06-01. Countdown to launch

In this programme, we show you how to use a counter controlled iteration, known as a for loop, which is an alternative to a condition controlled repetition or while loop.

We're going to make use of an extra command that will cause the programme to pause for one second before executing the next command.

This command is not built into C by default, so we'll import that from the system threading library with the using command.

As always, we then start with our boilerplate code.

Let's write a subprogram that will count down from 10 to 0.

Static void countdown.

No data is required.

So, open and close brackets, output T before the countdown begins.

So counting can be performed with the for command, we need a variable to store the counter and this can be called anything.

It's not unusual to use a single letter here, such as C, N or I, depending on what you're counting.

But here we'll identify our variable as counter, since that's what it's going to be used for.

We want the counter to start at 10, so int counter 10.

The first part of the command ends here, so we need a semicolon.

The next part is what will cause the programme to continue iterating.

We want to keep looping when the counter is greater than zero, followed by another semicolon.

Then finally, what causes the counter to go down in each iteration.

Here we want to say counter, counter minus one.

So the shorthand for that is counter.

Remember that increments or goes up by one and minus minus means decrement or go down by one.

The code for our iteration must be enclosed in curly brackets, just like while loops console, writeline the counter, with three dots to make the output look prettier.

Before we start the next iteration, we want to pause for one second.

This is where we make use of the sleep command in the threading library.

Thread sleep will require data, so open bracket followed by the number of milliseconds to wait, that's 1,000 for one second.

Close bracket.

That's all the code we want to loop, so close the curly bracket.

So what happens here is the variable counter is initialised to 10, and then it's subtracted by 1 at the end of these commands and it goes back up and checks if it's still greater than zero.

If it is, it loops again, and if not, it stops after the countdown from 10 to 1 we'll output the final number 0.

Of course, we could have done this in the loop if we'd made the condition counter greater than minus one.

We didn't do that because we don't want a one second wait after zero is reached.

Instead, we want to immediately output all engines running, followed by liftoff.

We have liftoff on Artemis 1.

Now wait or sleep for a second before outputting Towerclear.

That's it for the subprogram.

Let's now call that subprogram in the main programme section with countdown, which requires no data.

So open and close brackets give the programme a run and we can see the effects of the for loop and counter variable counting down.

Due to the way I've captured the screen, we don't see the weight in the video, but you should see it when you run the programme.

You might be thinking, I can achieve all this with a while command and a counter variable.

Why do I need a for loop?

Indeed you can, but generally, if we know how many times we want to iterate, we use a for instead of a while.

In future programmes, we'll also see how useful a for can be to iterate through arrays and lists without, indexing, making the loop more useful in those cases.