Golang in sixty seconds — tricks with maps

In a previous article I mentioned the strings.Count function. Here's an alternative for more complex use cases.

Lets say we have a slice or array of people:

type Person struct {
 name string
 age  int
}
people := []Person{{name: "Bob", age: 21}, {name: "Sam", age: 28}, {name: "Ann", age: 21}, {name: "Joe", age: 22}, {name: "Ben", age: 28}}

If we want to understand the frequency of ages of our people, we can make use of a map :

ageDistribution := map[int]int
for _, person := range people {
  ageDistribution[person.age]++
}
fmt.Println(ageDistribution) // map[21:2 22:1 28:2]

All we're doing here is looping over each person in our slice and incrementing the value that we have stored for that age, essentially counting them. You can try it here

Knowing the distribution of ages may not be all that useful, but you may want to be able to quickly get all the people of a certain age, in which case you could do this:

peopleByAge := make(map[int][]Person)
for _, person := range people {
  peopleByAge[person.age] = append(peopleByAge[person.age], person)
}
fmt.Println(peopleByAge[21]) // [{Bob 21} {Ann 21}]

You can try it here

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