I'm trying to write a program which task is to get rid of extra whitespace in a text stream.

#include <stdio.h>

int main()
{
    int c;
    
    while( (c = getchar()) != EOF)
    {
        if(c == ' ')
        {
            putchar(c);
            while( (c = getchar()) == ' ' );
            
            if(c == EOF) break;
        }
        putchar(c);
    }
}

I know that getchar() function returns a new character each iteration and we store the character in the c variable, but I can't figure out why do we need to assign c variable to getchar() in the second while loop again.

CodePudding user response：

The inner loop loops as long as it extracts spaces from the stream, effectively skipping them. The result is that, if you have a sequence of spaces, the first one will be printed, and the rest will be discarded.

You need to assign to c even there (as opposed to just doing something like while (getchar() == ' ');) because you'll reach a point where you extract a character that is not a space, so you need to remember what character it was to output it after the loop.

CodePudding user response：

I know that getchar() function returns a new character each iteration and we store the character in the c variable, but I can't figure out why do we need to assign c variable to getchar() in the second while loop again.

The reuse of variable c is of no particular consequence. A separate variable could have been used for the inner loop instead. Example:

#include <stdio.h>

int main()
{
    int c;
    
    while( (c = getchar()) != EOF)
    {
        putchar(c);
        if(c == ' ')
        {
            int c2;
            while( (c2 = getchar()) == ' ' );
            
            if (c2 == EOF) break;
            putchar(c2);
        }
    }
}

Note that a rearrangement of the putchar() calls was required. Either way, when the program encounters a run of spaces, it must both (i) print one space, AND (ii) avoid losing the first non-space, if any, after the run.

It would also have been possible to write the program with only one appearance of a call to getchar(), but then it would need to store more state explicitly (whether the previous character read was a space) and use that to decide whether to output each newly-read character. For example:

#include <stdio.h>

int main(void) {
    int c;
    _Bool was_space = 0;
    
    while ((c = getchar()) != EOF) {
        if (c != ' ' || !was_space) {
            putchar(c);
        }
        was_space = (c == ' ');
    }
}

In fact, I find both of those alternatives rather clearer than the original. The original seems to go out of its way to minimize the number of variable it declares.