Testing for Algorithm Method - swift

Question

I am relatively new to unit testing and was wondering how I can check to ensure my algorithm would correctly return any the most common word in any given string that's passed to my function?
var test = "Let's figure. out what the most comm$on wor!!!!d is in the sentence."
func checkForRandomChracters(string: String) -> String {
let charactersToRemove = Array(".:?!,##$%^*,")
let arrayOfChars = Array(string)
let filteredString = arrayOfChars.reduce("") {
let str = String($1)
return $0 + (charactersToRemove.contains($1) ? "" : str)
}
return filteredString
}
func mostFrequentWords(string: String) -> String {
let lowerString = string.lowercased()
let cleanedString = checkForRandomChracters(string: lowerString)
let array = cleanedString.components(separatedBy: " ")
var itemInDictionary: [String: Int] = [:]
for item in array {
itemInDictionary[item] = (itemInDictionary[item] ?? 0) + 1
}
var mostCommonNameInArray = ""
for key in itemInDictionary.keys {
if mostCommonNameInArray == "" {
mostCommonNameInArray = key
} else {
let count = itemInDictionary[key]!
if count > itemInDictionary[mostCommonNameInArray]! {
mostCommonNameInArray = key
}
}
}
return mostCommonNameInArray
}
mostFrequentWords(string: test) // returns 'the'

Answer 1

Unit test is not about ensuring that the answer will be correct for all strings. For that, you would need a formal proof, that is much more complex and related to mathematics and this kind of stuff (in a really high level description).
Unit testing is about developing tests to guarantee that your method will work for the cases you developed the tests for. And because of that, you will want to develop different cases to test different behaviours, for example:
What is the algorithm result when you use a string that you already know the result? Will It be correct?
What is the algorithm result when you send an empty string?
What is the algorithm result when there is more than one most frequent word?
Remember to always test corner cases (like empty or with more than one most frequent word), and develop a few tests for cases when you know the result, to guarantee.
Oh, and also, unit tests are really useful when you want to change the algorithm, but want to test if it's behaviour did not change.
In Swift, you can create an unit test class , then develop different tests methods with XCTest. A great tutorial you can follow to understand that is presented here.
I hope I've answered your question. Good luck.