K&R – Solution to Exercise 1.23

I’m working my way through The C Programming Language at the moment, and so far I’m loving C’s purity and leanness.

One of my few disappointments with the book is the lack of solutions to the exercises. The exercises are refreshingly challenging because the book isn’t aimed at beginning programmers. Unfortunately this means that the desire to validate one’s solutions is quite strong. Fortunately, Google had a solution to this problem: Richard Heathfield’s solutions site.

I just finished my solution to exercise 1.23 (“Remove all comments from a C program”) and I’m quite chuffed with it so thought I would share it here. Out of the multiple solutions provided on Richard’s site, mine was about the closest to following the guidelines as far as what knowledge one is supposed to have of C by that (early) point in the book (the only thing I cheated with is the use of `break` and `continue` – it would have just been quite ugly without those two small additions). Basically the solution is a state machine using `if / else` instead of more traditional `switch` methods as `switch` had not yet been introduced.

My solution deals with all the special cases I could think of, and even deals with the tricky sample input on Richard’s site, provided at the bottom of the solutions page for this exercise.

``````/*
* K&R Exercise 1-23
*
* "Write a program to remove all comments from a C program. Don't
*  forget to handle quoted strings and character constants properly.
*
*/

#include <stdio.h>

int main()
{
int c;
int c2;
int in_quotes = 0;
int in_comment = 0;
int current_quote;

c = getchar();
while (c != EOF)
{
if (!in_comment && !in_quotes)
{
if (c == '\'' || c == '"')
{
in_quotes = 1;
current_quote = c;
putchar(c);
}
else if (c == '/')
{
c2 = getchar();
if (c2 != EOF && c2 == '*')
{
in_comment = 1;
}
else
{
putchar(c); /* Just a regular '/', so output it. */
c = c2;
continue;
}
}
else
{
putchar(c);
}
}
else if (in_comment)
{
/* Check for a closing comment. */
if (c == '*')
{
c2 = getchar();
if (c2 != EOF && c2 == '/')
{
in_comment = 0;
}
else
{
/* Don't advance to next character in stream. */
c = c2;
continue;
}
}
}
else if (in_quotes)
{
/* Skip over escaped chars. */
if (c == '\\')
{
putchar(c);
c = getchar();
}
else
{
/* Check for closing quote. */
if (c == current_quote)
in_quotes = 0;
}
putchar(c);
}

c = getchar();
}

return 0;
}``````

One response to K&R – Solution to Exercise 1.23

Prashant kashyap says:

Actually I did the same logic with mine program too i.e., if we get a ‘/’ character then hold it in a variable and check if next character is ‘*’ or not if it is ,then don’t read next characters untill ‘/’ and ‘*’ is found again..but my logic is lacking something.when I put multiple ‘*’ characters inside the comment. It’s getting printed along with not in comment characters.

This site uses Akismet to reduce spam. Learn how your comment data is processed.