# K&R Exercise 1-13. Printing histogram of word lengths (horizontal variant)

I'm going through the K&R book (2nd edition, ANSI C ver.) and want to get the most from it. How does the following solution look to you? Note that, for the sake of exercise, I don't want to use techniques not introduced yet in the book. Also, I'm trying to reuse whatever code/philosophy already presented in the book.

/* Exercise 1-13. Write a program to print a histogram of the lengths of words in
* its input. It is easy to draw the histogram with the bars horizontal; a vertical
* orientation is more challenging.
* */
/* Solution 1: Horizontal Bars
* */
#include <stdio.h>

#define MAXLEN  10  /* Any word longer than this will get counted in the >MAXLEN histogram.
* For output formatting, the program assumes this won't be greater than 999.
* */
#define IN      1   /* inside a word */
#define OUT     0   /* outside a word */

int main()
{
int c, state;
int i, j;
int histo[MAXLEN+1];
int counter;

for (i = 0; i < MAXLEN+1; ++i)
histo[i] = 0;

/* Perform the counting */
state = OUT;
counter = 0;
while ((c = getchar()) != EOF)
if (c == ' ' || c == '\n' || c == '\t') {
if (state == IN && counter > 0) {
if (counter-1 < MAXLEN)
++histo[counter-1];
else
++histo[MAXLEN];
}
state = OUT;
counter = 0;
}
else {
++counter;
if (state == OUT)
state = IN;
}

/* Print horizontal histogram. I'd use a function for formatting
* with regards to max. number of digits, but functions haven't
* been introduced yet.
* */
for (i = 0; i < MAXLEN; ++i) {
if (MAXLEN < 10)
printf(" %1d | ", i+1);
else if (MAXLEN < 100)
printf(" %2d |", i+1);
else
printf(" %3d |", i+1);
for (j = 0; j < histo[i]; ++j)
putchar('*');
putchar('\n');
}
if (MAXLEN < 10)
printf(">%1d |", i);
else if (MAXLEN < 100)
printf(">%2d |", i);
else
printf(">%3d |", i);
for (j = 0; j < histo[i]; ++j)
putchar('*');
putchar('\n');
}


Output when run on the program code:

