Blog / Go Language Spec

For statements

May 29, 2024

Go is known as a simple language, with straightforward concepts. For loops may be one of rare exceptions to that straightforward character, with three different forms:

For statements

A “for” statement specifies repeated execution of a block. There are three forms: The iteration may be controlled by a single condition, a “for” clause, or a “range” clause.

ForStmt = "for" [ Condition | ForClause | RangeClause ] Block .
Condition = Expression .

Well, let’s dive on in, then, with the first of the three forms:

For statements with single condition

In its simplest form, a “for” statement specifies the repeated execution of a block as long as a boolean condition evaluates to true. The condition is evaluated before each iteration. If the condition is absent, it is equivalent to the boolean value true.

for a < b {
	a *= 2
}

A for loop of this form can operate on any boolean expression, whether it’s a single boolean variable, an expression that resolves to a boolean, or a function call:

for active { // active is of type bool

for a <= 3 {

for rows.Next() {

The text also mentions that the condition may be omitted, in which case it’s treated as true. This means the following two coe samples are functionally identical:

for true {

for {

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

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Iteration over strings

For statements with range clause … For a string value, the “range” clause iterates over the Unicode code points in the string starting at byte index 0. On successive iterations, the index value will be the index of the first byte of successive UTF-8-encoded code points in the string, and the second value, of type rune, will be the value of the corresponding code point. If the iteration encounters an invalid UTF-8 sequence, the second value will be 0xFFFD, the Unicode replacement character, and the next iteration will advance a single byte in the string.

Iteration over arrays and slices

For statements with range clause … 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.

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?

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