2013-07-29 12:57:44 +00:00
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---
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2013-07-29 22:59:12 +00:00
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language: Scala
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2013-08-02 16:43:54 +00:00
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filename: learnscala.scala
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2013-07-29 23:03:28 +00:00
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contributors:
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- ["George Petrov", "http://github.com/petrovg"]
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2013-08-02 16:43:54 +00:00
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- ["Dominic Bou-Samra", "http://dbousamra.github.com"]
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2014-11-11 02:24:00 +00:00
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- ["Geoff Liu", "http://geoffliu.me"]
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2013-07-29 14:14:37 +00:00
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filename: learn.scala
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2013-07-29 12:57:44 +00:00
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---
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2013-07-29 14:14:37 +00:00
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Scala - the scalable language
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2014-09-08 11:08:28 +00:00
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```scala
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2013-07-29 14:14:37 +00:00
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2013-07-28 19:52:32 +00:00
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/*
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Set yourself up:
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1) Download Scala - http://www.scala-lang.org/downloads
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2) unzip/untar in your favourite location and put the bin subdir on the path
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3) Start a scala REPL by typing scala. You should see the prompt:
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scala>
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2014-11-11 02:24:00 +00:00
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This is the so called REPL (Read-Eval-Print Loop). You may type any valid
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Scala expression into it, and the result will be printed. We will explain what
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Scala files look like further into this tutorial, but for now, let's start
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with some basics.
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2013-07-28 19:52:32 +00:00
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*/
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2014-11-12 08:36:19 +00:00
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/////////////////////////////////////////////////
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// 1. Basics
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/////////////////////////////////////////////////
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2013-06-30 07:52:21 +00:00
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2014-11-11 02:38:48 +00:00
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// Single line comments start with two forward slashes
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/*
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Multi line comments, as you can already see from above, look like this.
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*/
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2013-06-30 07:52:21 +00:00
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// Printing, and forcing a new line on the next print
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println("Hello world!")
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2014-11-11 02:38:48 +00:00
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println(10)
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2013-06-30 07:52:21 +00:00
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// Printing, without forcing a new line on next print
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print("Hello world")
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2014-11-11 02:38:48 +00:00
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// Declaring values is done using either var or val.
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2014-11-11 02:24:00 +00:00
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// val declarations are immutable, whereas var's are mutable. Immutability is
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2013-08-02 16:39:30 +00:00
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// a good thing.
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2013-06-30 07:52:21 +00:00
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val x = 10 // x is now 10
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x = 20 // error: reassignment to val
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2014-11-11 02:38:48 +00:00
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var y = 10
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y = 20 // y is now 20
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2013-06-30 07:52:21 +00:00
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2014-11-11 02:24:00 +00:00
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/*
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2014-11-14 23:01:16 +00:00
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Scala is a statically typed language, yet note that in the above declarations,
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we did not specify a type. This is due to a language feature called type
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inference. In most cases, Scala compiler can guess what the type of a variable
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is, so you don't have to type it every time. We can explicitly declare the
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type of a variable like so:
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2013-08-01 09:11:15 +00:00
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*/
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2014-11-11 02:38:48 +00:00
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val z: Int = 10
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val a: Double = 1.0
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2014-11-14 23:01:16 +00:00
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// Notice automatic conversion from Int to Double, result is 10.0, not 10
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val b: Double = 10
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2013-06-30 07:52:21 +00:00
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// Boolean values
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true
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false
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// Boolean operations
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!true // false
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!false // true
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true == false // false
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10 > 5 // true
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2013-08-01 09:11:15 +00:00
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// Math is as per usual
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1 + 1 // 2
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2 - 1 // 1
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5 * 3 // 15
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6 / 2 // 3
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2014-11-11 02:38:48 +00:00
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6 / 4 // 1
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6.0 / 4 // 1.5
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2013-06-30 07:52:21 +00:00
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2014-11-11 02:38:48 +00:00
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// Evaluating an expression in the REPL gives you the type and value of the result
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2013-07-28 19:52:32 +00:00
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1 + 7
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/* The above line results in:
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scala> 1 + 7
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res29: Int = 8
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2013-08-02 16:39:30 +00:00
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This means the result of evaluating 1 + 7 is an object of type Int with a
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value of 8
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2013-07-28 19:52:32 +00:00
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2014-11-14 23:01:16 +00:00
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Note that "res29" is a sequentially generated variable name to store the
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results of the expressions you typed, your output may differ.
