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Merge pull request #857 from geoffliu/master
[Scala/en] Reorganize the Scala tutorial
This commit is contained in:
Adam Bard
commit
493cff577c
@ -4,6 +4,7 @@ filename: learnscala.scala
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contributors:
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- ["George Petrov", "http://github.com/petrovg"]
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- ["Dominic Bou-Samra", "http://dbousamra.github.com"]
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- ["Geoff Liu", "http://geoffliu.me"]
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filename: learn.scala
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---
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@ -20,34 +21,47 @@ Scala - the scalable language
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scala>
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This is the so called REPL. You can run commands in the REPL. Let's do just
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that:
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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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*/
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println(10) // prints the integer 10
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println("Boo!") // printlns the string Boo!
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/////////////////////////////////////////////////
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// 1. Basics
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/////////////////////////////////////////////////
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// Single line comments start with two forward slashes
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// Some basics
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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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// Printing, and forcing a new line on the next print
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println("Hello world!")
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println(10)
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// Printing, without forcing a new line on next print
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print("Hello world")
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// Declaring values is done using either var or val
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// Declaring values is done using either var or val.
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// val declarations are immutable, whereas var's are mutable. Immutability is
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// a good thing.
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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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var x = 10
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x = 20 // x is now 20
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var y = 10
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y = 20 // y is now 20
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// Single line comments start with two forward slashes
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/*
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Multi line comments look like this.
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Scala is a statically typed language, yet note that in the above declarations, we did not specify
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a type. This is due to a language feature called type inference. In most cases, Scala compiler can
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guess what the type of a variable is, so you don't have to type it every time. We can explicitly
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declare the type of a variable like so:
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*/
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val z: Int = 10
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val a: Double = 1.0
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val b: Double = 10 // Notice automatic conversion from Int to Double, result is 10.0, not 10
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// Boolean values
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true
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@ -64,9 +78,11 @@ true == false // false
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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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6 / 4 // 1
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6.0 / 4 // 1.5
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// Evaluating a command in the REPL gives you the type and value of the result
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// Evaluating an expression in the REPL gives you the type and value of the result
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1 + 7
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@ -78,13 +94,53 @@ true == false // false
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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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1+7 will give you the same result
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Note that "res29" is a sequentially generated variable name to store the results of the
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expressions you typed, your output may differ.
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*/
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"Scala strings are surrounded by double quotes"
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'a' // A Scala Char
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// 'Single quote strings don't exist' <= This causes an error
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// Everything is an object, including a function. Type these in the REPL:
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// Strings have the usual Java methods defined on them
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"hello world".length
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"hello world".substring(2, 6)
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"hello world".replace("C", "3")
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7 // results in res30: Int = 7 (res30 is just a generated var name for the result)
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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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"hello world".drop(5)
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// String interpolation: notice the prefix "s"
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val n = 45
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s"We have $n apples" // => "We have 45 apples"
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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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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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// 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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f"Square root of 122: ${math.sqrt(122)}%1.4f" // "Square root of 122: 11.0454"
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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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// 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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// 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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<input type="submit">
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</form>"""
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/////////////////////////////////////////////////
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// 2. Functions
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/////////////////////////////////////////////////
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// The next line gives you a function that takes an Int and returns it squared
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(x:Int) => x * x
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@ -108,25 +164,65 @@ sq(10) // Gives you this: res33: Int = 100.
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// taking an Int and returning an Int.
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val add10: Int => Int = _ + 10
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// Scala allows methods and functions to return, or take as parameters, other
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// functions or methods.
