Methods

Why methods? A method gives a name to something that you want to do, so you don’t have to think about how to do it, you just do it The name should be descriptive and memorable Since a method is an action, it’s name is usually a verb Methods that do more than one thing are harder to name and harder to use Typical names: toChar, sort, sqrt, startsWith Example usages: scala> ('a' + 1).toChar res4: Char = b scala> "anteater".startsWith("ant") res7: Boolean = true 2

Two kinds of methods Classes describe objects (more about this later!) String is a class, and " anteater " is an object of that class Int is a class, and 'a' + 1 is an object of that class When we ask an object to compute something for us, we use dot notation ('a' + 1).toChar "anteater".startsWith("ant") We’ll learn more about classes later, but for now we will study “standalone” methods You have already met a couple of these methods val name = readLine("What is your name?" ) println("Hello, " + name) 3

Method syntax Here is the syntax for a method definition: def methodName ( parameters ): returnType = expression The parameters are a list of name : type pairs Scala can usually figure out the types of values for itself scala> val v = 3.6 v: Double = 3.6 Scala cannot determine parameter types, so you must specify them explicity Scala can usually determine the returnType, but sometimes you have to specify it Here is the syntax for a method call (or invocation): methodName ( arguments ) The arguments (sometimes called actual parameters) may be any expressions You do not specify the types of arguments, they just have to have the types expected by the method You can use a method anywhere a value of that type is required 4

Example: Palindromes A palindrome is a word or phrase that reads the same way backwards and forwards, for example, radar String objects have a reverse method, so we can define an isPalindrome method as follows: scala> def isPalindrome(s: String) = s == s.reverse isPalindrome: (s: String)Boolean and use it like this: scala> isPalindrome("radar") res16: Boolean = true We had to specify the type of the parameter s We always have to specify the type of each parameter We didn’t have to specify the return type (but we can if we want to) def isPalindrome(s: String): String = s == s.reverse Scala can usually figure out the return type 5

Palindromes II There is nothing wrong with writing very short methods (such as isPalindrome ) if it makes your program easier to understand In this case, I think just writing s == s.reverse is just as easy So let’s add some complexity! “Anna” is normally considered to be a palindrome, regardless of capitalization scala> isPalindrome("Anna") res18: Boolean = false Strings have a method toLowerCase to replace capital letters with their lowercase equivalents scala> def isPalindrome(s: String) = | s.toLowerCase == s.reverse.toLowerCase isPalindrome: (s: String)Boolean scala> isPalindrome("Anna") res20: Boolean = true Note: The vertical bar, |, is Scala’s prompt to tell us our expression isn’t finished 6

A little bit of string manipulation We can use a for loop to step through the characters of a string one at a time scala> for (ch <- "Anna") { | println(ch) | } A n n a We can use the + operator to add characters to a string scala> "abc" + 'd' res23: String = abcd We’ll use these in our next definition of isPalindrome 7

Removing non-letters from a string scala> var result = "" // empty string result: String = "" scala> var message = "One if by land, two if by sea" message: String = One if by land, two if by sea scala> for (ch result res25: String = Oneifbylandtwoifbysea 8

Palindromes III scala> :paste // Entering paste mode (ctrl-D to finish) def isPalindrome(s: String) = { var string1 = "" for (ch isPalindrome("Madam, I'm Adam!") res27: Boolean = true 9

Compound expressions The code for isPalindrome looked like this: def isPalindrome(s: String) = { // several lines of code omitted… string1.toLowerCase == string1.reverse.toLowerCase } The body of the method (the expression following the = sign) is a compound expression A compound expression consists of braces, {}, enclosing zero or more expressions If the braces are empty (zero expressions), the value is the unit, denoted () The value of a compound expression is the value of the last expression evaluated in it Hence, the value of the above method is the value of the expression string1.toLowerCase == string1.reverse.toLowerCase 10

Palindromes IV Just to neaten things up without changing how the program works (this is called refactoring), here’s one more version: def deleteNonLetters(s: String) = { var result = "" for (ch <- s) { if (ch.isLetter) { result = result + ch } } result } def isPalindrome(s: String) = { val temp = deleteNonLetters(s).toLowerCase temp == temp.reverse } 11

How I did it I wrote the earliest, simple versions in the REPL Once the method got long enough that I started making lots of mistakes, I wrote it as a script Here’s where I made most of my mistakes: Forgetting to lowercase both the original and the reversed string Forgetting to discard non-letters from both the original and the reversed string Simple typing errors (“typos”) Also I forgot that, in a script, the deleteNonLetters method must appear before it is used in isPalindrome This careful ordering of methods isn’t required in a program 12

Testing Of course, we have to test the method as we go along scala> isPalindrome("A man, a plan, a canal--Panama!") res5: Boolean = true scala> isPalindrome("Anna") res6: Boolean = true scala> isPalindrome("") res7: Boolean = true And don’t forget: scala> isPalindrome("success") res8: Boolean = false 13

AtomicTest in the REPL If you have installed the code supplied with the textbook, you have a very simple test framework Just enter this line: import com.atomicscala.AtomicTest._ Then you can use tests of the form: method-call is expected-result For example: scala> isPalindrome("Anna") is true true scala> isPalindrome("Anne") is false false scala> isPalindrome("Anne") is true false [Error] expected: true 14

AtomicTest in a script Put this line at the very beginning of your script: import com.atomicscala.AtomicTest._ Then put lines like this at the end of your script: isPalindrome("Anna") is true isPalindrome("A man, a plan, a canal--Panama!") is true isPalindrome("Anne") is false Here’s what is looks like in the REPL: scala> :load Palindrome.scala Loading Palindrome.scala... import com.atomicscala.AtomicTest._ deleteNonLetters: (s: String)String isPalindrome: (s: String)Boolean true true false 15

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