[go/en] missing map keys (#4413)

go.html.markdown

@@ -150,6 +150,13 @@ can include line breaks.` // Same string type.
 	// hash or dictionary types of some other languages.
 	m := map[string]int{"three": 3, "four": 4}
 	m["one"] = 1
+	// Looking up a missing key returns the zero value,
+	// which is 0 in this case, since it's a map[string]int
+	m["key not present"] // 0
+	// Check if a key is present in the map like this:
+	if val, ok := m["one"]; ok {
+		// Do something
+	}
 
 	// Unused variables are an error in Go.
 	// The underscore lets you "use" a variable but discard its value.