for-in

Learn to iterate over iterables with for-in.

for-in

To walk through each element of a collection in order you use for-in: for (var x in collection) { ... }. On each iteration x takes the next element; no counter or index is needed.

In this lesson we will use a List as the example collection: the form [1, 2, 3] is a List. You will see more about Lists in the next section; for now know them as comma-separated values inside square brackets.

Iterable

for-in works with any collection of type Iterable. List is the most common iterable. for-in manages the indices itself; so an i counter variable and a length check are not needed.

for-in is especially suited to processing elements in order. If you need to skip by index or walk backwards, a classic for loop may fit better. Practical tip: pick for-in when you walk all elements from start to end.

Exercise Print List elements

Declare a List with var numbers = [1, 2, 3];. Walk each element with for (var x in numbers) and print it with print(x);. The output must be 3 lines: 1, 2, 3 (one number per line).

Show example solution

var numbers = [1, 2, 3];
for (var x in numbers) {
  print(x);
}

Test yourself

What does for (var x in [1, 2, 3]) walk over?
```
for (var x in [1, 2, 3]) {
  print(x);
}
```
- Indices (0, 1, 2)
- Length (3)
- First element (1)
- Elements (1, 2, 3)
Show answer

Elements (1, 2, 3) — for-in walks the elements themselves, not the indices.
Match each part to its job

for (var x in ...)

Walk start
var x

Current element
[1, 2, 3]

List example
print(x)

Prints the element

Write the for-in header

var numbers = [1, 2, 3];
____ {
  print(x);
}

Show answer

var numbers = [1, 2, 3];
for (var x in numbers) {
  print(x);
}

Next lesson: break and continue

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