Fix reusing the url variable in concurrency tutorial

concurrency.md

@@ -363,8 +363,9 @@ func CheckWebsites(wc WebsiteChecker, urls []string) map[string]bool {
 	resultChannel := make(chan result)
 
 	for _, url := range urls {
+		u := url
 		go func() {
-			resultChannel <- result{url, wc(url)}
+			resultChannel <- result{u, wc(u)}
 		}()
 	}
 
@@ -385,7 +386,11 @@ The new type, `result` has been made to associate the return value of the
 within the struct; this can be useful in when it's hard to know what to name
 a value.
 
-Now when we iterate over the urls, instead of writing to the `map` directly
+Now when we iterate over the urls, at first, we create a copy of the `url`
+variable as `u` and pass it to the goroutine. Without this step, all gorouties
+will reuse the same `url` variable, leading to unreliable results.
+
+Next, instead of writing to the `map` directly
 we're sending a `result` struct for each call to `wc` to the `resultChannel`
 with a _send statement_. This uses the `<-` operator, taking a channel on the
 left and a value on the right:

concurrency/v3/check_websites.go

@@ -14,8 +14,9 @@ func CheckWebsites(wc WebsiteChecker, urls []string) map[string]bool {
 	resultChannel := make(chan result)
 
 	for _, url := range urls {
+		u := url
 		go func() {
-			resultChannel <- result{url, wc(url)}
+			resultChannel <- result{u, wc(u)}
 		}()
 	}