Today we’ll look at the second, of three, types of for statements: Those with a for clause.

For statements with for clause

A “for” statement with a ForClause is also controlled by its condition, but additionally it may specify an init and a post statement, such as an assignment, an increment or decrement statement. The init statement may be a short variable declaration, but the post statement must not.

ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] .
InitStmt = SimpleStmt .
PostStmt = SimpleStmt .

for i := 0; i < 10; i++ {
	f(i)
}

So a for statement with for clause differs from the simpler type we saw yesterday, the for statement with a single condition, in that it contains two additional statements. This works a lot like the simple statements we’ve already seen in if, switch, and select statements, with the addition of a post statement, not just an init statement.

If non-empty, the init statement is executed once before evaluating the condition for the first iteration; the post statement is executed after each execution of the block (and only if the block was executed).

Let’s look at the execution order here, in detail. Given a simple for loop like this:

for i := 0; i < 3; i++ {
	fmt.Println(i)
}

based on the above, the execution would look something like this, if broken out of a loop:

i := 0           // initial statement
if i < 3 {       // i == 0, 0 < 3 so execute the block
	fmt.Println(i) // Prints 0
	i++            // Post statement only executes if the block executes
}
if i < 3 {       // i == 1, 1 < 3 so execute the block
	fmt.Println(i) // Prints 1
	i++            // Post statement only executes if the block executes
}
if i < 3 {       // i == 2, 2 < 3 so execute the block
	fmt.Println(i) // Prints 2
	i++            // Post statement only executes if the block executes
}
if i < 3 {       // i == 3, 3 is not less than 3, so the block does not execute, and the loop ends
	/* not executed */
}

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