I don't understand how how this code works

Question

I'm doing Exercise 1-9 in the K&R Book, while trying to find solutions I came across this code:

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

Why does the first if statement work if even if I input a letter. From my understanding it will only execute if the character I input is a blank space? Btw the exercise is making a program replace multiple consecutive blank spaces to a single one.

Answer 1

I've formatted out and comented the code. Hope that'll help. The code actually covert all sequence of spaces within stdin into one space:

 "123    456 a  b c  " -> "123 456 a b c "

Code:

int main() {
    int c;
 
    /* we read stdin character after character */
    while ((c = getchar()) != EOF) {
        /* if we have read space */   
        if (c == ' ') {
            /* we skip ALL spaces */
            while ((c = getchar()) == ' ')
                ; /* skipping ALL spaces: we do nothing */

            /* and then we print just ONE space instead of many skipped */
            putchar(' ');

            /* if we at the end of stdin, we have nothing more to print */
            if (c == EOF) 
                break;
        }

        /* we print every non space character */
        putchar(c);
    }
}