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2013-07-28 19:52:32 +00:00
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*/
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2014-11-11 02:38:48 +00:00
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"Scala strings are surrounded by double quotes"
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2014-11-11 02:24:00 +00:00
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'a' // A Scala Char
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2014-11-12 01:42:28 +00:00
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// 'Single quote strings don't exist' <= This causes an error
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2013-07-29 17:33:03 +00:00
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2014-11-11 02:38:48 +00:00
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// Strings have the usual Java methods defined on them
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"hello world".length
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2014-11-11 02:58:14 +00:00
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"hello world".substring(2, 6)
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"hello world".replace("C", "3")
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2013-07-29 12:53:55 +00:00
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2014-11-11 02:38:48 +00:00
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// They also have some extra Scala methods. See also: scala.collection.immutable.StringOps
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"hello world".take(5)
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2014-11-11 02:58:14 +00:00
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"hello world".drop(5)
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2013-07-29 12:53:55 +00:00
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2014-11-11 02:38:48 +00:00
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// String interpolation: notice the prefix "s"
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2014-11-11 02:24:00 +00:00
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val n = 45
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2014-11-11 02:38:48 +00:00
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s"We have $n apples" // => "We have 45 apples"
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2013-07-29 12:53:55 +00:00
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2014-11-11 02:24:00 +00:00
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// Expressions inside interpolated strings are also possible
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val a = Array(11, 9, 6)
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2014-11-11 02:38:48 +00:00
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s"My second daughter is ${a(0) - a(2)} years old." // => "My second daughter is 5 years old."
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s"We have double the amount of ${n / 2.0} in apples." // => "We have double the amount of 22.5 in apples."
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s"Power of 2: ${math.pow(2, 2)}" // => "Power of 2: 4"
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2013-07-29 12:53:55 +00:00
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2014-11-11 02:38:48 +00:00
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// Formatting with interpolated strings with the prefix "f"
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f"Power of 5: ${math.pow(5, 2)}%1.0f" // "Power of 5: 25"
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2014-11-11 02:46:26 +00:00
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f"Square root of 122: ${math.sqrt(122)}%1.4f" // "Square root of 122: 11.0454"
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2013-07-29 12:53:55 +00:00
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2014-11-11 02:38:48 +00:00
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// Raw strings, ignoring special characters.
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raw"New line feed: \n. Carriage return: \r." // => "New line feed: \n. Carriage return: \r."
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2013-06-30 07:52:21 +00:00
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2014-11-11 02:38:48 +00:00
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// Some characters need to be "escaped", e.g. a double quote inside a string:
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"They stood outside the \"Rose and Crown\"" // => "They stood outside the "Rose and Crown""
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2013-06-30 07:52:21 +00:00
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2014-11-11 02:24:00 +00:00
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// Triple double-quotes let strings span multiple rows and contain quotes
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val html = """<form id="daform">
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<p>Press belo', Joe</p>
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2014-11-11 02:38:48 +00:00
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<input type="submit">
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2014-11-11 02:24:00 +00:00
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</form>"""
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2013-06-30 07:52:21 +00:00
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2014-11-12 08:36:19 +00:00
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/////////////////////////////////////////////////
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// 2. Functions
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/////////////////////////////////////////////////
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2013-08-01 09:11:15 +00:00
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2014-11-14 23:01:16 +00:00
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// Functions are defined like so:
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//
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// def functionName(args...): ReturnType = { body... }
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//
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// If you come from more traditional languages, notice the omission of the
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// return keyword. In Scala, the last expression in the function block is the
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// return value.
