Difference between for loop construct vs range keyword in go

Question

Consider the following code which just prints all the ENV vars

package main

import (
    "fmt"
    "os"
)

func main() {
    for i, env := range os.Environ() {
        fmt.Println(i, env)
    }
}

Here, os.Environ() is supposed to return array of strings([] string), to loop over it. I need to to use range keyword & also for loop. Question is:

Why are both for & range required? is it possible to use for loop for this as []string is already an array & we can iterate over arrays right?
In the above code what does range do? and what does for loop does?

Sorry if this question is too stupid, I am just starting with Go

Replace every instance of "array" with "slice" in your question. The two are very different things in Go. — Dave C
– Dave C, Commented Jun 20, 2015 at 16:45

VonC · Accepted Answer · 2015-06-20 15:41:02Z

8

As mentioned in Range Clauses:

A range clause provides a way to iterate over a array, slice, string, map, or channel.

If you want to iterate over an []string, you need range.

A For statement doesn't always use range.

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

You have:

In its simplest form, a "for" statement specifies the repeated execution of a block as long as a boolean condition evaluates to true

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

A "for" statement with a "range" clause iterates through all entries of an array, slice, string or map, or values received on a channel. For each entry it assigns iteration values to corresponding iteration variables if present and then executes the block.

answered Jun 20, 2015 at 15:41

VonC

1.4m569 gold badges4.8k silver badges5.7k bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

CuriousMind Over a year ago

It makes sense now, Thank you!

Sam Over a year ago

The quotation from golang.org/ref/spec#For_statements was exactly what I was looking for. Thank you!

hookenz · Accepted Answer · 2024-11-21 20:59:50Z

I find the accepted answer slightly confusing as it doesn't directly directly address the questions.

So to answer your questions.

for range is one loop style you can use to access the elements in the environment slice returned from os.Environ(), but as I show below you can use the other for loop style. You will see the differences below.
range in this for loop is the construct you use that will provide a copy and the index for each of the elements in the slice.

To summarize, there are basically two forms of the for loop construct in go (actually 3, but we're only concerned with two here).

One uses the range keyword, and the other does not.

The for ... range version creates a copy of the item in the slice you are iterating over. The other, does not and you must access it by reference.

func main() {
    for i, env := range os.Environ() {
        fmt.Println(i, env)
    }
}

Here is how you would write the for loop without using range construct:

func main() {
    env := os.Environ()
    for i := 0; i < len(env); i++ {
        fmt.Println(i, env[i])
    }
}

I also found a third option which looks tidy and has the same advantage as option 2.

func main() {
    env := os.Environ()
    for i := range env {
        fmt.Println(i, env[i])
    }
}

The first approach is nice and clean, but the second provides better performance because it doesn't do a copy. Both have their place. I would generally use range, but where you need it, the traditional C-like loop is available.

Note: Whenever you are dealing with strings you should always use the range loop as it will return a rune instead of a char. This is particularly important when dealing with internationalisation. See this blog post about string iteration here: Strings, bytes, runes and characters in Go

Nik · Accepted Answer · 2022-10-31 14:05:40Z

1

One notable difference if for range creates copy of each the objects from slice/array. Have a look at following code snippet. https://go.dev/play/p/MHaCXj-Y5WD

answered Oct 31, 2022 at 14:05

Nik

391 bronze badge

Collectives™ on Stack Overflow

Difference between for loop construct vs range keyword in go

3 Answers 3

2 Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related