ios - Alphabetical sections in table table view in swift

Question

I have a list of names sorted alphabetically, and now I want display these names in a table view. I'm struggling with grouping these names for each letter.

My code looks like this:

let sections:Array<AnyObject> = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]
var usernames = [String]()

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell{
    
    let cellID = "cell"
    
    let cell: UITableViewCell = self.tv.dequeueReusableCellWithIdentifier(cellID) as UITableViewCell
    
    cell.textLabel?.text = usernames[indexPath.row]
    
return cell
}

func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int{
    
    return usernames.count
}
func numberOfSectionsInTableView(tableView: UITableView) -> Int{
    
    return 26
}


func sectionIndexTitlesForTableView(tableView: UITableView) -> [AnyObject]!{
    
    return self.sections
}

func tableView(tableView: UITableView,
    sectionForSectionIndexTitle title: String,
    atIndex index: Int) -> Int{
        
        return index
}

func tableView(tableView: UITableView,
    titleForHeaderInSection section: Int) -> String?{
        
        return self.sections[section] as? String
}

and it all works pretty good except for the grouping which makes my table view end up like this:

So I know you should be able to use the filtered function in an Array, but I did not understand how to implement it.

Any suggestions on how to proceed would be appreciated.

Answer 1

In Swift 4 Dictionary(grouping:by:) was introduced to group a sequence to a dictionary by an arbitrary predicate.

This example maps the grouped dictionary to a custom struct Section

struct Section {
    let letter : String
    let names : [String]
}

...

let usernames = ["John", "Nancy", "James", "Jenna", "Sue", "Eric", "Sam"]

var sections = [Section]()

override func viewDidLoad() {
    super.viewDidLoad()

    // group the array to ["N": ["Nancy"], "S": ["Sue", "Sam"], "J": ["John", "James", "Jenna"], "E": ["Eric"]]
    let groupedDictionary = Dictionary(grouping: usernames, by: {String($0.prefix(1))})
    // get the keys and sort them
    let keys = groupedDictionary.keys.sorted()
    // map the sorted keys to a struct
    sections = keys.map{ Section(letter: $0, names: groupedDictionary[$0]!.sorted()) }
    self.tableView.reloadData()
}


func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {

    let cellID = "cell"
    let cell = tableView.dequeueReusableCell(withIdentifier: cellID, for: indexPath)
    let section = sections[indexPath.section]
    let username = section.names[indexPath.row]
    cell.textLabel?.text = username
    return cell
}

func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    return sections[section].names.count
}

func numberOfSections(in tableView: UITableView) -> Int {
    return sections.count
}

func sectionIndexTitles(for tableView: UITableView) -> [String]? {
    return sections.map{$0.letter}
}

func tableView(_ tableView: UITableView, titleForHeaderInSection section: Int) -> String? {
    return sections[section].letter
}