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def sumOfSquares(x: Int, y: Int): Int = {
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val x2 = x * x
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val y2 = y * y
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x2 + y2
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}
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2013-07-29 12:53:55 +00:00
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2014-11-14 23:01:16 +00:00
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// The { } can be omitted if the function body is a single expression:
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def sumOfSquaresShort(x: Int, y: Int): Int = x * x + y * y
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2013-07-29 12:53:55 +00:00
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2014-11-14 23:01:16 +00:00
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// Syntax for calling functions is familiar:
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sumOfSquares(3, 4) // => 25
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2013-07-29 12:53:55 +00:00
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2014-11-14 23:01:16 +00:00
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// In most cases (with recursive functions the most notable exception), function
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// return type can be omitted, and the same type inference we saw with variables
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// will work with function return values:
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def sq(x: Int) = x * x // Compiler can guess return type is Int
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2013-07-29 12:53:55 +00:00
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2014-11-14 23:01:16 +00:00
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// Functions can have default parameters:
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def addWithDefault(x: Int, y: Int = 5) = x + y
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addWithDefault(1, 2) // => 3
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addWithDefault(1) // => 6
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2013-07-29 12:53:55 +00:00
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2014-11-14 23:01:16 +00:00
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// Anonymous functions look like this:
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(x:Int) => x * x
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2013-07-29 17:33:03 +00:00
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2014-11-14 23:01:16 +00:00
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// Unlike defs, even the input type of anonymous functions can be omitted if the
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// context makes it clear. Notice the type "Int => Int" which means a function
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// that takes Int and returns Int.
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val sq: Int => Int = x => x * x
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// Anonymous functions can be called as usual:
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sq(10) // => 100
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// If your anonymous function has one or two arguments, and each argument is
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// used only once, Scala gives you an even shorter way to define them. These
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// anonymous functions turn out to be extremely common, as will be obvious in
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// the data structure section.
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val addOne: Int => Int = _ + 1
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val weirdSum: (Int, Int) => Int = (_ * 2 + _ * 3)
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addOne(5) // => 6
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weirdSum(2, 4) // => 16
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// The return keyword exists in Scala, but it only returns from the inner-most
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2015-01-03 18:48:53 +00:00
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// def that surrounds it.
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// WARNING: Using return in Scala is error-prone and should be avoided.
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// It has no effect on anonymous functions. For example:
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2014-12-20 22:03:25 +00:00
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def foo(x: Int): Int = {
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2014-11-14 23:01:16 +00:00
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val anonFunc: Int => Int = { z =>
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if (z > 5)
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return z // This line makes z the return value of foo!
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else
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z + 2 // This line is the return value of anonFunc
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}
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anonFunc(x) // This line is the return value of foo
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}
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2013-07-29 17:33:03 +00:00
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2014-11-12 08:36:19 +00:00
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/////////////////////////////////////////////////
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// 3. Flow Control
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/////////////////////////////////////////////////
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2013-07-29 17:33:03 +00:00
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1 to 5
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val r = 1 to 5
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r.foreach( println )
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2014-11-11 02:24:00 +00:00
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r foreach println
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2013-08-02 16:39:30 +00:00
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// NB: Scala is quite lenient when it comes to dots and brackets - study the
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// rules separately. This helps write DSLs and APIs that read like English
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2013-07-29 17:33:03 +00:00
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(5 to 1 by -1) foreach ( println )
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2013-08-01 09:11:15 +00:00
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// A while loops
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2013-07-29 17:33:03 +00:00
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var i = 0
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while (i < 10) { println("i " + i); i+=1 }
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while (i < 10) { println("i " + i); i+=1 } // Yes, again. What happened? Why?
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2013-08-02 16:39:30 +00:00
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i // Show the value of i. Note that while is a loop in the classical sense -
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// it executes sequentially while changing the loop variable. while is very
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2014-11-14 23:01:16 +00:00
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// fast, faster that Java loops, but using the combinators and
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2013-08-02 16:39:30 +00:00
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// comprehensions above is easier to understand and parallelize
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2013-07-29 17:33:03 +00:00
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2013-08-01 09:11:15 +00:00
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// A do while loop
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2013-08-02 16:39:30 +00:00
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do {
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2014-11-11 02:24:00 +00:00
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println("x is still less than 10");
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2013-08-01 09:11:15 +00:00
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x += 1
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} while (x < 10)
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2013-08-02 16:39:30 +00:00
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// Tail recursion is an idiomatic way of doing recurring things in Scala.
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// Recursive functions need an explicit return type, the compiler can't infer it.
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// Here it's Unit.
