How katas can help you learn

18 Aug 2015

Baton_long by Alain Delmas

In the last couple of weeks I have spent my time doing katas to improve my Objective-C. As you may remember, I had an issue, that you can revisit here.

I've learned a lot from other katas too: for instance the RomanNumerals kata... You'd may think it's a simple task, that you wan't learn much from. It's a simple algorithm, a good exercise for your red -> green -> refactoring cycle. So nothing new will come from here... Wasn't I wrong!!!!!

First I created some tests. Next I implemented the kata and refactored. Whilst I was refactoring, I decided to use NSDictionary to map from a decimal to a roman number. Here is an example:

NSDictionary *mapper = @{@10: @"X", @5:@"V", @1: @"I"};

That's when I discovered that the NSDictionary does not guarantee insertion order. What???? So my keys were all mixed up... And I needed them to maintain their order!!!! That's ok! I decided to create a class to map between decimals and romans. That's cool! Done! In the converter I just added a private property of type NSMutableArray. In it's constructor I added the class to the array for all elements I needed. But now I have to do:

[[DecimalToRomanMapper alloc] initWithDecimal: andRoman];

for every entry in the mapper. God!!!! So much work! Maybe that's why developers solve problems. They definitely don't like to do things by hand so they automate everything. So I decided that I could create a factory method. I really didn't know how to create this factory method, so I just looked in Apple docs. Here is another thing i learned. Even thought I knew that it existed, I never used it because I never needed to. But as I was in learning mode, I think I was more interested in finding a different way to do it. When you are at a client, you don't always have the opportunity to experiment with new things. Having this time to learn new ways of doing things is really rewarding. So here is my class:

@interface DecimalToRomanMapper : NSObject

@property (nonatomic, assign, readonly) NSInteger decimal;
@property (nonatomic, copy, readonly) NSString* roman;

+ (instancetype)mappDecimal:(NSInteger)decimal toRoman:(NSString*)roman;


It's even more readable then the initialiser. And here is how I initialised it in my converter:

   self.mapper = @[
                      [DecimalToRomanMapper mappDecimal:1000 toRoman:@"M"]

So my class was looking pretty but then I look at my test wasn't good:


- (void)testShouldConvertFourToIV {
    NSString *result = [converter convert:4];
   XCTAssertTrue([result isEqualToString:@"IV"]);


- (void)testShouldConvert1000ToM {
   NSString *result = [converter convert:1000];
    XCTAssertTrue([result isEqualToString:@"M"]);


So many tests. They remind me of the DRY principle. I really don't like to repeat myself. It's like a broken CD that doesn't move from the same music track... I've done the same kata in C# and my test class was all parameterised and I really liked it. After all this is a simple kata, right? ;).

(C# code)
[TestCase(1, "I")]
[TestCase(2, "II")]
[TestCase(3, "III")]
[TestCase(4, "IV")]
[TestCase(5, "V")]
[TestCase(9, "IX")]
[TestCase(10, "X")]
[TestCase(40, "XL")]
[TestCase(50, "L")]
[TestCase(90, "XC")]
[TestCase(100, "C")]
[TestCase(400, "CD")]
[TestCase(900, "CM")]
[TestCase(1000, "M")]
[TestCase(2499, "MMCDXCIX")]
[TestCase(3949, "MMMCMXLIX")]
public void convertDecimalToRoman(int decimalNumber, string expectedRomanNumber)
     var converter = new DecimalToRomanConverter();

      string result = converter.Convert(decimalNumber);

    Assert.AreEqual(expectedRomanNumber, result);

Well wouldn't it be nice if I could have that in Objective-C? Well, after some help from Franzi I found a little library that does it. You install the pod, you inherit from it and you create an array of inputs and expected values like this:

+ (NSArray *)testCaseData {
    return @[
             [XCTestCaseData createWithInputValue:@1 withExpectedValue:@"I"]

And then you just have to use the properties input and expected. So my test class looks like this:

@interface DecimalToRomanConverterTests : XCParameterizedTestCase


@implementation DecimalToRomanConverterTests

+(NSArray *)testCaseData {
    return @[
             [XCTestCaseData createWithInputValue:@1 withExpectedValue:@"I"],
             [XCTestCaseData createWithInputValue:@2 withExpectedValue:@"II"],
             [XCTestCaseData createWithInputValue:@3 withExpectedValue:@"III"],
             [XCTestCaseData createWithInputValue:@5 withExpectedValue:@"V"],
             [XCTestCaseData createWithInputValue:@8 withExpectedValue:@"VIII"],
             [XCTestCaseData createWithInputValue:@10 withExpectedValue:@"X"],
             [XCTestCaseData createWithInputValue:@18 withExpectedValue:@"XVIII"],
             [XCTestCaseData createWithInputValue:@4 withExpectedValue:@"IV"],
             [XCTestCaseData createWithInputValue:@9 withExpectedValue:@"IX"],
             [XCTestCaseData createWithInputValue:@50 withExpectedValue:@"L"],
             [XCTestCaseData createWithInputValue:@100 withExpectedValue:@"C"],
             [XCTestCaseData createWithInputValue:@500 withExpectedValue:@"D"],
             [XCTestCaseData createWithInputValue:@2499 withExpectedValue:@"MMCDXCIX"],
             [XCTestCaseData createWithInputValue:@3949 withExpectedValue:@"MMMCMXLIX"]

-(void)testShouldConvertADecimalIntoARoman {

    DecimalToRomanConverter *converter = [[DecimalToRomanConverter alloc]init];
    NSString* result = [converter convert:[self.input integerValue]];

    XCTAssertEqualObjects(self.expected, result);

Here is my final solution: RomanNumerals.

So please don't underestimate what you can learn from a kata. They are a good opportunity to stretch your knowledge and add some more to it!

This post was cross-posted to my personal blog.

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