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List(1, 2, 3) map add10 // List(11, 12, 13) - add10 is applied to each element
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/////////////////////////////////////////////////
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// 3. Flow Control
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/////////////////////////////////////////////////
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// Anonymous functions can be used instead of named functions:
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List(1, 2, 3) map (x => x + 10)
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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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// And the underscore symbol, can be used if there is just one argument to the
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// anonymous function. It gets bound as the variable
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List(1, 2, 3) map (_ + 10)
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r foreach println
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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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// If the anonymous block AND the function you are applying both take one
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// argument, you can even omit the underscore
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List("Dom", "Bob", "Natalia") foreach println
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(5 to 1 by -1) foreach ( println )
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// A while loops
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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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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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// fast, faster that Java // loops, but using the combinators and
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// comprehensions above is easier to understand and parallelize
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// A do while loop
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do {
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println("x is still less than 10");
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x += 1
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} while (x < 10)
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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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showNumbersInRange(1,14)
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// Conditionals
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// Data structures
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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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if (x == 11) println ("yeah") else println("nay")
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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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/////////////////////////////////////////////////
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// 4. Data Structures
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/////////////////////////////////////////////////
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val a = Array(1, 2, 3, 5, 8, 13)
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a(0)
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@ -175,110 +271,25 @@ d._1
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d._2
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/////////////////////////////////////////////////
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// 5. Object Oriented Programming
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/////////////////////////////////////////////////
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// Combinators
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/*
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Aside: Everything we've done so far in this tutorial has been simple
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expressions (values, functions, etc). These expressions are fine to type into
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the command-line interpreter for quick tests, but they cannot exist by
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themselves in a Scala file. For example, you cannot have just "val x = 5" in
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a Scala file. Instead, the only top-level constructs allowed in Scala are:
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s.map(sq)
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- objects
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- classes
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- case classes
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- traits
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val sSquared = s. map(sq)
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And now we will explain what these are.
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*/
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sSquared.filter(_ < 10)
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sSquared.reduce (_+_)
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// The filter function takes a predicate (a function from A -> Boolean) and
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// selects all elements which satisfy the predicate
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List(1, 2, 3) filter (_ > 2) // List(3)
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case class Person(name: String, age: Int)
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List(
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Person(name = "Dom", age = 23),
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Person(name = "Bob", age = 30)
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).filter(_.age > 25) // List(Person("Bob", 30))
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// Scala a foreach method defined on certain collections that takes a type
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// returning Unit (a void method)
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val aListOfNumbers = Set(1,2,3,45,234)
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aListOfNumbers foreach (x => println(x))
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aListOfNumbers foreach println
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// For comprehensions
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for { n <- s } yield sq(n)
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val nSquared2 = for { n <- s } yield sq(n)
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for { n <- nSquared2 if n < 10 } yield n
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for { n <- s; nSquared = n * n if nSquared < 10} yield nSquared
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/* NB Those were not for loops. The semantics of a for loop is 'repeat', whereas
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a for-comprehension defines a relationship between two sets of data. */
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// Loops and iteration
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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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r foreach println
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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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(5 to 1 by -1) foreach ( println )
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// A while loops
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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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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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// fast, faster that Java // loops, but using the combinators and
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// comprehensions above is easier to understand and parallelize
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// A do while loop
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do {
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println("x is still less than 10");
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x += 1
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} while (x < 10)
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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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showNumbersInRange(1,14)
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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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if (x == 11) println ("yeah") else println("nay")
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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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// Object oriented features
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// Classname is Dog
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class Dog(br: String) {
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var breed: String = br
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//A method called bark, returning a String
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@ -296,8 +307,6 @@ println(mydog.bark) // => "Woof, woof!"
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// Classes can contain nearly any other construct, including other classes,
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// functions, methods, objects, case classes, traits etc.
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// Case classes
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case class Person(name:String, phoneNumber:String)
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@ -305,8 +314,12 @@ case class Person(name:String, phoneNumber:String)
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Person("George", "1234") == Person("Kate", "1236")
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// Objects and traits coming soon!
