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Arrays

Futhark arrays are written as values enclosed in square brackets.

let arr = [1,2,3]

The type of arr is []i32 (or [3]i32 to be pedantic). The element type of an array can be another array:

let marr = [[1,2,3], [4,5,6]]

The type of marr is [][]i32. Arrays must be regular, meaning that all elements must have the same shape. An array such as [[1,2], [3] is illegal.

Arrays can be indexed from zero:

let first = arr[0]

And sliced:

let tail = arr[1:]

We can even slice or index only some dimensions:

let innermost = marr[:,1:2]

This produces [[2], [5]]. Slices are inclusive in the starting position and exclusive in the ending position as Dijkstra wills it.

Strides are also supported. Generally, The expression a[i:j:s] returns a slice of the array a from index i (inclusive) to j (exclusive) with a stride of s. If the stride is positive, then i <= j must hold, and if the stride is negative, then j <= i must hold. For example, we can reverse an array like this:

let arr_reversed = arr[::-1]

Some syntactic sugar is provided for concisely specifying arrays of intervals of integers. The expression x...y produces an array of the integers from x to y, both inclusive. The upper bound can be made exclusive by writing x..<y. For example:

let arr_range = 1...3

It is usually necessary to enclose a range expression in parentheses, because they bind very loosely. A stride can be provided by writing x..y...z, with the interpretation “first x, then y, up to z”. For example:

let strided = 1..3...7

Now strided contains [1, 3, 5, 7].

See also

Reference manual: slicing.