program reducing numerous spaces to one

Here is the code

Code:

#include <stdio.h>

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

That's an example from K&R, but I can't understand how it can work.
The line with lastc is incomprehensible to me.
How can it check if lastc (which is actually nothing) is different from space?
Thanks in advance !

You are right: lastc should have been initialised to say, a null character. It is not that lastc is nothing at that point in the first iteration, but that it was not initialised.

Originally Posted by laserlight

You are right: lastc should have been initialised to say, a null character. It is not that lastc is nothing at that point in the first iteration, but that it was not initialised.

Thanks, but when I'd introduce eg. lastc = 0;
how can it be compared with blank space? 0 is always different than ' '
What's the point of that if then?

Originally Posted by Pole

how can it be compared with blank space? 0 is always different than ' '
What's the point of that if then?

Refer to this line:

Code:

lastc = c;

Maybe it would be easier to understand like this:

Code:

#include <stdio.h>

int main(void)
{
    int lastc = '\0'; /* previous character */
    int c; /* current character */

    while ((c = getchar()) != EOF) {
        if (c != ' ' || lastc != ' ') {
            putchar(c);
        }
        lastc = c;
    }

    return 0;
}

If the current character is not a space, then obviously you should print it because it would not be a consecutive space. Or, if the previous character was not a space, then printing the current character (a space) is fine because it would not be a consecutive space. Hence, consecutive spaces are not printed, and thus it appears that they have been reduced to a single space.