iPhone UITableView. How do turn on the single letter alphabetical list like the Music App?

In the iPhone music app, selecting Artist, Songs, or Albums presents a tableView with a verticl list of single letters at the righthand side of the UI that enables rapid scrolling. How do I enable this functionality in my app?

Cheers, Doug

Supply your own index characters:

- (NSArray *)sectionIndexTitlesForTableView:(UITableView *)tableView {
	return[NSArray arrayWithObjects:@"a", @"e", @"i", @"m", @"p", nil];
}

and then:

- (NSInteger)tableView:(UITableView *)tableView sectionForSectionIndexTitle:(NSString
    *)title atIndex:(NSInteger)index {
        return <yourSectionIndexForTheSectionForSectionIndexTitle >;
}

You will need sections.

Something else you have to consider is localizing the sections for each language. After digging around a bit, I found UILocalizedIndexedCollation to be quite useful:

- (NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section {
    return [[[UILocalizedIndexedCollation currentCollation] sectionTitles] objectAtIndex:section];
}

- (NSArray *)sectionIndexTitlesForTableView:(UITableView *)tableView {
    return [[UILocalizedIndexedCollation currentCollation] sectionIndexTitles];
}
 
- (NSInteger)tableView:(UITableView *)tableView sectionForSectionIndexTitle:(NSString *)title atIndex:(NSInteger)index {
    return [[UILocalizedIndexedCollation currentCollation] sectionForSectionIndexTitleAtIndex:index];
}

https://developer.apple.com/documentation/uikit/uilocalizedindexedcollation

I came up with an alternative approach to handling a single letter alphabet list without using sections. It's similar to Zaph's answer but instead of getting any value from returning a new index (since we'll always have 1 section), we calculate the index for the location of the first item in the array that begins with a certain character, then scroll to it.

The downside is this requires searching the array every time (is this absolutely terrible?), however I didn't notice any lag or slow behavior in the iOS simulator or on my iPhone 4S.

- (NSArray *)sectionIndexTitlesForTableView:(UITableView *)tableView {
  return[NSArray arrayWithObjects:@"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", nil];
}

- (NSInteger)tableView:(UITableView *)tableView sectionForSectionIndexTitle:(NSString *)title atIndex:(NSInteger)index {
  
  NSInteger newRow = [self indexForFirstChar:title inArray:self.yourStringArray];
  NSIndexPath *newIndexPath = [NSIndexPath indexPathForRow:newRow inSection:0];
  [tableView scrollToRowAtIndexPath:newIndexPath atScrollPosition:UITableViewScrollPositionTop animated:NO];
  
  return index;
}

// Return the index for the location of the first item in an array that begins with a certain character
- (NSInteger)indexForFirstChar:(NSString *)character inArray:(NSArray *)array
{
  NSUInteger count = 0;
  for (NSString *str in array) {
    if ([str hasPrefix:character]) {
      return count;
    }
    count++;
  }
  return 0;
}

---

adding property to store last selected index like

 @property (assign, nonatomic) NSInteger previousSearchIndex;

and storing this property every time like:

- (NSInteger)indexForFirstChar:(NSString *)character inArray:(NSArray *)array
{
    NSUInteger count = 0;
    for (NSString *str in array) {
        if ([str hasPrefix:character]) {
            self.previousSearchIndex = count;
            return count;
        }
        count++;
    }
    return self.previousSearchIndex;
}

and updating scrollToRow code like:

 [tableView scrollToRowAtIndexPath:newIndexPath atScrollPosition:UITableViewScrollPositionTop animated:YES];

Do this method even better and with nice animation.

A bunch of people asked if it was possible to do this without sections. I wanted the same thing and I found a solution which might be a little shady and doesn't return a value to sectionForSectionIndexTitle but if you are in a corner and don't want to have to make a section for every letter of the alphabet this is a sure fix. Sorry to any code Nazis in advance. :P

- (NSArray *)sectionIndexTitlesForTableView:(UITableView *)tableView {
    if (thisTableDataIsShowing)
    {
        NSMutableArray *charactersForSort = [[NSMutableArray alloc] init];
        for (NSDictionary *item in d_itemsInTable)
        {
            if (![charactersForSort containsObject:[[item valueForKey:@"character_field_to_sort_by"] substringToIndex:1]])
            {
                [charactersForSort addObject:[[item valueForKey:@"character_field_to_sort_by"] substringToIndex:1]];
            }
        }
        return charactersForSort;
    }
    return nil;
}

- (NSInteger)tableView:(UITableView *)tableView sectionForSectionIndexTitle:(NSString *)title atIndex:(NSInteger)index {
    BOOL found = NO;
    NSInteger b = 0;
    for (NSDictionary *item in d_itemsInTable)
    {
        if ([[[item valueForKey:@"character_field_to_sort_by"] substringToIndex:1] isEqualToString:title])
            if (!found)
            {
                [d_yourTableView scrollToRowAtIndexPath:[NSIndexPath indexPathForRow:b inSection:0] atScrollPosition:UITableViewScrollPositionTop animated:NO];
                found = YES;
            }
        b++;
    }
}

It works great if you are getting a large amount of data and sectioning it would take a bunch of work. :) Tried to use generic variables so you knew what I was doing. d_itemsInTable is an NSArray of NSDictionaries that I'm listing out to the UITableView.

Here is a modified version of Kyle's function that handles the case of clicking an index for which you do not have a string:

- (NSInteger)indexForFirstChar:(NSString *)character inArray:(NSArray *)array
{
    char testChar = [character characterAtIndex:0];
    __block int retIdx = 0;
    __block int lastIdx = 0;
    
    [array enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        char firstChar = [obj characterAtIndex:0];
        
        if (testChar == firstChar) {
            retIdx = idx;
            *stop = YES;
        }
        
        //if we overshot the target, just use whatever previous one was
        if (testChar < firstChar) {
            retIdx = lastIdx;
            *stop = YES;
        }
        
        lastIdx = idx;
    }];
    return retIdx;
}

If you're using a NSFetchedResultsController, you can just do:

- (NSArray *)sectionIndexTitlesForTableView:(UITableView *)tableView {
    return [frc sectionIndexTitles];
}
 
- (NSInteger)tableView:(UITableView *)tableView sectionForSectionIndexTitle:(NSString *)title atIndex:(NSInteger)index {
    return [frc sectionForSectionIndexTitle:title atIndex:index];
}

Implement the delegate methods -sectionIndexTitlesForTableView: and -tableView:sectionForSectionIndexTitle:atIndex:

See the UITableViewDataSource documentation for more info.

Here's a simple solution in Swift, assuming you have your title headers in an array. If the title couldn't be found, it will return the previous index in the array.

func sectionIndexTitlesForTableView(tableView: UITableView) -> [String]? {
    return "ABCDEFGHIJKLMNOPQRSTUVWXYZ".characters.flatMap{String($0)}
}

func tableView(tableView: UITableView, sectionForSectionIndexTitle title: String, atIndex index: Int) -> Int {
    return self.headerTitles.filter{$0 <= title}.count - 1
}

If you're using MonoTouch, override the SectionIndexTitles(UITableView) method in the UITableViewDataSource class. Just return an array of strings and the subclass takes care of the rest.

class TableViewDataSource : UITableViewDataSource
{
  public override string[] SectionIndexTitles(UITableView tableView) 
  { 
    return new string[] { /*your string values */};
  }
}

*just a hint for those of us using C# and Mono (.NET) to write iPhone apps. :)