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def showNumbersInRange(a:Int, b:Int):Unit = {
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print(a)
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if (a < b)
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showNumbersInRange(a + 1, b)
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}
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2014-11-12 01:42:28 +00:00
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showNumbersInRange(1,14)
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2013-07-29 17:33:03 +00:00
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2013-07-29 12:53:55 +00:00
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// Conditionals
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val x = 10
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if (x == 1) println("yeah")
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if (x == 10) println("yeah")
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if (x == 11) println("yeah")
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2013-08-01 09:11:15 +00:00
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if (x == 11) println ("yeah") else println("nay")
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2013-07-29 12:57:44 +00:00
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println(if (x == 10) "yeah" else "nope")
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val text = if (x == 10) "yeah" else "nope"
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2013-08-01 09:11:15 +00:00
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2014-11-12 08:36:19 +00:00
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/////////////////////////////////////////////////
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// 4. Data Structures
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/////////////////////////////////////////////////
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2013-08-01 09:11:15 +00:00
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2014-11-11 02:24:00 +00:00
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val a = Array(1, 2, 3, 5, 8, 13)
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a(0)
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a(3)
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a(21) // Throws an exception
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val m = Map("fork" -> "tenedor", "spoon" -> "cuchara", "knife" -> "cuchillo")
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m("fork")
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m("spoon")
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m("bottle") // Throws an exception
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val safeM = m.withDefaultValue("no lo se")
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safeM("bottle")
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val s = Set(1, 3, 7)
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s(0)
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s(1)
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/* Look up the documentation of map here -
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* http://www.scala-lang.org/api/current/index.html#scala.collection.immutable.Map
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* and make sure you can read it
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*/
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// Tuples
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(1, 2)
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(4, 3, 2)
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(1, 2, "three")
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|
|
(a, 2, "three")
|
|
|
|
|
|
|
|
// Why have this?
|
|
|
|
val divideInts = (x:Int, y:Int) => (x / y, x % y)
|
|
|
|
|
|
|
|
divideInts(10,3) // The function divideInts gives you the result and the remainder
|
|
|
|
|
|
|
|
// To access the elements of a tuple, use _._n where n is the 1-based index of
|
|
|
|
// the element
|
|
|
|
val d = divideInts(10,3)
|
|
|
|
|
|
|
|
d._1
|
|
|
|
|
|
|
|
d._2
|
|
|
|
|
|
|
|
|
2014-11-12 08:36:19 +00:00
|
|
|
/////////////////////////////////////////////////
|
|
|
|
// 5. Object Oriented Programming
|
|
|
|
/////////////////////////////////////////////////
|
2014-11-11 02:24:00 +00:00
|
|
|
|
|
|
|
/*
|
|
|
|
Aside: Everything we've done so far in this tutorial has been simple
|
|
|
|
expressions (values, functions, etc). These expressions are fine to type into
|
|
|
|
the command-line interpreter for quick tests, but they cannot exist by
|
|
|
|
themselves in a Scala file. For example, you cannot have just "val x = 5" in
|
|
|
|
a Scala file. Instead, the only top-level constructs allowed in Scala are:
|
|
|
|
|
|
|
|
- objects
|
|
|
|
- classes
|
|
|
|
- case classes
|
|
|
|
- traits
|
|
|
|
|
|
|
|
And now we will explain what these are.
|
|
|
|
*/
|
2013-07-29 12:53:55 +00:00
|
|
|
|
2014-11-14 23:01:16 +00:00
|
|
|
// classes are similar to classes in other languages. Constructor arguments are
|
|
|
|
// declared after the class name, and initialization is done in the class body.
|
2014-11-07 02:47:38 +00:00
|
|
|
class Dog(br: String) {
|
2014-11-14 23:01:16 +00:00
|
|
|
// Constructor code here
|
2014-11-05 22:06:34 +00:00
|
|
|
var breed: String = br
|
2014-11-14 23:01:16 +00:00
|
|
|
|
|
|
|
// Define a method called bark, returning a String
|
|
|
|
def bark = "Woof, woof!"
|
|
|
|
|
|
|
|
// Values and methods are assumed public. "protected" and "private" keywords
|
|
|
|
// are also available.
|
|
|
|
private def sleep(hours: Int) =
|
|
|
|
println(s"I'm sleeping for $hours hours")
|
|
|
|
|
|
|
|
// Abstract methods are simply methods with no body. If we uncomment the next
|
|
|
|
// line, class Dog would need to be declared abstract
|
|
|
|
// abstract class Dog(...) { ... }
|
|
|
|
// def chaseAfter(what: String): String
|
2013-08-01 09:11:15 +00:00
|
|
|
}
|
|
|
|
|
2014-11-05 22:06:34 +00:00
|
|
|
val mydog = new Dog("greyhound")
|
|
|
|
println(mydog.breed) // => "greyhound"
|
|
|
|
println(mydog.bark) // => "Woof, woof!"
