4 basic range loop (for-each) patterns

Basic for-each loop (slice or array)

a := []string{"Foo", "Bar"}
for i, s := range a {
    fmt.Println(i, s)
}

0 Foo
1 Bar

• The range expression, a, is evaluated once before beginning the loop.
• The iteration values are assigned to the respective iteration variables, i and s, as in an assignment statement.
• The second iteration variable is optional.
• For a nil slice, the number of iterations is 0.

String iteration: runes or bytes

For strings, the range loop iterates over Unicode code points.

for i, ch := range "日本語" {
    fmt.Printf("%#U starts at byte position %d\n", ch, i)
}

U+65E5 '日' starts at byte position 0
U+672C '本' starts at byte position 3
U+8A9E '語' starts at byte position 6

• The index is the first byte of a UTF-8-encoded code point; the second value, of type rune, is the value of the code point.
• For an invalid UTF-8 sequence, the second value will be 0xFFFD, and the iteration will advance a single byte.

To loop over individual bytes, simply use a normal for loop and string indexing:

const s = "日本語"
for i := 0; i < len(s); i++ {
    fmt.Printf("%x ", s[i])
}

e6 97 a5 e6 9c ac e8 aa 9e

Map iteration: keys and values

The iteration order over maps is not specified and is not guaranteed to be the same from one iteration to the next.

m := map[string]int{
    "one":   1,
    "two":   2,
    "three": 3,
}
for k, v := range m {
    fmt.Println(k, v)
}

two 2
three 3
one 1

• If a map entry that has not yet been reached is removed during iteration, this value will not be produced.
• If a map entry is created during iteration, that entry may or may not be produced.
• For a nil map, the number of iterations is 0.

Channel iteration

For channels, the iteration values are the successive values sent on the channel until closed.

ch := make(chan int)
go func() {
    ch <- 1
    ch <- 2
    ch <- 3
    close(ch)
}()
for n := range ch {
    fmt.Println(n)
}

1
2
3

• For a nil channel, the range loop blocks forever.

Gotchas

Here are two traps that you want to avoid when using range loops:

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