LESSON 122 OF 414 · WHILE LOOPS & FOR LOOPS

Printing Numbers with a While Loop in Scala: Counting Down

Practice makes perfect! Let's check two more examples of printing numbers using the while loop.

Example 1:

Let's print all the even numbers from 4 through 14:

var i = 4
while(i <= 14) {
	println(i)
	i += 2
}

The output of this program is:

Example 2:

We can also loop in reverse. Let's print all even numbers from 12 to 4 in decreasing order:

var i = 12
while(i >= 4) {
	println(i)
	i -= 2
}

The output of this program is:

Assignment
Let's print all the odd numbers from 23 to 7 in decreasing order!

Hint
Look at the examples above if you get stuck.

STEP 1 OF 4

Guided

Ctrl + Enter to check

Printing Numbers 2 in Scala, explained

What this lesson covers

A while loop repeats its body as long as its condition is true. Two things decide what it prints: where the counter starts, and how the counter changes on each pass. This lesson changes both. It steps by 2 instead of 1, and it counts down instead of up.

Stepping by 2

To print the even numbers from 4 through 14, start the counter at 4 and add 2 on every pass:

var i = 4
while (i <= 14) {
    println(i)
    i += 2
}

It prints 4, 6, 8, 10, 12 and 14, one per line. The condition i <= 14 uses <= because 14 itself must be printed. After 14 is printed, i becomes 16, the condition is false, and the loop ends.

Counting down

To go in reverse, start high, subtract in the body, and flip the comparison:

var i = 12
while (i >= 4) {
    println(i)
    i -= 2
}

This prints 12, 10, 8, 6 and 4. A condition written as i <= 4 would be false on the very first check, so the loop would never run.

Scala has no ++ or -- operators. Use i += 1 and i -= 1, or a larger step as above. The counter has to be a var, because a val can't be reassigned.

The assignment: odd numbers from 23 down to 7

The task is to print 23, 21, 19, and so on down to 7. Build it from the two patterns above.

Declare the counter. var counter = 23 starts at the first number to print. It is odd, so stepping by 2 keeps it odd.
Write the loop with the right condition. The loop goes down and must include 7, so the condition is counter >= 7.
Print the counter with println(counter) inside the body.
Move to the previous odd number. counter -= 2 does it. Without this line the counter would never change and the loop would never end.

var counter = 23
while (counter >= 7) {
    println(counter)
    counter -= 2
}

The output is 23, 21, 19, 17, 15, 13, 11, 9 and 7, each on its own line. After 7 is printed the counter becomes 5, the condition fails and the loop stops.

Common mistakes

Using > instead of >=. With counter > 7 the loop stops before printing 7.
Starting on the wrong parity. Starting at 22 and subtracting 2 would print even numbers. The starting value decides which numbers appear.
Adding instead of subtracting. counter += 2 makes the counter grow away from 7, so the condition stays true forever.
Printing before or after the update. Moving counter -= 2 above println would print 21 first and skip 23.

Key takeaways

The start value, the condition and the update together decide what a while loop prints.
Count down by subtracting in the body and using >= or > in the condition.
Scala uses += and -=; it has no ++.
Always check that the update moves the counter toward the point where the condition becomes false.

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