Table of Contents

Overview

There are two cases of for loop being outside a select statement

Finite for loop outside select statement
Infinite for loop outside select statement

Let’s look at each one by one

Finite for loop

In case of finite for loop, the select statement will execute equal to the number of iterations in the loop. Let’s see a program for that

package main

import "fmt"

func main() {
    ch1 := make(chan string)
    ch2 := make(chan string)
    go goOne(ch1)
    go goTwo(ch2)
    for i := 0; i < 2; i++ {
        select {
        case msg1 := <-ch1:
            fmt.Println(msg1)
        case msg2 := <-ch2:
            fmt.Println(msg2)
        }
    }
}

func goOne(ch chan string) {
    ch <- "From goOne goroutine"
}

func goTwo(ch chan string) {
    ch <- "From goTwo goroutine"
}

Output

From goOne goroutine
From goTwo goroutine

In the above program we put a for loop of length two across the select statement. Hence the select statement is executed twice and prints the received value from each of the case statement.

Infinite for loop

We can have infinite for loop outside a select statement. This will cause the select statement to execute indefinite number of times.So when using for statement with infinite loop outside the select statement, we need to have a way yo break out of the for loop. One of the use case of having infinite for loop outside select statement could be that you are waiting for multiple operations to receive on the same channel for a certain time. See below example

package main

import (
	"fmt"
	"time"
)

func main() {
	news := make(chan string)
	go newsFeed(news)

	printAllNews(news)
}

func printAllNews(news chan string) {
	for {
		select {
		case n := <-news:
			fmt.Println(n)
		case <-time.After(time.Second * 1):
			fmt.Println("Timeout: News feed finished")
			return
		}
	}
}

func newsFeed(ch chan string) {
	for i := 0; i < 2; i++ {
		time.Sleep(time.Millisecond * 400)
		ch <- fmt.Sprintf("News: %d", i+1)
	}
}

Output

News: 1
News: 2
Timeout: News feed finished

In the above program, we have created a channel named news which will hold data of string type. Then we pass this channel to the newsfeed function which is pushing the news feed to this channel . In the select statement, we are receiving the news feed from the news channel. This select statement is inside an infinite for loop so the select statement will be executed multiple times until we exit out of for loop . We also have time.After with a duration for 1 second as one of the case statements. So this set up will receive all the news from the news channel for 1 second and then exit.

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