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Eruhenon's solution

to Nucleotide Count in the Objective-C Track

Published at Jul 13 2018 · 1 comment
Test suite

Given a single stranded DNA string, compute how many times each nucleotide occurs in the string.

The genetic language of every living thing on the planet is DNA. DNA is a large molecule that is built from an extremely long sequence of individual elements called nucleotides. 4 types exist in DNA and these differ only slightly and can be represented as the following symbols: 'A' for adenine, 'C' for cytosine, 'G' for guanine, and 'T' thymine.

Here is an analogy:

  • twigs are to birds nests as
  • nucleotides are to DNA as
  • legos are to lego houses as
  • words are to sentences as...


There are two different methods of getting set up to run the tests with Objective-C:

  • Create an Xcode project with a test target which will run the tests.
  • Use the ruby gem objc as a test runner utility.

Both are described in more detail here: http://exercism.io/languages/objective-c

Submitting Exercises

When submitting an exercise, make sure your solution file is in the same directory as the test code.

The submit command will look something like:

exercism submit <path-to-exercism-workspace>/objective-c/nucleotide-count/NucleotideCount.m

You can find the Exercism workspace by running exercism debug and looking for the line beginning with Workspace.


The Calculating DNA Nucleotides_problem at Rosalind http://rosalind.info/problems/dna/

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


#import <XCTest/XCTest.h>

#if __has_include("NucleotideCountExample.h")
# import "NucleotideCountExample.h"
# else
# import "NucleotideCount.h"


@interface NucleotideCountTest : XCTestCase


@implementation NucleotideCountTest

- (void)testEmptyDNAStringHasNoAdenosine {
  NucleotideCount *dna = [[NucleotideCount alloc] initWithStrand:@""];
  NSUInteger result = [dna count:@"A"];
  NSUInteger expected = 0;

- (void)testEmptyNucleotideCountStringHasNoNucleotides {
  NucleotideCount *dna = [[NucleotideCount alloc] initWithStrand:@""];
  NSDictionary<NSString *, NSNumber *> *results = [dna nucleotideCounts];
  NSDictionary<NSString *, NSNumber *> *expected = @{ @"A": @0, @"T" : @0, @"C" : @0, @"G" : @0 };
  XCTAssertEqualObjects(results, expected);

- (void)testRepetitiveCytidineGetsCounted {
  NucleotideCount *dna = [[NucleotideCount alloc] initWithStrand:@"CCCCC"];
  NSUInteger result = [dna count:@"C"];
  NSUInteger expected = 5;

- (void)testRepetitiveSequenceHasOnlyGuanosine {
  NucleotideCount *dna = [[NucleotideCount alloc] initWithStrand:@"GGGGGGGG"];
  NSDictionary<NSString *, NSNumber *> *results = [dna nucleotideCounts];
  NSDictionary<NSString *, NSNumber *> *expected = @{ @"A": @0, @"T" : @0, @"C" : @0, @"G" : @8 };
  XCTAssertEqualObjects(results, expected);

- (void)testCountsByThymidine {
  NucleotideCount *dna = [[NucleotideCount alloc] initWithStrand:@"GGGGGTAACCCGG"];
  NSUInteger result = [dna count:@"T"];
  NSUInteger expected = 1;

- (void)testCountsANucleotideOnlyOnce {
  NucleotideCount *dna = [[NucleotideCount alloc] initWithStrand:@"CGATTGGG"];
  NSUInteger result = [dna count:@"T"];
  NSUInteger expected = 2;

- (void)testValidatesNucleotideCount {
  XCTAssertThrows([[NucleotideCount alloc] initWithStrand:@"John"]);

- (void)testCountsAllNucleotides {
  NucleotideCount *dna = [[NucleotideCount alloc] initWithStrand:longStrand];
  NSDictionary<NSString *, NSNumber *> *results = [dna nucleotideCounts];
  NSDictionary<NSString *, NSNumber *> *expected = @{ @"A": @20, @"T" : @21, @"C" : @12, @"G" : @17 };
  XCTAssertEqualObjects(results, expected);

#import "DNA.h"

@implementation NSString (DNA)

- (NSArray *)characters {
    NSMutableArray *characters = [NSMutableArray new];
    for (int i = 0; i < self.length; i++) {
        [characters addObject:[NSString stringWithFormat:@"%c", [self characterAtIndex:i]]];
    return [characters copy];


@interface DNA ()

@property (nonatomic, strong) NSString *strand;


@implementation DNA

- (instancetype)initWithStrand:(NSString *)strand {
    self = [super init];
    if (self) {
        [self validateStrand:strand againstComponents:[self nucleotides]];
        self.strand = strand;
    return self;

- (NSUInteger)count:(NSString *)nucleotide {
    [self validateStrand:nucleotide againstComponents:[[self nucleotides] stringByAppendingString:@"U"]];
    return [[self nucleotideBucket] countForObject:nucleotide];

- (NSDictionary *)nucleotideCounts {
    NSMutableDictionary *results = [NSMutableDictionary new];
    NSArray *nucleotideElements = [[self nucleotides] characters];
    NSCountedSet *bucket = [self nucleotideBucket];
    [nucleotideElements enumerateObjectsUsingBlock:^(NSString *nucleotide, NSUInteger idx, BOOL *stop) {
        NSUInteger count = [bucket countForObject:nucleotide];
        results[nucleotide] = @(count);
    return [results copy];

- (void)validateStrand:(NSString *)strand againstComponents:(NSString *)components {
    NSMutableCharacterSet *validateCharacterSet = [NSMutableCharacterSet characterSetWithCharactersInString:components];
    NSCharacterSet *strandCharacterSet = [NSCharacterSet characterSetWithCharactersInString:strand];
    if (! [validateCharacterSet isSupersetOfSet:strandCharacterSet]) {
        [NSException raise:@"Invalid DNA input" format:@"Strand %@ is not a valid nucleotide strand", strand];

- (NSString *)nucleotides {
    return @"ATCG";

- (NSCountedSet *)nucleotideBucket {
    return [NSCountedSet setWithArray:[self.strand characters]];


Community comments

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Avatar of Eruhenon
Solution Author
commented about 6 years ago

I'm always seeing solutions in which one iterates over the whole string and picks a single character. Since I'm using NSCountedSet, I'm wondering which is actually better.

What can you learn from this solution?

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