How to make alphabetically section headers in table view with a mutable data source

I would change the way you store your contacts to a dictonary with the initial letters as keys and put the names that correspond to that initial letter into a subarray:

contacts = ["A": ["Anton", "Anna"], "C": ["Caesar"]]

I simplified the way of the contacts here (in form of strings), but you get the concept.

I would also save the section number of the letter in a seperate array like this:

letters = ["A", "C"]

Keep the array sorted and organized, so check after each insertion/deletion/update. This is not part of the table view implementation. I would make the Viewcontroller a delegate of the phonebook, so you can fire an update-like method from the phonebook to update the table.

How to get the data for the data source:

the number of sections:

letters.count

the section title for section at index i is

letters[i]

the number of cells in a section i is

contacts[letters[i]].count

and the content for a specific cell c in section i is:

contacts[letters[i]][c]

Feel free to ask further questions if anything is still not clear.

UPDATE - How to generate the arrays:

I don't require the data to be sorted, if you pass it already sorted, you can delete the sorting lines below ...

let data = ["Anton", "Anna", "John", "Caesar"] // Example data, use your phonebook data here.// Build letters array:var letters: [Character]letters = data.map { (name) -> Character in    return name[name.startIndex]}letters = letters.sort()letters = letters.reduce([], combine: { (list, name) -> [Character] in    if !list.contains(name) {        return list + [name]    }    return list})// Build contacts array:var contacts = [Character: [String]]()for entry in data {    if contacts[entry[entry.startIndex]] == nil {        contacts[entry[entry.startIndex]] = [String]()    }    contacts[entry[entry.startIndex]]!.append(entry)}for (letter, list) in contacts {    list.sort()}

For Swift 3:

let data = ["Anton", "Anna", "John", "Caesar"] // Example data, use your phonebook data here.// Build letters array:var letters: [Character]letters = data.map { (name) -> Character in    return name[name.startIndex]}letters = letters.sorted()letters = letters.reduce([], { (list, name) -> [Character] in    if !list.contains(name) {        return list + [name]    }    return list})// Build contacts array:var contacts = [Character: [String]]()for entry in data {    if contacts[entry[entry.startIndex]] == nil {        contacts[entry[entry.startIndex]] = [String]()    }    contacts[entry[entry.startIndex]]!.append(entry)}for (letter, list) in contacts {    contacts[letter] = list.sorted()}

I ran the code in playground and got the following outputs for

letters:

["A", "C", "J"]

contacts:

["J": ["John"], "C": ["Caesar"], "A": ["Anton", "Anna"]]

For Swift 3. Thank you @Stefan! Here is my version with Set

var tableViewSource: [Character : [String]]!var tableViewHeaders: [Character]!let data = ["Anton", "Anna", "John", "Caesar"]func createTableData(wordList: [String]) -> (firstSymbols: [Character], source: [Character : [String]]) {    // Build Character Set    var firstSymbols = Set<Character>()    func getFirstSymbol(word: String) -> Character {        return word[word.startIndex]    }    wordList.forEach {_ = firstSymbols.insert(getFirstSymbol(word: $0)) }    // Build tableSourse array    var tableViewSourse = [Character : [String]]()    for symbol in firstSymbols {        var words = [String]()        for word in wordList {            if symbol == getFirstSymbol(word: word) {                words.append(word)            }        }        tableViewSourse[symbol] = words.sorted(by: {$0 < $1})    }    let sortedSymbols = firstSymbols.sorted(by: {$0 < $1})    return (sortedSymbols, tableViewSourse)}func getTableData(words: [String]) {    tableViewSource = createTableData(wordList: words).source    tableViewHeaders = createTableData(wordList: words).firstSymbols}getTableData(words: data)print(tableViewSource)  // ["J": ["John"], "C": ["Caesar"], "A": ["Anna", "Anton"]]print(tableViewHeaders) // ["A", "C", "J"]

I did it within one loop, not few (Swift 4):

struct ContactData {    let longName: String    let phones: [String]    let thumbnailImageData: Data?}var contacts = [ContactData]()var tableViewSource = [Character : [ContactData]]()var headerTitles = [Character]()func createContactsData(completionHandler: @escaping () -> Swift.Void) {    contacts = extractContacts() // convert CNContact to custom ContactData    tableViewSource.removeAll()    var prevChar: Character?    var currentBatch: [ContactData]!    contacts.forEach { contact in        guard let firstChar = contact.longName.first else {            return        }        if prevChar != firstChar {            if prevChar != nil {                tableViewSource[prevChar!] = currentBatch            }            prevChar = firstChar            currentBatch = [ContactData]()        }        currentBatch.append(contact)    }    let allKeys = Array(tableViewSource.keys)    let sortedSymbols = allKeys.sorted(by: {$0 < $1})    headerTitles = sortedSymbols    completionHandler()}