Swift Dictionary Grouping

     There is the comment in the Dictionary souce code.

    /// Creates a new dictionary whose keys are the groupings returned by the

    /// given closure and whose values are arrays of the elements that returned

    /// each key.

    ///

    /// The arrays in the "values" position of the new dictionary each contain at

    /// least one element, with the elements in the same order as the source

    /// sequence.

    ///

    /// The following example declares an array of names, and then creates a

    /// dictionary from that array by grouping the names by first letter:

let students = ["Kofi", "Abena", "Efua", "Kweku", "Akosua"]
let studentsByLetter = Dictionary(grouping: students, by: { $0.first! })