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Iteration over integers

July 3, 2024

I’m back from my holiday! I thought I’d send at least a few dalies over the last week while I was on vacation, but it wasn’t meant to be. But now that I’m back, we’ll pick up where we left off…

For statements with range clause

…

  1. For an integer value n, the iteration values 0 through n-1 are produced in increasing order. If n <= 0, the loop does not run any iterations.

This form of range was added recently, in Go 1.22, and it serves as a simple short cut. These two code snippets are functionally equivalent:

for x := 0; x < 10; x++ {

for x := range 10 {

It might be a bit counter-intuitive that you can use a negative integer value as the argument to range, without a runtime panic: “If n <= 0, the loop does not run any iterations.”

But I think this makes sense, if we consider it to be equivalent to the earlier form:

for x := 0; x < -3; x++ {

would also iterate 0 times, and would not panic.

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

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Range conclusion

Today we finish up with some final notes, and a big example, from the spec section on for statements. For statements with range clause … If the iteration variables are not explicitly declared by the “range” clause, they must be preexisting. In this case, the iteration values are assigned to the respective variables as in an assignment statement. I actually discussed this case yesterday, so won’t go into it again.

Scope and type of range variables

For statements with range clause … The iteration variables may be declared by the “range” clause using a form of short variable declaration (:=). In this case their scope is the block of the “for” statement and each iteration has its own new variables [Go 1.22] (see also “for” statements with a ForClause). We’ve already talked at some length about the scope of variables in other forms of for loops.

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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