Lesson 4 — Repeating things

You are here

1. Your first program ✓
2. Values and variables ✓
3. Making decisions ✓
4. Repeating things ←
5. Functions
6. Lists and hashes
7. Strings
8. Organizing programs
9. Classes

What are we learning?

A loop repeats instructions while a condition stays true.

Smallest useful loop: while

i: int = 1;

while i <= 3 {
    say(i);
    i = i + 1;
}

Output

1
2
3

What happens

1. Start with i as 1.
2. Ask: is i less than or equal to 3? Yes → print i, then add 1 to i.
3. Repeat until the answer is no.

Do not forget to change the condition

If you never change i, the loop can run forever. Always move toward finishing the condition.

Counting with for

When you want a range of whole numbers, for is handy:

for n: int in 1..3 {
    say(n);
}

Output

1
2
3

1..3 means from 1 through 3 (both ends included).

Try it

Change the loop so it prints 1 through 5.

Predict it

i: int = 0;

while i < 2 {
    say("hi");
    i = i + 1;
}

Show answer

hi
hi

Fix it

This never ends (and will hang until stopped):

while true {
    say("loop");
}

Use a condition that becomes false, like i <= 3, and update i inside the loop.

Deeper (later)

break, continue, foreach, and ... (exclusive ranges) are in Control Flow (full) and Loops Reference.

Next

→ Lesson 5 — Functions