cat ch1-ex-1-13-01.c | ./ch1-ex-1-13-01 1 |********************************************************* 2 |**************************************************************** 3 |*************************************** 4 |************************************ 5 |************** 6 |********** 7 |************************ 8 |*********** 9 |******* 10 |********* >10 |**************  • By the way, which Version of the C Standard are you targeting? Premordial, K&R, C90, C99, C11? Oct 18 '18 at 15:59 • I didn't think about it until I saw the answer here using C99 & C90. I guess I'll be sticking to K&R 2nd edition as I go through the book, but it certainly wouldn't hurt to be aware of new features where relevant. Oct 18 '18 at 16:11 • I earnestly suggest that you take a close look at your book: Is it really K&R-C, or (mostly) C90? It depends on the revision. In any case, I suggest targeting at least C90, which might be ancient but is still ubiquitous. For Background, consider reading this post on meta.SO and this tag-wiki on SO. Oct 18 '18 at 16:24 • Since it's 2nd ed. I interpret it as being C90, right? The second edition, "K&R2", was first published in 1988 with some updates of the book to meet the version of the language standardized by ANSI in 1989. and The 1989 ANSI C standard was republished by ISO in 1990 Oct 18 '18 at 16:28 ## 2 Answers K&R C requires all variables to be declared at the start of their enclosing block. This is a bad habit to learn: it's much safer to declare variables where they can be initialised: /* these can already be initialised where they are declared */ int state = OUT; int histo[MAXLEN+1] = { 0 }; /* C90 */ int counter = 0;  /* reduce scope, and never have uninitialized value */ for (int j = 0; j < histo[i]; ++j) /* C99 */  We could remove the need to subtract 1 from the length when updating histo if we reserve the first element for long words instead of the last:  if (state == IN && counter > 0) { if (counter <= MAXLEN) ++histo[counter]; else ++histo[0]; }  We obviously need to make a corresponding change to the printing: for (int i = 1; i <= MAXLEN; ++i) { if (MAXLEN < 10) printf(" %1d | ", i); else if (MAXLEN < 100) printf(" %2d |", i); else printf(" %3d |", i); for (int j = 0; j < histo[i]; ++j) putchar('*'); putchar('\n'); }  We can avoid the if/else chain on MAXLEN in this loop, by choosing the formatting string in advance: const char *format = ""; if (MAXLEN < 10) { format = " %1d | "; } else if (MAXLEN < 100) { format = " %2d | "; } else { format = " %3d | "; } for (int i = 1; i <= MAXLEN; ++i) { printf(format, i); for (int j = 0; j < histo[i]; ++j) putchar('*'); putchar('\n'); }  That might not look like an improvement, but with a small change, we can use the same format string to insert > for the over-long words, too: const char *format = ""; if (MAXLEN < 10) { format = "%c%1d | "; } else if (MAXLEN < 100) { format = "%c%2d | "; } else { format = "%c%3d | "; } for (int i = 1; i <= MAXLEN; ++i) { printf(format, ' ', i); for (int j = 0; j < histo[i]; ++j) putchar('*'); putchar('\n'); } /* over-long words */ printf(format, '>', MAXLEN); for (int j = 0; j < histo[0]; ++j) putchar('*'); putchar('\n');  Going beyond the expected level of knowledge, we could even compute the necessary length, and pass it using %*d: int width = 1; for (int i = MAXLEN; i >= 10; i /= 10) { ++width; } for (int i = 1; i <= MAXLEN; ++i) { printf(" %*d | ", width, i); for (int j = 0; j < histo[i]; ++j) putchar('*'); putchar('\n'); } /* over-long words */ printf(">%*d | ", width, MAXLEN); for (int j = 0; j < histo[0]; ++j) putchar('*'); putchar('\n');  That allows us to have non-constant MAXLEN, which will be to our advantage in future when we make this a function. Whilst the state machine is a good idea, it turns out we don't need to explicitly store state, because counter is always zero outside a word and non-zero once we're in one: while ((c = getchar()) != EOF) { if (c == ' ' || c == '\n' || c == '\t') { if (counter > 0) { ++histo[counter <= MAXLEN ? counter : 0]; counter = 0; } } else { ++counter; } }  Did you spot the bug in this loop? Probably not, because the bug is missing code immediately after the loop. If the input stream ends during a word, we fail to count it. We need to repeat the if (counter > 0) test just after the loop, to catch that case. We have an unusual definition of "word" - in our source code, strings such as ++histo[counter-1]; count as single words. Even in English text, we'd expect text, to register as a four-character word. To fix this, we could consider only alphanumerics as word