|
|
|
|
|
|
|
|
|
2014-11-14 23:01:16 +00:00
|
|
|
// The "object" keyword creates a type AND a singleton instance of it. It is
|
|
|
|
// common for Scala classes to have a "companion object", where the per-instance
|
|
|
|
// behavior is captured in the classes themselves, but behavior related to all
|
|
|
|
// instance of that class go in objects. The difference is similar to class
|
|
|
|
// methods vs static methods in other languages. Note that objects and classes
|
|
|
|
// can have the same name.
|
|
|
|
object Dog {
|
|
|
|
def allKnownBreeds = List("pitbull", "shepherd", "retriever")
|
|
|
|
def createDog(breed: String) = new Dog(breed)
|
|
|
|
}
|
2013-08-01 09:11:15 +00:00
|
|
|
|
2013-07-29 13:09:11 +00:00
|
|
|
|
2014-11-14 23:01:16 +00:00
|
|
|
// Case classes are classes that have extra functionality built in. A common
|
|
|
|
// question for Scala beginners is when to use classes and when to use case
|
|
|
|
// classes. The line is quite fuzzy, but in general, classes tend to focus on
|
|
|
|
// encapsulation, polymorphism, and behavior. The values in these classes tend
|
|
|
|
// to be private, and only methods are exposed. The primary purpose of case
|
|
|
|
// classes is to hold immutable data. They often have few methods, and the
|
|
|
|
// methods rarely have side-effects.
|
|
|
|
case class Person(name: String, phoneNumber: String)
|
|
|
|
|
|
|
|
// Create a new instance. Note cases classes don't need "new"
|
|
|
|
val george = Person("George", "1234")
|
|
|
|
val kate = Person("Kate", "4567")
|
|
|
|
|
|
|
|
// With case classes, you get a few perks for free, like getters:
|
|
|
|
george.phoneNumber // => "1234"
|
|
|
|
|
|
|
|
// Per field equality (no need to override .equals)
|
|
|
|
Person("George", "1234") == Person("Kate", "1236") // => false
|
|
|
|
|
|
|
|
// Easy way to copy
|
|
|
|
// otherGeorge == Person("george", "9876")
|
|
|
|
val otherGeorge = george.copy(phoneNumber = "9876")
|
2013-07-29 13:09:11 +00:00
|
|
|
|
2014-11-14 23:01:16 +00:00
|
|
|
// And many others. Case classes also get pattern matching for free, see below.
|
2013-07-29 13:09:11 +00:00
|
|
|
|
|
|
|
|
2014-11-14 23:01:16 +00:00
|
|
|
// Traits coming soon!
|
2014-11-11 02:24:00 +00:00
|
|
|
|
2013-07-29 13:09:11 +00:00
|
|
|
|
2014-11-12 08:36:19 +00:00
|
|
|
/////////////////////////////////////////////////
|
|
|
|
// 6. Pattern Matching
|
|
|
|
/////////////////////////////////////////////////
|
2013-07-29 13:09:11 +00:00
|
|
|
|
2013-07-29 14:18:24 +00:00
|
|
|
val me = Person("George", "1234")
|
|
|
|
|
2013-08-02 16:39:30 +00:00
|
|
|
me match { case Person(name, number) => {
|
|
|
|
"We matched someone : " + name + ", phone : " + number }}
|
2013-07-29 14:18:24 +00:00
|
|
|
|
|
|
|
me match { case Person(name, number) => "Match : " + name; case _ => "Hm..." }
|
|
|
|
|
|
|
|
me match { case Person("George", number) => "Match"; case _ => "Hm..." }
|
|
|
|
|
|
|
|
me match { case Person("Kate", number) => "Match"; case _ => "Hm..." }
|
|
|
|
|
|
|
|
me match { case Person("Kate", _) => "Girl"; case Person("George", _) => "Boy" }
|
|
|
|
|
|
|
|
val kate = Person("Kate", "1234")
|
|
|
|
|
|
|
|
