Go: Anonymous Func Only Taking Last Value in For Range

I imagine this is a pretty easy fix. Just trying loop through the string list urls and print out each url as the loop iterates. But when I add in concurrency (with go and WaitGroup) all it prints out is the last url in the list.

Why is it only printing out the last url (when printed from the anon function) rather than each url individually as the loop iterates?

Go Playground: http://play.golang.org/p/z4IZLY7Mt_

Code:

package main

import (
	"fmt"
	"sync"
)

var urls = []string{
	"http://google.com",
	"http://facebook.com",
	"http://youtube.com",
	"http://yahoo.com",
	"http://baidu.com",
	"http://wikipedia.org",
	"http://twitter.com",
	"http://live.com",
	"http://amazon.com",
	"http://linkedin.com",
	"http://google.co.in",
}

func main() {
	var wg sync.WaitGroup
	for _, url := range urls {
		fmt.Println("correct: " + url)
		wg.Add(1)
		go func() {
			fmt.Println("wrong: " + url)
			wg.Done()
		}()
	}

	wg.Wait()
}

Here is the working code:

package main

import (
	"fmt"
	"sync"
)

var urls = []string{
	"http://google.com",
	"http://facebook.com",
	"http://youtube.com",
	"http://yahoo.com",
	"http://baidu.com",
	"http://wikipedia.org",
	"http://twitter.com",
	"http://live.com",
	"http://amazon.com",
	"http://linkedin.com",
	"http://google.co.in",
}

func main() {
	var wg sync.WaitGroup
	for _, url := range urls {
		fmt.Println("correct: " + url)
		wg.Add(1)
		go func(url string) {
			fmt.Println("wrong: " + url)
			wg.Done()
		}(url)
	}

	wg.Wait()
}

Also on go playground: http://play.golang.org/p/zFSDvfdIDS

Adding my answer because the accepted one does not explain WHY.

A closure in Go has access to all the variables of the scope where it's created. So every closure being created in the example below has access to the loop variable url.

func main() {
    var wg sync.WaitGroup
    for _, url := range urls {
        fmt.Println("correct: " + url)
        wg.Add(1)
        go func() {
            fmt.Println("wrong: " + url)
            wg.Done()
        }()
    }

    wg.Wait()
}

Therefore, when every closure starts running concunrrently, value of url can be any element of urls slice(in most cases the last element, but not every time), because it's not blocking the main go routine. To make use of proper values for each closures, we need to copy the proper value to use. For example, you can do this:

func main() {
    var wg sync.WaitGroup
    for _, url := range urls {
        fmt.Println("correct: " + url)
        url2 = url
        wg.Add(1)
        go func() {
            fmt.Println("wrong: " + url2)
            wg.Done()
        }()
    }

    wg.Wait()
}

Or can do what's already in the previous answer, which is more explicit and readable:

func main() {
    var wg sync.WaitGroup
    for _, url := range urls {
        fmt.Println("correct: " + url)
        wg.Add(1)
        go func(url string) {
            fmt.Println("wrong: " + url)
            wg.Done()
        }(url)
    }

    wg.Wait()
}