2 · Model and validate · 25 MIN

Structs, maps and missing values

A map lookup can distinguish a missing key from a stored zero.

Structs describe named fields, while maps index records by a comparable key. The two-value lookup returns both a value and a presence flag. Without that flag, a learner who has earned zero points can look identical to a learner who does not exist. This example deliberately includes a zero value. Reading a nil map works, but writing to it panics, so initialize maps before mutation. Map iteration order is unspecified; sort keys when you need deterministic output for reports or tests.

Use value, ok := records[id] and test ok before reading business fields.

Read the example

package main
import "fmt"
type Learner struct { Name string; Points int }
func main() {
  learners := map[string]Learner{"ada": {Name:"Ada", Points:0}}
  learner, found := learners["ada"]
  fmt.Println(learner.Name, learner.Points, found)
  _, missing := learners["lin"]
  fmt.Println(missing)
}

Ada 0 true
false

Your challenge

Write a lookup function that returns a learner and an error for an unknown ID.

Solution cost: Expected O(1) lookup; sorted reporting adds O(k log k). time · Account for collection storage separately from the returned result. space

Common trap

Do not infer existence from the zero value of a field.

package main
import "fmt"
type Learner struct { Name string; Points int }
func main() {
  learners := map[string]Learner{"ada": {Name:"Ada", Points:0}}
  learner, found := learners["ada"]
  fmt.Println(learner.Name, learner.Points, found)
  _, missing := learners["lin"]
  fmt.Println(missing)
}

Further reading: Official documentation