Countdown

Writing a line once and running it many times

A countdown is one sentence said over and over with a different number in it. You could write five separate printf calls by hand, but five is the gentle case: a program printing a thousand rows cannot be typed out.

A for loop is how you write the line once and say how many times to run it.

for (int n = 1; n <= 4; n++) {
    /* the lines in here run four times */
}

Everything the loop needs sits between the round brackets, in three parts separated by semicolons. Read them as three questions.

• Where does it start? int n = 1 declares a counter called n, local to the loop, and puts 1 in it once, before the first pass.
• How long does it keep going? n <= 4 is checked before every pass. True and the block runs again; false and the loop ends, and the program carries on with whatever comes after the closing brace.
• What changes each time? n++ runs at the end of every pass. Two plus signs mean add one; two minus signs, n--, mean take one away.

A worked example

Seats rather than a rocket, so the countdown stays yours to write:

#include <stdio.h>

int main(void) {
    for (int seat = 1; seat <= 3; seat++) {
        printf("Seat %d ready\n", seat);
    }
    printf("All seats checked.\n");
    return 0;
}

Seat 1 ready
Seat 2 ready
Seat 3 ready
All seats checked.

Three passes, and seat holds a different number on each one without you ever setting it again by hand. The last line sits outside the braces, so it runs once, after the loop has finished.

Your turn

The editor holds a launch sequence that already compiles and runs. Press Run before changing anything and the trouble shows itself: the count reads 0, 1, 2 and then announces liftoff. No rocket has ever left the ground that way.

One line needs rewriting, the one beginning with for:

for (int i = 0; i <= 2; i++) {

The banners, the rocket and the printf between the braces are finished — leave them alone.

Rewrite the three parts so the count reads 5, 4, 3, 2, 1 and then hands over to the liftoff banner. Start at five, keep going while the counter has not yet dropped below one, and subtract one each pass instead of adding one.

If something goes wrong

A countdown one number too long or too short is the likeliest result, and it is usually the middle part. The condition is checked before each pass, so the last number printed is the last one for which it was still true. Let it survive zero and a T minus 0 line sneaks in before the banner. If the count starts on the wrong number instead, it is the first part rather than the condition.

The next likeliest mistake is a loop that never finishes, which happens when the counter moves away from the finish rather than towards it. This site notices and stops a run that takes too long, and reports it as a timeout rather than letting your browser tab hang.

And if nothing appears between the two banners, the loop took no passes at all: the condition was already false on the very first check. Read the three parts left to right as start, keep going, change, then run it again.