How katas can help you learn
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:
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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:
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[[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:
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@interface DecimalToRomanMapper : NSObject @property (nonatomic, assign, readonly) NSInteger decimal; @property (nonatomic, copy, readonly) NSString* roman; + (instancetype)mappDecimal:(NSInteger)decimal toRoman:(NSString*)roman; @end |
It’s even more readable then the initialiser. And here is how I initialised it in my converter:
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self.mapper = @[ [DecimalToRomanMapper mappDecimal:1000 toRoman:@"M"] ]; |
So my class was looking pretty but then I look at my test class…it wasn’t good:
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... - (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? ;).
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(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:
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+ (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:
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@interface DecimalToRomanConverterTests : XCParameterizedTestCase @end @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!
image: Baton_long by Alain Delmas