Collapse runs of blanks

K&R Exercise 1-9: write a program that reads characters from input and writes them back, squeezing any run of spaces down to a single space.

A blank here is a space character. Tabs stay tabs. Newlines stay newlines.

You already counted spaces in Count blanks, tabs, and newlines. Same read loop; this time you copy, and you squeeze consecutive spaces down to one.

Remember the last character

The copy loop is the one from the earlier exercises: while ((c = getchar()) != EOF). Each time around, look at c. If it is not a space, print it. If it is a space, print it only when the previous character was not a space too.

Keep the previous character in a variable. That is enough to collapse a run. You do not need a nested loop.

Write the program

Create a file called collapse-blanks.c:

#include <stdio.h>

int main(void)
{
    int c;
    int last;

    last = 0;

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

    return 0;
}

last starts as 0, which is not a space, so the first character always prints. After that, a space is printed only when the previous character was something else. putchar writes one character — the same job as the book's copy program.

Compile and run

In Terminal, from the folder that holds collapse-blanks.c:

gcc collapse-blanks.c -o collapse-blanks
./collapse-blanks

Type this exactly — three spaces between the words, then Return:

hello   world

You should see:

hello world

One space between the words. Press Ctrl+D on an empty line to end.

If you type a line with mixed single and double spaces, every run collapses to one. A tab between words stays a tab — this program only squeezes ' '.

If gcc is missing, finish Getting your macOS ready for C first.

How to send EOF

On macOS, Ctrl+D after a newline is end-of-file — same as getchar() and EOF. The program then stops copying and exits.

Check the output

Type a b (two spaces) and you should get a b. Type a b (one space) and it should stay a b. If extra spaces still appear, last is not being compared to ' '.

What you should take away

getchar still gives you one character at a time. Remembering the last one is enough to collapse a run of blanks. A tab is not a blank.

Next

→ Make escapes visible — Exercise 1-10