Using Ranges

Ranges have two everyday jobs: driving a counted loop and carving a sub-slice out of a sequence.

Iteration

A for loop over a range walks its values in order, binding each to the loop variable. Use .. for a half-open count and ..= when the last value should be included:

for i in 0..5 {
    Print(i);    // 0, 1, 2, 3, 4
}

for i in 1..=5 {
    Print(i);    // 1, 2, 3, 4, 5
}

Only ranges that have a start can be iterated — a..b, a..=b, and a... A range with no start (..b, ..=b, ..) has no first value, so iterating one is a compile-time error; those forms exist for slicing. A a.. range with no end is unbounded, so a loop over it must break out itself.

The loop variable takes the range's bound type — for i in 0..n gives i of type int.

Slicing

Indexing an array or slice with a range — rather than a single position — produces a slice over that sub-interval. Here every range form is allowed: an omitted start defaults to 0, and an omitted end defaults to the sequence's length.

let data = [10, 20, 30, 40, 50];   // int[5]

let a = data[1..4];    // 20, 30, 40   — explicit bounds
let b = data[2..];     // 30, 40, 50   — from index 2 to the end
let c = data[..3];     // 10, 20, 30   — start through index 2
let d = data[..];      // 10, 20, 30, 40, 50 — the whole thing

The result is a Slice<T> view into the original storage — no elements are copied, and writing through it writes back to the source (when that source is a var). See Arrays as Slices for more.

See Also

• Ranges — the range forms and their bounds
• for / in — the loop that iterates a range
• Arrays as Slices — sub-slicing in the context of arrays
• break / continue — leaving an unbounded loop