The easiest approach.
NSArray *rows = ...;
NSMutableDictionary *map = [NSMutableDictionary dictionary];
for (NSString *value in rows) {
NSString *firstLetter = [value substringToIndex:1];
if (!map[firstLetter]) {
map[firstLetter] = @[];
}
NSMutableArray *values = [map[firstLetter] mutableCopy];
[values addObject:value];
map[firstLetter] = values;
}
NSArray *finalRows = [map allValues];
Note that finalRows
is not sorted.