Create index for UITableView from an NSArray

Question

I've read that the best way of creating an index (the a-z at the side of a uitableview) is to set up an array of nsdictionaries, where each dictionary corresponds to a section, and a rowValue key contains an array of the rows.

NSDictionary
    headerTitle => ‘A’
    rowValues => {”Aardvark”, “Ape”, “Aquaman”}
NSDictionary
    headerTitle => ‘B’
    rowValues => {”Bat”, “Boot”, “Bubbles”} etc

But how can this be created from an array of all the row titles - {”Aardvark”, “Ape”, “Aquaman”, ”Bat”, “Boot”, “Bubbles”, "Cat", "Cabbage" etc} ...?

Answer 1

I recently had a similar objective and this is how I solved it. The advantage of this over Robin's solution is that it creates the index title array dynamically based on the content of your array and it won't include indices for empty sections (plus it's a little cleaner).

I created a category of NSMutableDictionary that takes an array of data as a parameter and returns an NSMutableDictionary (we'll call it indexDictionary and it should be an instance variable):

// if your data is static, you can call this in `viewDidLoad`
indexDictionary = [NSMutableDictionary createDictionaryForSectionIndex:arrayOfStrings];

The category method:

@implementation NSMutableDictionary (DictionaryForSectionIndex)

+(NSMutableDictionary *)createDictionaryForSectionIndex:(NSArray *)array
{
    NSMutableDictionary *dict = [NSMutableDictionary dictionary];
    for (char firstChar = 'a'; firstChar <= 'z'; firstChar++)
    {
        //NSPredicates are fast
        NSString *firstCharacter = [NSString stringWithFormat:@"%c", firstChar];
        NSArray *content = [array filteredArrayUsingPredicate:[NSPredicate predicateWithFormat:@"SELF beginswith[cd] %@", firstCharacter]];
        NSMutableArray *mutableContent = [NSMutableArray arrayWithArray:content];

        if ([mutableContent count] > 0)
        {
            NSString *key = [firstCharacter uppercaseString];
            [dict setObject:mutableContent forKey:key];
            NSLog(@"%@: %u", key, [mutableContent count]);
        }
    }

    return dict;
}

@end

/*

**Input:** 
{"Aardvark", "Cabbage", "Boot", "Eggs", "Ape", "Aquaman", "Elephant", "Cat", "Bat", "Bubbles"}

**Output:**
NSMutableDictionary
    key => 'A'
    object => {"Aardvark", "Ape", "Aquaman"}

    key => 'B'
    object => {"Bat", "Boot", "Bubbles"}

    key => 'C'
    object => {"Cat", "Cabbage"}

    key => 'E'
    object => {"Elephant", "Eggs"}

*/

Then I create an NSArray instance variable to sort and store all the keys from indexDictionary:

// this line should follow the creation of `indexDictionary`

sortedKeys = [[indexDictionary allKeys] sortedArrayUsingSelector:@selector(localizedCaseInsensitiveCompare:)];

// Output, in this case, is {'A', 'B', 'C', 'E'}

You now have everything you need to set up the index for your table. Implement the following methods (if something isn't self explanatory, just let me know):

//this code assumes `sortedKeys` is not empty

#pragma mark - UITableViewDataSource

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return ([sortedKeys count]);
}


- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    NSString *key = [sortedKeys objectAtIndex:section];
    return [[indexDictionary valueForKey:key] count];
}


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


-(NSArray *)sectionIndexTitlesForTableView:(UITableView *)tableView
{
    return sortedKeys;
}


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


- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    // preceding code...

    NSString *key = [sortedKeys objectAtIndex:indexPath.section];
    NSArray *array = [indexDictionary objectForKey:key];
    NSString *yourString = [array objectAtIndex:indexPath.row];

    cell.textLabel.text = yourString;

    // following code...
}

The result is a table with an index that skips letters that have no associated data.