Slices, Maps and Error Handling

Go's two everyday collection types, and a closer look at the error-handling pattern you've already seen in action.

Slices

A slice is a resizable, dynamically-sized view over an underlying array, Go's equivalent of a list:

scores := []int{90, 85, 77}
scores = append(scores, 100)    // append returns a new slice, always reassign it
fmt.Println(scores[0])           // 90
fmt.Println(len(scores))          // 4

subset := scores[1:3]              // elements at index 1 and 2

append might allocate a new underlying array behind the scenes if the old one ran out of room, that's exactly why you always write scores = append(scores, ...) instead of just calling it and discarding the result.

Maps

scoresByName := map[string]int{"Ada": 95, "Grace": 98}
scoresByName["Bob"] = 70

value, ok := scoresByName["Missing"]
fmt.Println(value, ok)     // 0 false, ok tells you whether the key existed

delete(scoresByName, "Bob")

The comma-ok idiom (value, ok := ...) is how Go distinguishes "the key exists and its value is the zero value" from "the key doesn't exist at all", both would otherwise look identical.

Error handling recap

file, err := os.Open("data.txt")
if err != nil {
    fmt.Println("Could not open file:", err)
    return
}
defer file.Close()   // runs when the surrounding function returns, guaranteed

defer schedules a call to run right before the enclosing function returns, no matter which return statement triggers it, exactly like a finally block, and the idiomatic way to guarantee cleanup (closing files, unlocking mutexes) happens.