how to capture the first letter of each array entry - ios

Question

I have a sorted array of NSString values, I would like to know how to capture the first letter of each string only when the first letter is different and put it in a new NSArray.
For instance if I have an array that was like like
"a, aaa, aaaa, b, c, d, dd, ddd"
it would be like this in the new NSArray
"a, b, c, d"
Any help would be greatly appreciated.

Answer 1

Something like this:
- (NSArray *)indexLettersForStrings:(NSArray *)strings {
NSMutableArray *letters = [NSMutableArray array];
NSString *currentLetter = nil;
for (NSString *string in strings) {
if (string.length > 0) {
NSString *letter = [string substringToIndex:1];
if (![letter isEqualToString:currentLetter]) {
[letters addObject:letter];
currentLetter = letter;
}
}
}
return [NSArray arrayWithArray:letters];
}

Answer 2

NSString+LetterIndex.h
#interface NSString (LetterIndex)
#property (nonatomic, readonly) NSString * firstLetter;
#end
NSString+LetterIndex.m
#implementation NSString (LetterIndex)
- (NSString *)firstLetter
{
return self.length ? [self substringToIndex:1] : #"";
}
your method:
- (NSArray *)indexLettersForStrings:(NSArray *)strings {
NSSet * distinctValues = [NSSet setWithArray:[strings valueForKey:#"firstLetter"]];
return [[distinctValues allObjects] sortedArrayUsingSelector:#selector(compare:)]
}
also if you have some objects of custom class and want to group them by first letters of some string parameter, you can use this:
NSSet * distinctValues = [NSSet setWithArray:[objects valueForKeyPath:#"myStringParam.firstLetter"]];

Answer 3

NSArray *array = [NSArray arrayWithObjects:#"a", #"aaa", #"aaaa",#"b", #"c", #"d", #"dd", #"ddd", nil];
BOOL control = YES;
NSMutableArray *array2 = [NSMutableArray array];
for (int i = 0; i<array.count; i++) {
for (int j = 0; j<array2.count;j++){
if ([[array2 objectAtIndex:j]isEqualToString:[[array objectAtIndex:i]substringToIndex:1]]){
control = NO;
}
else
control = YES;
}
if (control)
[array2 addObject:[[array objectAtIndex:i]substringToIndex:1]];
}

Answer 4

Try this:
NSArray *arr = #[#"a", #"aaa", #"aaaa", #"b", #"c", #"d", #"dd", #"ddd"];
NSMutableArray *newArr = [NSMutableArray array];
NSMutableSet *set = [NSMutableSet set];
for (NSString *str in arr)
{
if (![set containsObject:[str substringToIndex:1]])
[newArr addObject:[str substringToIndex:1]];
[set addObject:[str substringToIndex:1]];
}
NSLog(#"%#", newArr);
This uses a Set to keep track of occurrences that already past threw. When it doesnt exist it places them into a new array.