Indexing and Iteration

The elements of an array are reached the same way as any sequence — by position or in a loop.

Indexing

Use [i] to read or write a single element. The index is zero-based, so valid indices run from 0 to length - 1:

var row = [10, 20, 30];
let first = row[0];   // 10
row[2] = 99;          // row == [10, 20, 99]

Writing an element requires the array to be mutable — bound with var. Through a let binding the elements are read-only, matching the deep mutability rule for value types.

Iteration

for … in walks every element in order, binding each to the loop variable:

let nums = [1, 2, 3, 4, 5];
var sum = 0;
for n in nums {
    sum += n;
}
// sum == 15

The loop covers exactly length elements — and because an array's length is a compile-time constant, the compiler knows the trip count up front. See for for the full loop syntax.

Indexing with a Range

Indexing with a range instead of a single position produces a slice over part of the array rather than one element:

let data = [1, 2, 3, 4, 5];
let middle = data[1..4];   // Slice<int> viewing 2, 3, 4

This is covered in Arrays as Slices.

See Also

• Arrays — the array type and its value semantics
• Arrays as Slices — range sub-slicing and passing arrays to functions
• for — iterating over a sequence
• Mutability of Structs — why element writes need var