kate match { case Person("Kate", _) => "Girl"; case Person("George", _) => "Boy" }
|
|
|
|
|
|
|
|
|
2013-07-29 13:09:11 +00:00
|
|
|
|
|
|
|
// Regular expressions
|
2013-08-02 16:39:30 +00:00
|
|
|
val email = "(.*)@(.*)".r // Invoking r on String makes it a Regex
|
2014-08-08 13:23:46 +00:00
|
|
|
val serialKey = """(\d{5})-(\d{5})-(\d{5})-(\d{5})""".r // Using verbatim (multiline) syntax
|
2014-08-08 13:19:14 +00:00
|
|
|
|
|
|
|
val matcher = (value: String) => {
|
|
|
|
println(value match {
|
|
|
|
case email(name, domain) => s"It was an email: $name"
|
|
|
|
case serialKey(p1, p2, p3, p4) => s"Serial key: $p1, $p2, $p3, $p4"
|
|
|
|
case _ => s"No match on '$value'" // default if no match found
|
|
|
|
})
|
2013-07-29 17:33:03 +00:00
|
|
|
}
|
|
|
|
|
2014-08-15 13:47:12 +00:00
|
|
|
matcher("mrbean@pyahoo.com") // => "It was an email: mrbean"
|
|
|
|
matcher("nope..") // => "No match on 'nope..'"
|
|
|
|
matcher("52917") // => "No match on '52917'"
|
|
|
|
matcher("52752-16432-22178-47917") // => "Serial key: 52752, 16432, 22178, 47917"
|
2013-07-29 14:18:24 +00:00
|
|
|
|
|
|
|
|
2014-11-12 08:36:19 +00:00
|
|
|
/////////////////////////////////////////////////
|
|
|
|
// 7. Functional Programming
|
|
|
|
/////////////////////////////////////////////////
|
2013-07-29 13:09:11 +00:00
|
|
|
|
2014-11-11 02:24:00 +00:00
|
|
|
// Scala allows methods and functions to return, or take as parameters, other
|
|
|
|
// functions or methods.
|
2013-08-02 16:39:30 +00:00
|
|
|
|
2014-11-11 02:24:00 +00:00
|
|
|
List(1, 2, 3) map add10 // List(11, 12, 13) - add10 is applied to each element
|
2013-08-01 09:11:15 +00:00
|
|
|
|
2014-11-11 02:24:00 +00:00
|
|
|
// Anonymous functions can be used instead of named functions:
|
|
|
|
List(1, 2, 3) map (x => x + 10)
|
2013-07-29 15:03:49 +00:00
|
|
|
|
2014-11-11 02:24:00 +00:00
|
|
|
// And the underscore symbol, can be used if there is just one argument to the
|
|
|
|
// anonymous function. It gets bound as the variable
|
|
|
|
List(1, 2, 3) map (_ + 10)
|
2013-07-29 15:03:49 +00:00
|
|
|
|
2014-11-11 02:24:00 +00:00
|
|
|
// If the anonymous block AND the function you are applying both take one
|
|
|
|
// argument, you can even omit the underscore
|
|
|
|
List("Dom", "Bob", "Natalia") foreach println
|
2014-08-08 13:19:14 +00:00
|
|
|
|
|
|
|
|
2014-11-11 02:24:00 +00:00
|
|
|
// Combinators
|
2013-07-29 15:03:49 +00:00
|
|
|
|
2014-11-11 02:24:00 +00:00
|
|
|
s.map(sq)
|
2013-07-29 17:33:03 +00:00
|
|
|
|
2014-11-11 02:24:00 +00:00
|
|
|
val sSquared = s. map(sq)
|
|
|
|
|
|
|
|
sSquared.filter(_ < 10)
|
|
|
|
|
|
|
|
sSquared.reduce (_+_)
|
|
|
|
|
|
|
|
// The filter function takes a predicate (a function from A -> Boolean) and
|
|
|
|
// selects all elements which satisfy the predicate
|
|
|
|
List(1, 2, 3) filter (_ > 2) // List(3)
|
2014-12-21 12:08:21 +00:00
|
|
|
case class Person(name:String, age:Int)
|
2014-11-11 02:24:00 +00:00
|
|
|
List(
|
|
|
|
Person(name = "Dom", age = 23),
|
|
|
|
Person(name = "Bob", age = 30)
|
|
|
|
).filter(_.age > 25) // List(Person("Bob", 30))
|
|
|
|
|
|
|
|
|
|
|
|
// Scala a foreach method defined on certain collections that takes a type
|
|
|
|
// returning Unit (a void method)
|
