Countdown

Repeating a line with for and range

A countdown is one line said over and over with a different number in it each time. You could write five separate print lines by hand. A loop is how you write the line once and tell Python how many numbers to feed through it.

The shape looks like this:

• for opens the loop.
• n is a name. Each time round, Python puts the next number into it for you, so n is a variable you never have to set yourself.
• range(5) is the supply of numbers the loop draws from.
• The colon ends the line, and the line underneath is indented. That indent is how Python knows which instructions belong to the loop. Indented lines run once per number. Lines back at the left edge run once, after the loop has finished.

The part that catches everybody

range(5) does not mean one to five. It hands over five numbers beginning at zero: 0, 1, 2, 3, 4. The number you write is where counting stops, and it is not one of the numbers you get.

Think of a shop open until six. Six o'clock is when it shuts, not another hour of business.

range also accepts three numbers: where to start, where to stop, and how far to move each time round.

• range(2, 6) gives 2, 3, 4, 5. It starts at 2 and still stops before 6.
• range(6, 2, -1) gives 6, 5, 4, 3. The -1 means take one off each time, which walks the count downwards. It still stops before 2.

That last rule holds whichever direction you travel: the stopping number is the one number you never see. Counting down, that means it sits one below the last number you actually want printed.

A worked example

for step in range(3):
    print("step", step)
print("done")

Running that prints four lines:

step 0
step 1
step 2
done

Three numbers, so the indented print ran three times, holding a different value in step on each pass — 0 first, 2 last, no 3 anywhere. The final line is not indented, so it sits outside the loop and prints once at the end, after the counting is over.

Your turn

The editor holds a launch sequence that already works. Press Run before changing anything and you can see the problem for yourself: it counts 0, 1, 2 and then announces liftoff. No rocket has ever left the ground that way.

One line needs changing, the one with range on it. The banners, the rocket and the print inside the loop are all finished — leave them alone.

Put three numbers inside range so the count reads 5, 4, 3, 2, 1 and then hands over to the liftoff banner: where it begins, where it stops, and the step that carries it downwards instead of up.

If something goes wrong

The most likely result is a gap. Both banners print, the rocket prints, and between them there is nothing at all — no countdown and no error message. That happens when the step is missing. Asked to count upwards from 5 until it reaches 0, Python sees that 0 is already behind it, takes no steps, and moves on. There is no complaint because nothing went wrong; the loop simply had nothing to do.

The other likely result is a countdown one number short at one end — starting at 4, or finishing on 2. Both come from the same rule: the stopping number is never printed. If the count ends early, that number is one too high.

Nothing you type here can break anything. The worst case is a countdown that reads oddly, and then you change a number and run it again.