Lets say we have a record which defines a person type with the properties name and age:

type person = {name:string ; age:int };;

and initialize a list with different types:

let personlist = [{name="alexander";age=21};{name="benjamin";age=30};{name="claudia";age=21}];;

How can I count the amount of types with a specific age, lets say 21 (alexander and claudia) so that the output in this case would be two?

CodePudding user response：

The most straightforward way would be to combine List.length and List.filter.

List.(personlist |> filter (fun p -> p.age = 21) |> length)

However, this is somewhat less efficient than it could be. We can do it in one pass because we don't really need to build a list of the matching people, and then iterate over that list to find its length.

List.fold_left will let us iterate over personlist, updating the initial value of 0 on each iteration dependent on whether the age field of each record is 21 or not.

List.fold_left (fun i {age; _} -> i   if age = 21 then 1 else 0) 0 personlist