Go for

External

• Language Specification for Statements https://golang.org/ref/spec#For_statements

Internal

• Go Language

Overview

There are four for syntactical forms: the generic, C-like syntax with an init section, a condition and post section, a form where the iteration is controlled by a range keyword, a form with just one condition, and a form that iterates forever, unless the body contains statements that exit the loop. There are no do or while loops in Go. In all four causes, the for statement specifies repeated execution of a block.

for init; condition; post {}

The most generic syntax of the for statement is a C-like syntax, with an init, condition and post statements:

for <init>; <condition>; <post> {
  <statements>
}

The for loop executes the init statement and then iterates while the condition evaluates to true. The condition is an expression evaluated on each iteration. condition must evaluate to a boolean. If the expression evaluates to true, the block is executed. At the end of each block execution, the post statement is executed.

for i := 0; i < 3; i ++ {
  println(i)
}

The init statement is usually a variable declaration and initialization. Multiple variables can be declared at the same time (see Short Variable Declaration):

for i, j := 0, 0; ... {
  ...
}

All of the init, condition and post sections are optional and can be dropped. If we drop the init and the post section, we get the for <condition> form. If we drop all of them, we get the for {} form.

for range {}

To iterate over an iterable structure, such as an array, slice, string or map, or values received on a channel, use range. For each entry, range assigns iteration values to corresponding iteration variables if present and then executes the block. For arrays, maps and slices, for exits when all values are returned. For channels, for exists when the channel is closed.

// map
ss := []string{"a", "b", "c"}
for key, value := range ss {
  ...
}

If you only need the first item in a range (the key or the index), the second can be dropped:

// map
ss := ...
for key := range ss {
  ...
}

If you only need the second item in the range (the value), use a blank identifier to discard the first:

// slice
ss := ...
for _, value := range ss {
  ...
}

If the values of the iterable structure are not important, and only the number of elements matters, this syntax can also be used:

ss := []string{"a", "b", "c"}
for range ss {
  ...
}

To get consecutive integers in the loop, use this syntax (Go 1.22 and newer):

for i := range 10 {
  println(i)
}

for can be used to iterate over values received from a channel.

For strings, range breaks out individual Unicode code points by parsing UTF-8. Erroneous encodings consume one byte and produce the replacement rune U+FFFD.

for condition {}

for <condition> {
  <statements>
}

The condition is evaluated before each iteration. If the condition is absent, is equivalent with true, and the form syntax becomes for {}. for condition {} is equivalent with while in other languages.

i := 0
for i < 3 {
  println(i)
  i ++
}

for {}

i := 0
for  {
  if i == 3 {
    break
  }
  println(i)
  i ++
}

Iteration Variable Scope

TODO.

Use Cases

Iterating over a slice from a non-zero index

for index := 5; index < len(slice); index ++ {
    // slice[index] ...
}

Reverse a Slice

for i, j := 0, len(a) - 1; i < j; i, j = i+1, j-1 {
    a[i], a[j] = a[j], a[i]
}