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Range Clauses

Spec: https://go.dev/ref/spec#For_statements

Summary

A range clause provides a way to iterate over an array, slice, string, map, or channel.

Example

for k, v := range myMap {
	log.Printf("key=%v, value=%v", k, v)
}

for v := range myChannel {
	log.Printf("value=%v", v)
}

for i, v := range myArray {
	log.Printf("array value at [%d]=%v", i, v)
}

Reference

If only one value is used on the left of a range expression, it is the 1st value in this table.

Range expression 1st value 2nd value (optional) notes
array or slice a [n]E, *[n]E, or []E index i int a[i] E
string s string type index i int rune int range iterates over Unicode code points, not bytes
map m map[K]V key k K value m[k] V
channel c chan E element e E none

Gotchas

When iterating over a slice or map of values, one might try this:

items := make([]map[int]int, 10)
for _, item := range items {
	item = make(map[int]int, 1) // Oops! item is only a copy of the slice element.
	item[1] = 2                 // This 'item' will be lost on the next iteration.
}

The make and assignment look like they might work, but the value property of range (stored here as item) is a copy of the value from items, not a pointer to the value in items. The following will work:

items := make([]map[int]int, 10)
for i := range items {
	items[i] = make(map[int]int, 1)
	items[i][1] = 2
}

Last update: March 1, 2022