ANSI C - Replacing blank characters without using arrays

Question

I've started studying "The ANSI C Programming Language" by Dennis Ritchie and Brian W.Kernighan. So far I've just learned getchar(),putchar(),while, for, if. There is an exercise that I have to do only using what I've learned by now. The above are the only things I know. Here's the exercise as it appears in the book:

Exercise 1-9. Write a program to copy its input to its output, replacing each string of one or more blanks by a single blank.

I know C#, Pascal, Objective-C and some Java but I cannot understand how this problem can be solved without using arrays. I know arrays but as the author has not covered arrays yet I assume I must not use them. Arrays are taught in the next chapters.

Answer 1

Implementing Nikolai's solution, as it might be easier to understand in code,

#include <stdio.h>
#include <stdbool.h> /* C99, but a good idea to use nonetheless */

int main() {
    bool was_space; /* state variable */
    char c; /* one-character buffer */

    while ( ( c = getchar() ) != EOF ) {
        if ( ! was_space || c != ' ' ) putchar( c );
        was_space = c == ' ';
    }
    return 0;
}

http://ideone.com/LLmBh

If you really want to avoid bool, you can just use int instead.

Answer 2

After some time I realised that I'd misunderstood the question and this misunderstanding was because I did not fully understand the mechanism of getchar() and putchar() functions. At first I thought I had to take the whole string and then itearte through it. But then I understood that "while ((input_char = getchar()) != EOF)" does some kind of iteration (maybe a simulation of iteration, don't know for sure). After that the rest is just a normal logic. When you look at the code you'll probably notice that I did not use if-else pattern. That's simply because "If" is the only thing that I've got so far and I should not use anything else that I know from other programming languages(although I'm sure if-else is found in any language). We have not seen &&,||,boolean either. And also I used ++counter instead of counter++. That's also because we have not yet come to post increment. I know it's the simplest logic that everyone knows but I wanted to share anyway.

#include <stdio.h>
int main()
{
   int input_char;
   int counter = 0;
   while ((input_char = getchar()) != EOF)      
   {
     if (input_char == ' ')
     {
        ++counter;
        if (counter == 1)
          putchar(input_char);
     }

     if (input_char != ' ')
     {
        putchar(input_char);
        counter = 0;
     }
   }
  return 0;
}          

Answer 3

Example based on my comment:

#include <stdio.h>

int main()
{
    int previous_char = 0;
    int latest_char;

    while ((latest_char = getchar()) != EOF)
    {
        if (' ' != previous_char || ' ' != latest_char)
        {
            putchar(latest_char);
        }
        previous_char = latest_char;
    }
    return 0;
}

Note that getchar() returns, and putchar() accepts, an int, not char.

Answer 4

Print the first whitespace (blank) character you see, skip the rest. That means having a boolean variable that is set to false as soon you see a non-whitespace character, and set to true when you hit and print first whitespace. Do not print if that variable is true. Kind of a two-state "state machine".