }
  printf("      +");
  for(thisidx = 0; thisidx <= NUM_CHARS; thisidx++)
  {
    if(freqarr[thisidx] > 0)
    {
      printf("-");
    }
  }
  printf("\n       ");
  for(thisidx = 0; thisidx < NUM_CHARS; thisidx++)
  {
    if(freqarr[thisidx] > 0)
    {
      printf("%d", thisidx / 100);
    }
  }
  printf("\n       ");
  for(thisidx = 0; thisidx < NUM_CHARS; thisidx++)
  {
    if(freqarr[thisidx] > 0)
    {
      printf("%d", (thisidx - (100 * (thisidx / 100))) / 10 );
    }
  }
  printf("\n       ");
  for(thisidx = 0; thisidx < NUM_CHARS; thisidx++)
  {
    if(freqarr[thisidx] > 0)
    {
      printf("%d", thisidx - (10 * (thisidx / 10)));
    }
  }
  if(freqarr[NUM_CHARS] > 0)
  {
    printf(">%d\n", NUM_CHARS);
  }

  printf("\n");

  return 0;
}



Here's the output of the program when given its own source as input: 
 474  | *                                                                    
 473  | *                                                                    
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  97  |**                                                 *                  
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  82  |**                                                 *                  
  81  |**                                                 *                  
  80  |**                                                 *                  
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  76  |**                                                 *                  
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  74  |**                                                 *                  
  73  |**                                                 *                  
  72  |**                                                 *                  
  71  |**                                                 *         *        
  70  |**                                                 *         *        
  69  |**                                                 *         *        
  68  |**                                                 *         *        
  67  |**                                                 *         *        
  66  |**                                                 *         *        
  65  |**                                                 *         *        
  64  |**                                                 *         *        
  63  |**                                                 *         *        
  62  |**                                                 *         *        
  61  |**                                                 *         *        
  60  |**                                                 *         *        
  59  |**                                                 *       * *        
  58  |**                                                 *       * *        
  57  |**                                                 *       * *        
  56  |**                                                 *       * *        
  55  |**                                                 *       * *        
  54  |**                                                 *       * *        
  53  |**                                                 *       * *        
  52  |**                                                 *       * *        
  51  |**                                                **       * *        
  50  |**                                                **       * *        
  49  |**                                                **       ***        
  48  |**                                                **       ***        
  47  |**                                                **       ***        
  46  |**                                                **       ***        
  45  |**                                                **       ***        
  44  |**                                                **       ***        
  43  |**                                              * **       ***        
  42  |**                                         *  * * **       ***        
  41  |**                                         *  * * **       ***        
  40  |**      **                                 *  * * **       ***        
  39  |**      **            *                    *  * * **       ***        
  38  |**      **            *                    *  * * **       ***        
  37  |**      **            *                    *  * * **       ***   *    
  36  |**      **            *                    *  * * **       ***   *    
  35  |**      **            *                    *  * * **       ***   *    
  34  |**      **            *                    *  * * **       ***   *    
  33  |**      **            *                    *  * * **       ***   *    
  32  |**      **            *                    *  * * **   *   ***   *    
  31  |**      **            *                    *  * * **   *   ***   *    
  30  |**      **            *                    *  * * **   *   ***   *    
  29  |**      **            *                    *  * * **   *   ***   *    
  28  |** *    **            *                    *  *** ** * *   ***   *    
  27  |** *    **      *     *                    *  *** ** * *   ***   *    
  26  |** *    **      *     *                    *  *** ** * *   ***   *    
  25  |** *    **      *     *                    *  *** ** * *   ***   *    
  24  |** *    **      *     *                    *  *** ** * *   ***   *    
  23  |** *    **      *     *                    *  *** ** * *   ***   *    
  22  |** *    **      *     *                    *  *** ** * *   ***   *    
  21  |** *    **      *     * *                  *  *** ** * *   *** * *    
  20  |** *    **      *     * *                  *  *** ** * *   *** * * * *
  19  |** *    **      *     * *                  *  *** ** * *   *** * * * *
  18  |** *    ** *    *     * *                  *  *** ** * *   *** * * * *
  17  |** *    ** *    *     * *                  *  *** ** * *   *** * * * *
  16  |** *    ** *    *     * *                  *  *** ** * **  *** * * * *
  15  |** *    ** *    *     * * *        *       *  *** ** * **  *** * * * *
  14  |** *    ** *    *     * * **  * *  *      **  *** ** * *** *** * * * *
  13  |** *    ** *    *     * * **  * ** ***    **  *** ** * *** *** * * * *
  12  |** *    ** *    *     * ****  * ** ***    **  *** ** * *** *** * * * *
  11  |** *    ** *    *     * ****  * ** *** * ***  *** ** * ******* * * * *
  10  |** *    ** *    *     * ****  * ** *** * ***  *** ** * ******* * * * *
   9  |** *    ** *    *     * ****  * ** *** * ***  *** ** * ******* * * * *
   8  |** *    ** *    *     ******  * ** *** * ***  *** ** * ******* * * * *
   7  |** *    ** *    **    ******  * ** *** * *** **** ** ********* * * * *
   6  |** *    ** *   ***    ******  * ** *** ***** **** ** ********* * * * *
   5  |** * *  ****** ***    ******  * ** *** ***** ******* ********* * * * *
   4  |** * *  ****** ***    ******  * ** *** ***** ******* *********** * * *
   3  |** * *  ****** ***    ******  * ** *** ************* *********** * * *
   2  |** ***  ****** ***    ******  * ** *** ************* *********** * * *
   1  |**********************************************************************
      +----------------------------------------------------------------------
       0000000000000000000000000000000000000000000000111111111111111111111111
       1333333344444444445555566666677777788889999999000000000111111111122222
       0234578901234567890234901257902578923581235789012345789012345678901345


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