Count the number of letter occurrences in a given string.

gistfile1.m

    //initial string
    NSString *str = @"the quick brown fox jumped over the fence";
    
    //convert string to array of characters
    NSMutableArray *stringArray = [NSMutableArray array];
    [str enumerateSubstringsInRange:NSMakeRange(0, str.length)
                            options:NSStringEnumerationByComposedCharacterSequences
                         usingBlock:^(NSString *substring, NSRange substringRange, NSRange enclosingRange, BOOL *stop) {
                             [stringArray addObject:[substring lowercaseString]];
    }];
    
    //make an array of unique characters sorted alphabetically
    NSArray *letterArray = [[[NSSet setWithArray:stringArray] allObjects] sortedArrayUsingSelector:@selector(localizedCaseInsensitiveCompare:)];
    
    //find the number of occurences
    [letterArray enumerateObjectsUsingBlock:^(NSString *letter, NSUInteger idx, BOOL *stop) {
        
        NSInteger occurrences = [[stringArray indexesOfObjectsPassingTest:^(NSString *obj, NSUInteger idx, BOOL *stop) {
            
            return [[obj lowercaseString] isEqualToString:[letter lowercaseString]];
        }] count];
        
        NSLog(@"letter %@, %@ times", [letter lowercaseString], @(occurrences));
    }];
    
    //  , 7 times
    // b, 1 times
    // c, 2 times
    // d, 1 times
    // e, 6 times
    // f, 2 times
    // h, 2 times
    // i, 1 times
    // j, 1 times
    // k, 1 times
    // m, 1 times
    // n, 2 times
    // o, 3 times
    // p, 1 times
    // q, 1 times
    // r, 2 times
    // t, 2 times
    // u, 2 times
    // v, 1 times
    // w, 1 times
    // x, 1 times