2014-11-12 01:42:28 +00:00
|
|
|
val aListOfNumbers = List(1, 2, 3, 4, 10, 20, 100)
|
2014-11-11 02:24:00 +00:00
|
|
|
aListOfNumbers foreach (x => println(x))
|
|
|
|
aListOfNumbers foreach println
|
|
|
|
|
|
|
|
// For comprehensions
|
|
|
|
|
|
|
|
for { n <- s } yield sq(n)
|
|
|
|
|
|
|
|
val nSquared2 = for { n <- s } yield sq(n)
|
|
|
|
|
|
|
|
for { n <- nSquared2 if n < 10 } yield n
|
|
|
|
|
|
|
|
for { n <- s; nSquared = n * n if nSquared < 10} yield nSquared
|
|
|
|
|
|
|
|
/* NB Those were not for loops. The semantics of a for loop is 'repeat', whereas
|
|
|
|
a for-comprehension defines a relationship between two sets of data. */
|
|
|
|
|
|
|
|
|
2014-11-12 08:36:19 +00:00
|
|
|
/////////////////////////////////////////////////
|
|
|
|
// 8. Implicits
|
|
|
|
/////////////////////////////////////////////////
|
2013-07-29 17:33:03 +00:00
|
|
|
|
2014-11-27 07:38:54 +00:00
|
|
|
/* WARNING WARNING: Implicits are a set of powerful features of Scala, and
|
|
|
|
* therefore it is easy to abuse them. Beginners to Scala should resist the
|
|
|
|
* temptation to use them until they understand not only how they work, but also
|
|
|
|
* best practices around them. We only include this section in the tutorial
|
|
|
|
* because they are so commonplace in Scala libraries that it is impossible to
|
|
|
|
* do anything meaningful without using a library that has implicits. This is
|
|
|
|
* meant for you to understand and work with implicts, not declare your own.
|
|
|
|
*/
|
|
|
|
|
|
|
|
// Any value (vals, functions, objects, etc) can be declared to be implicit by
|
|
|
|
// using the, you guessed it, "implicit" keyword. Note we are using the Dog
|
|
|
|
// class from section 5 in these examples.
|
|
|
|
implicit val myImplicitInt = 100
|
|
|
|
implicit def myImplicitFunction(breed: String) = new Dog("Golden " + breed)
|
|
|
|
|
|
|
|
// By itself, implicit keyword doesn't change the behavior of the value, so
|
|
|
|
// above values can be used as usual.
|
|
|
|
myImplicitInt + 2 // => 102
|
|
|
|
myImplicitFunction("Pitbull").breed // => "Golden Pitbull"
|
|
|
|
|
2014-11-27 07:40:32 +00:00
|
|
|
// The difference is that these values are now eligible to be used when another
|
2014-11-27 07:38:54 +00:00
|
|
|
// piece of code "needs" an implicit value. One such situation is implicit
|
|
|
|
// function arguments:
|
|
|
|
def sendGreetings(toWhom: String)(implicit howMany: Int) =
|
|
|
|
s"Hello $toWhom, $howMany blessings to you and yours!"
|
|
|
|
|
|
|
|
// If we supply a value for "howMany", the function behaves as usual
|
|
|
|
sendGreetings("John")(1000) // => "Hello John, 1000 blessings to you and yours!"
|
|
|
|
|
|
|
|
// But if we omit the implicit parameter, an implicit value of the same type is
|
|
|
|
// used, in this case, "myImplicitInt":
|
|
|
|
sendGreetings("Jane") // => "Hello Jane, 100 blessings to you and yours!"
|
|
|
|
|
|
|
|
// Implicit function parameters enable us to simulate type classes in other
|
|
|
|
// functional languages. It is so often used that it gets its own shorthand. The
|
|
|
|
// following two lines mean the same thing:
|
|
|
|
def foo[T](implicit c: C[T]) = ...
|
|
|
|
def foo[T : C] = ...
