Sectioning of Data

Yeah, there's a lot going on in those few lines:

static var sectionedContacts: [[Contact]] {
     return sortedUniqueFirstLetters.map { firstLetter in
          let filteredContacts = contacts.filter { $0.firstLetterForSort == firstLetter }
          return filteredContacts.sorted(by:  { $0.firstName < $0.firstName} }
     }
}

Take the example:

contacts = ["Dan", "Bob", "April", "Cathy", "Abe"]

... where I'm representing each Contact object by a string which is the Contact's first name for simplicity ... in actuality, it would be Contact(firstName = "Dan", lastName = "Smith", etc ...) for each element.

Then, sortedUniqueFirstLetters is an array of sorted first letters of contacts. So, it would be ["A", "B", "C"].

Now, we are going to apply the .map transformation on sortedUniqueFirstLetters, that is, for every element in the array, we are going to apply the function in the closure.

For the first element in sortedUniqueFirstLetters, which is "A" (assigned to the local variable "firstLetter"), we apply the filter transformation on the contacts array, where we filter on the condition defined in the closure that the first letter of each Contact equals whatever firstLetter is (in this case, "A"). For "A", the elements which satisfy this condition are "April" and "Abe", so the local variable filteredContacts is now ["April", "Abe"]. Finally, we sort this array based on first name in ascending order (as defined in the closure passed into .sorted), so the resulting array which we return is ["Abe", "April"].

So, so far, our return array has one element, and it is [ ["Abe", "April"]].

Likewise, we traverse through all the elements in sortedUniqueFirstLetters, "B" creating the resulting array ["Bob"], "C", ["Cathy"], "D", ["Dan"] ... and so the final array of arrays which is returned as the variable sectionedContacts is:

[["Abe", "April"], ["Bob"], ["Cathy"], ["Dan"]]