characters, and that could be a good introduction to the functions in <ctype.h>. while ((c = getchar()) != EOF) { if (isalnum(c)) { ++counter; } else { if (counter > 0) { ++histo[counter <= MAXLEN ? counter : 0]; counter = 0; } } } if (counter > 0) { ++histo[counter <= MAXLEN ? counter : 0]; }  Enhancement - we could avoid bars going off the right edge of the display, by calculating a scale factor first. Here's my suggestion: #define MAXWIDTH 72 /* screen width available for bars */   double scale = 1.0; for (int i = 0; i <= MAXLEN; ++i) { if (histo[i] * scale > MAXWIDTH) { scale = 1.0 * MAXWIDTH / histo[i]; } }   for (int j = 0; j < histo[i] * scale; ++j) { putchar('*'); }  Finally: it's not part of your C code, but there's no need for cat in the test. We can simply redirect input like this: ./ch1-ex-1-13-01 <ch1-ex-1-13-01.c  # Modified code Applying all my suggestions, I got: /* Exercise 1-13. Write a program to print a histogram of the lengths of words in its input. It is easy to draw the histogram with the bars horizontal; a vertical orientation is more challenging. */ /* Solution 1: Horizontal Bars * */ #include <ctype.h> #include <stdio.h> #define MAXLEN 10 /* Any word longer than this will get counted in the >MAXLEN histogram. */ #define MAXWIDTH 72 /* screen width available for bars */ int main() { int histo[MAXLEN+1] = { 0 }; /* element 0 counts over-length words */ /* Perform the counting */ int counter = 0; int c; while ((c = getchar()) != EOF) { if (isalnum(c)) { ++counter; } else { if (counter > 0) { ++histo[counter <= MAXLEN ? counter : 0]; counter = 0; } } } if (counter > 0) { ++histo[counter <= MAXLEN ? counter : 0]; } /* Calculate sensible scale */ double scale = 1.0; for (int i = 0; i <= MAXLEN; ++i) { if (histo[i] * scale > MAXWIDTH) { scale = 1.0 * MAXWIDTH / histo[i]; } } /* how wide are the labels? */ int width = 1; for (int i = MAXLEN; i >= 10; i /= 10) { ++width; } /* Write the output */ for (int i = 1; i <= MAXLEN; ++i) { printf(" %*d | ", width, i); for (int j = 0; j < histo[i] * scale; ++j) { putchar('*'); } putchar('\n'); } /* over-long words */ printf(">%*d | ", width, MAXLEN); for (int j = 0; j < histo[0] * scale; ++j) { putchar('*'); } putchar('\n'); }  • Wow, great, thank you! Is your array initialization also C99? I thought about using [0] for the over-length but wasn't sure if it's good for code readability. I do like it, tho. Cool idea for formatting string with the %c%d, I'll remember that (char array assignment hasn't been introduced yet, tho). Didn't know about the %*d trick, I'd probably go with sprintf instead. /= hasn't been introduced, but I get the idea. For the bug, nice catch, I didn't notice. Or, we could say we're counting complete words :) As for word definition, I agree tho I followed the book example 1.5.4 there. Oct 16 '18 at 13:52 • That initialization is plain C90 (C99 adds support for non-constant initializers, but incomplete initializer lists go back to at least the first Standard C). The use of element [0] is clearer in my (subjective) opinion, but others may differ (or might want more comments). As for the rest, I don't have my K&R with me here, so had to guess what's in scope and what's not! Oct 16 '18 at 14:07 • Nice to know, I'm yet to learn about different standards. For now I'm just following K&R as if we're still living in the 80's. I've implemented your comments myself and posted it as another answer. Fixed some scaling issues, too. Oct 17 '18 at 13:21 • If the number of stars is bounded and acceptably small, they can all be printed at once using part of the same string-literal. And that can be fused with the preceding printf() and trailing printing of a newline. Oct 18 '18 at 11:55 • Yes @Deduplicator, that's probably what I'd do with this. I was already in too deep, though... Oct 18 '18 at 12:10 # Take 2 I've reworked my code according to the comments received here but sticking to the techniques which have been introduced up to this point in K&R, which means the following is NOT used: • variable declaration where first initialized, • assignment of char array constants, • %*d format specifier, • ctype.h. Some other improvements: • avoided repeating some code by introducting readEOF state, • improved scaling, • check whether MAXWIDTH is sufficient to print the smallest possible chart, • printing numerical value next to histogram, • figured out how to print labels better, while sticking to the techniques introduced so far, • limited source code width to 72, • fixed a scaling bug due to flating point precision, • reverted to storing >MAXLEN count in