|
|
|
|
|
|
|
|
|
|
|
|
// Another situation in which the compiler looks for an implicit is if you have
|
|
|
|
// obj.method(...)
|
|
|
|
// but "obj" doesn't have "method" as a method. In this case, if there is an
|
|
|
|
// implicit conversion of type A => B, where A is the type of obj, and B has a
|
|
|
|
// method called "method", that conversion is applied. So having
|
|
|
|
// myImplicitFunction above in scope, we can say:
|
|
|
|
"Retriever".breed // => "Golden Retriever"
|
|
|
|
"Sheperd".bark // => "Woof, woof!"
|
|
|
|
|
|
|
|
// Here the String is first converted to Dog using our function above, and then
|
|
|
|
// the appropriate method is called. This is an extremely powerful feature, but
|
|
|
|
// again, it is not to be used lightly. In fact, when you defined the implicit
|
|
|
|
// function above, your compiler should have given you a warning, that you
|
|
|
|
// shouldn't do this unless you really know what you're doing.
|
2013-07-29 17:33:03 +00:00
|
|
|
|
2013-08-01 09:11:15 +00:00
|
|
|
|
2014-11-12 08:36:19 +00:00
|
|
|
/////////////////////////////////////////////////
|
|
|
|
// 9. Misc
|
|
|
|
/////////////////////////////////////////////////
|
2013-08-01 09:11:15 +00:00
|
|
|
|
|
|
|
// Importing things
|
|
|
|
import scala.collection.immutable.List
|
|
|
|
|
|
|
|
// Import all "sub packages"
|
|
|
|
import scala.collection.immutable._
|
|
|
|
|
|
|
|
// Import multiple classes in one statement
|
|
|
|
import scala.collection.immutable.{List, Map}
|
|
|
|
|
|
|
|
// Rename an import using '=>'
|
2013-09-28 13:33:32 +00:00
|
|
|
import scala.collection.immutable.{ List => ImmutableList }
|
2013-08-01 09:11:15 +00:00
|
|
|
|
|
|
|
// Import all classes, except some. The following excludes Map and Set:
|
|
|
|
import scala.collection.immutable.{Map => _, Set => _, _}
|
|
|
|
|
2013-08-02 16:39:30 +00:00
|
|
|
// Your programs entry point is defined in an scala file using an object, with a
|
|
|
|
// single method, main:
|
2013-08-01 09:11:15 +00:00
|
|
|
object Application {
|
|
|
|
def main(args: Array[String]): Unit = {
|
|
|
|
// stuff goes here.
|
|
|
|
}
|
|
|
|
}
|
|
|
|
|
|
|
|
// Files can contain multiple classes and objects. Compile with scalac
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
2013-07-29 15:03:49 +00:00
|
|
|
// Input and output
|
|
|
|
|
2013-08-01 09:11:15 +00:00
|
|
|
// To read a file line by line
|
|
|
|
import scala.io.Source
|
2013-12-30 07:45:15 +00:00
|
|
|
for(line <- Source.fromFile("myfile.txt").getLines())
|
2013-08-01 09:11:15 +00:00
|
|
|
println(line)
|
|
|
|
|
|
|
|
// To write a file use Java's PrintWriter
|
2014-08-08 13:19:14 +00:00
|
|
|
val writer = new PrintWriter("myfile.txt")
|
|
|
|
writer.write("Writing line for line" + util.Properties.lineSeparator)
|
|
|
|
writer.write("Another line here" + util.Properties.lineSeparator)
|
|
|
|
writer.close()
|
2013-07-29 15:03:49 +00:00
|
|
|
|
|
|
|
```
|
|
|
|
|
|
|
|
## Further resources
|
|
|
|
|
|
|
|
[Scala for the impatient](http://horstmann.com/scala/)
|
|
|
|
|
2013-08-07 21:31:49 +00:00
|
|
|
[Twitter Scala school](http://twitter.github.io/scala_school/)
|
2013-07-29 15:03:49 +00:00
|
|
|
|
2013-08-07 21:31:49 +00:00
|
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[The scala documentation](http://docs.scala-lang.org/)
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[Try Scala in your browser](http://scalatutorials.com/tour/)
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2013-07-29 13:09:11 +00:00
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2013-08-01 09:11:15 +00:00
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Join the [Scala user group](https://groups.google.com/forum/#!forum/scala-user)
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