Blog / Go Language Spec

Iteration over arrays and slices

June 17, 2024

For statements with range clause

…

  1. For an array, pointer to array, or slice value a, the index iteration values are produced in increasing order, starting at element index 0. If at most one iteration variable is present, the range loop produces iteration values from 0 up to len(a)-1 and does not index into the array or slice itself. For a nil slice, the number of iterations is 0.

From this above paragraph, we can be assured that ranging over an array, pointer to array, or slice, will operate in a defined order–from index 0, upward. While this is probably what anyone would expect, it’s good to know this is part of the spec, and that someone can’t one day surprise us by writing a Go compiler that arbitrarily operates in reverse order, or random order.

What’s probably more interesting from this paragraph, though, is that when there’s at most a single iteration variable, that is, a loop in the form of for x := range a (as opposed to for x, y := range a, with two variables), we never actually index into the array/slice. This is because all that’s needed to assign to the loop variables is a generated iteration from 0 to len(a)-1, which can be done without indexing into a. Functionally, of course, it doesn’t matter that we don’t index into a, it’s purely an optimization in performance.

Quotes from The Go Programming Language Specification Language version go1.22 (Feb 6, 2024)

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The types of iteration variables

For statements with range clause … For each iteration, iteration values are produced as follows if the respective iteration variables are present: Range expression 1st value 2nd value array or slice a [n]E, *[n]E, or []E index i int a[i] E string s string type index i int see below rune map m map[K]V key k K m[k] V channel c chan E, <-chan E element e E integer n integer type value i see below How do we read this table?

Evaluation of range expressions

For statements with range clause … The range expression x is evaluated once before beginning the loop, with one exception: if at most one iteration variable is present and len(x) is constant, the range expression is not evaluated. The first part of this seems pretty self-evident, as it follows the same pattern as a for statement with a for clause: The expression needs to be evaluated once before the loop executes.

Rangeable values

For statements with range clause A “for” statement with a “range” clause iterates through all entries of an array, slice, string or map, values received on a channel, or integer values from zero to an upper limit [Go 1.22]. Wow, that’s a long list. And it’s about to get longer! Go 1.23 is scheduled to add iterator functions to that list, but we’ll talk about that when the time comes (in August).

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