the last element. ### Modified code /* Exercise 1-13. Write a program to print a histogram of the lengths of * words in its input. It is easy to draw the histogram with the bars * horizontal; a vertical orientation is more challenging. * */ /* Solution 1: Horizontal Bars * Take 2: Implementing some comments received from SE: * https://codereview.stackexchange.com/questions/205674#205680 * but sticking to the techniques which have been introduced up to this * point in K&R, which means the following is NOT used: * - variable declaration where first initialized, * - assignment of char array constants, * - %*d format specifier, * - <ctype.h>. * Some other improvements: * - avoided repeating some code by introducing readEOF state, * - improved scaling, * - check whether MAXWIDTH is sufficient to print the smallest * possible chart, * - printing numerical value next to histogram, * - figured out how to print labels better, while sticking to the * techniques introduced so far, * - limited source code width to 72, * - fixed a scaling bug due to flating point precision, * - reverted to storing >MAXLEN count in the last element. * */ #include <stdio.h> #define MAXLEN 10 /* Any word longer than this will get counted in * the >MAXLEN histogram. */ #define MAXWIDTH 72 /* Max. width for chart printout, the whole * chart will be scaled to fit. */ int main() { int c; int i, j; int histogram[MAXLEN+1]; /* last element will count the over-length * words */ int alnumCount; int readEOF; int labelW; int maxLabelW; int maxWordCount; int maxWordCountW; float scale; for (i = 0; i <= MAXLEN; ++i) histogram[i] = 0; /* Perform the counting */ alnumCount = 0; readEOF = 0; c = getchar(); while (readEOF == 0) { if ((c >= '0' && c <= '9') || (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z')) ++alnumCount; else { if (alnumCount > 0) { if (alnumCount <= MAXLEN) ++histogram[alnumCount-1]; else ++histogram[MAXLEN]; alnumCount = 0; } } if ((readEOF = (c == EOF)) == 0) c = getchar(); } /* Chart Printing Section * */ /* Each histogram will look like: * ">NNN |******************** MMM", * showing both the label and the histogram value, so we can display * accurate information even if the chart is scaled. * To be able to plot on a fixed screen width, we must calculate the * max. histogram value, the widths of NNN & MMM fields, and finally * the scale. * */ /* Find max. histogram value */ maxWordCount = 0; for (i = 0; i <= MAXLEN; ++i) if (maxWordCount < histogram[i]) maxWordCount = histogram[i]; /* Calculate histogram value max. width */ maxWordCountW = 1; for (i = maxWordCount; i >= 10; i = i/10) ++maxWordCountW; /* Calculate label max. width */ maxLabelW = 1; for (i = MAXLEN; i >= 10; i = i/10) ++maxLabelW; /* Calculate the scale */ /* Note - histogram layout: ">NNN |******************** MMM". */ scale = 1.0*(MAXWIDTH - 1 - maxLabelW - 2 - 1 - maxWordCountW) / maxWordCount; /* Fix a scale rounding bug */ if (scale * maxWordCount < MAXWIDTH - 1 - maxLabelW - 2 - 1 - maxWordCountW) scale = scale + 0.000001; /* Print the chart only if MAXWIDTH is sufficient to print at least * the values */ if (maxWordCount * scale >= 0) /* Print horizontal histogram. */ /* Row loop */ for (i = 0; i <= MAXLEN; ++i) { /* Print either the " " or ">" for the last histogram */ if (i < MAXLEN) putchar(' '); else putchar('>'); /* Calculate the label width */ labelW = 1; for (j = i+1; j >= 10; j = j/10) ++labelW; /* Print blanks as required for lavel alignment and print * the label */ for (j = maxLabelW - labelW; j > 0; --j) putchar(' '); if (i < MAXLEN) printf("%d |", i+1); else printf("%d |", MAXLEN); /* Column loop, print the histogram */ for (j = 1; j <= scale*histogram[i]; ++j) putchar('*'); /* Print the histogram value */ printf(" %d", histogram[i]); putchar('\n'); } else printf("Error: insufficient screen width for print-out.\n"); }  ### Output when run on the program code  ./ch1-ex-1-13-01_Take2 < ch1-ex-1-13-01_Take2.c
1 |*************************************************************** 103
2 |*************************************** 64
3 |************************************************* 81
4 |***************************** 49
5 |********************************************* 74
6 |************************** 43
7 |****************** 31
8 |************** 24
9 |************************* 41
10 |*********** 18
>10 |************** 23

• If you want to get that reviewed, make it a new question. Oct 18 '18 at 11:49
• Didn't intend to go through round 2, just posting here for info. I'll post the vertical variant as a new Q once I'm done with it. Oct 18 '18 at 14:02