LESSON 121 OF 415 · WHILE LOOPS & FOR LOOPS

Printing Numbers with a While Loop in Swift

We can achieve many different outcomes with while loops.

For example, we can print all the numbers from 0 through 4:

var i = 0
while i < 5 {
	print(i)
	i += 1
}

The output of this program is:

And this is what the computer does behind the scenes during this loop:

0. Creates and initializes a variable i = 0

1. First iteration:
	a. Is i < 5 true? <=> Is 0 < 5 true? Yes.
	b. print(i) => Output: 0
	c. i += 1 => i = 1

1. Second iteration:
	a. Is i < 5 true? <=> Is 1 < 5 true? Yes.
	b. print(i) => Output: 1
	c. i += 1 => i = 2

2. Third iteration:
	a. Is i < 5 true? <=> Is 2 < 5 true? Yes.
	b. print(i) => Output: 2
	c. i += 1 => i = 3

3. Fourth iteration:
	a. Is i < 5 true? <=> Is 3 < 5 true? Yes.
	b. print(i) => Output: 3
	c. i += 1 => i = 4

4. Fifth iteration:
	a. Is i < 5 true? <=> Is 4 < 5 true? Yes.
	b. print(i) => Output: 4
	c. i += 1 => i = 5

5. Sixth iteration:
	a. Is i < 5 true? <=> Is 5 < 5 true? No.
	b. Exit the loop.

Assignment
Let's print all numbers from 3 to 10 using a while loop.

Hint
Look at the examples above if you get stuck.

STEP 1 OF 4

Guided

Ctrl + Enter to check

Printing Numbers in Swift, explained

Counting with a while loop

A while loop can print a run of numbers. The loop variable holds the current number, the body prints it, and an update moves to the next one. Here are the numbers 0 through 4:

var i = 0
while i < 5 {
    print(i)
    i += 1
}

What the computer does

The variable i starts at 0. Before each pass the condition i < 5 is checked.

With i equal to 0, 1, 2, 3 and 4 the condition is true. The body prints the value and adds 1.
After the fifth pass i is 5. The condition 5 < 5 is false, so the loop stops without printing 5.

Two choices shape the output: the starting value and the condition. With < the last number is excluded. With <= it is included. Choosing between them is the main thing to get right.

The exercise

The assignment is to print all the numbers from 3 to 10, both included.

Start at the first number. Declare var counter = 3. It must be a var, because the loop changes it.
Choose the condition. Ten must be printed, so use counter <= 10. With < the output would stop at 9. Swift takes no parentheses around the condition, but the braces are required.
Print the counter in the body with print(counter). Each call ends with a newline, so every number is on its own line.
Update the counter with counter += 1. Without it, counter stays 3, the condition is always true and the loop never ends.

var counter = 3
while counter <= 10 {
    print(counter)
    counter += 1
}

The output is the eight numbers from 3 to 10, one per line. After the loop, counter is 11, the first value that failed the condition.

Reading the loop as a table

When a loop misbehaves, write down the variable's value at the start of each pass. For the loop above, counter is 3 on the first pass and 10 on the last, so there are eight passes, one per number from 3 to 10. After the update on the eighth pass it is 11, and the condition 11 <= 10 is false. The number of passes is the last value minus the first plus one, which is why a range that includes both ends has one more element than their difference.

Swift details

No ++. Swift removed the increment operator. Use counter += 1.
The type is inferred. var counter = 3 makes counter an Int, so you don't write the type.
A different tool. For a fixed range, for counter in 3...10 does the same without a manual update. The while form is useful when you need to control the update yourself.

Common mistakes

Off by one at the end. counter < 10 misses the 10. Count the numbers you expect: 3 to 10 is eight lines.
Wrong start. Starting at 0 or 1 prints extra numbers.
Update before print. If counter += 1 comes first, the first line is 4 and the last is 11.
Using let. A constant can't be increased.

Key takeaways

The initial value sets the first number printed.
The condition sets the last. Use <= to include it.
The update must move the variable toward ending the loop.

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