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// Pattern matching
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/////////////////////////////////////////////////
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// 6. Pattern Matching
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/////////////////////////////////////////////////
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val me = Person("George", "1234")
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@ -345,49 +358,77 @@ matcher("52917") // => "No match on '52917'"
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matcher("52752-16432-22178-47917") // => "Serial key: 52752, 16432, 22178, 47917"
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// Strings
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/////////////////////////////////////////////////
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// 7. Functional Programming
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/////////////////////////////////////////////////
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"Scala strings are surrounded by double quotes" //
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'a' // A Scala Char
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'Single quote strings don't exist' // Error
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"Strings have the usual Java methods defined on them".length
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"They also have some extra Scala methods.".reverse
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// Scala allows methods and functions to return, or take as parameters, other
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// functions or methods.
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// Seealso: scala.collection.immutable.StringOps
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List(1, 2, 3) map add10 // List(11, 12, 13) - add10 is applied to each element
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println("ABCDEF".length)
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println("ABCDEF".substring(2, 6))
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println("ABCDEF".replace("C", "3"))
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// Anonymous functions can be used instead of named functions:
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List(1, 2, 3) map (x => x + 10)
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// String interpolation
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val n = 45
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println(s"We have $n apples") // => "We have 45 apples"
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// And the underscore symbol, can be used if there is just one argument to the
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// anonymous function. It gets bound as the variable
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List(1, 2, 3) map (_ + 10)
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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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println(s"My second daughter is ${a(0) - a(2)} years old.") // => "My second daughter is 5 years old."
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println(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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println(s"Power of 2: ${math.pow(2, 2)}") // => "Power of 2: 4"
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// Formatting with interpolated strings (note the prefixed f)
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println(f"Power of 5: ${math.pow(5, 2)}%1.0f") // "Power of 5: 25"
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println(f"Square root of 122: ${math.sqrt(122)}%1.4f") // "Square root of 122"
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// Ignoring special characters.
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println(raw"New line feed: \n. Carriage return: \r.") // => "New line feed: \n. Carriage return: \r."
|
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|
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// Some characters need to be 'escaped', e.g. a double quote inside a string:
|
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val a = "They stood outside the \"Rose and Crown\"" // => "They stood outside the "Rose and Crown""
|
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|
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// Triple double-quotes let strings span multiple rows and contain quotes
|
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val html = """<form id="daform">
|
||||
<p>Press belo', Joe</p>
|
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| <input type="submit">
|
||||
</form>"""
|
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// If the anonymous block AND the function you are applying both take one
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// argument, you can even omit the underscore
|
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List("Dom", "Bob", "Natalia") foreach println
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|
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// Combinators
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// Application structure and organization
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s.map(sq)
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val sSquared = s. map(sq)
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sSquared.filter(_ < 10)
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sSquared.reduce (_+_)
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// The filter function takes a predicate (a function from A -> Boolean) and
|
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// selects all elements which satisfy the predicate
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List(1, 2, 3) filter (_ > 2) // List(3)
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case class Person(name:String, phoneNumber:String)
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List(
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Person(name = "Dom", age = 23),
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Person(name = "Bob", age = 30)
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).filter(_.age > 25) // List(Person("Bob", 30))
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|
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|
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// Scala a foreach method defined on certain collections that takes a type
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// returning Unit (a void method)
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val aListOfNumbers = List(1, 2, 3, 4, 10, 20, 100)
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aListOfNumbers foreach (x => println(x))
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aListOfNumbers foreach println
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// For comprehensions
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for { n <- s } yield sq(n)
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val nSquared2 = for { n <- s } yield sq(n)
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for { n <- nSquared2 if n < 10 } yield n
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for { n <- s; nSquared = n * n if nSquared < 10} yield nSquared
|
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|
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/* NB Those were not for loops. The semantics of a for loop is 'repeat', whereas
|
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a for-comprehension defines a relationship between two sets of data. */
|
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|
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|
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/////////////////////////////////////////////////
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// 8. Implicits
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/////////////////////////////////////////////////
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Coming soon!
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/////////////////////////////////////////////////
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// 9. Misc
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/////////////////////////////////////////////////
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// Importing things
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import scala.collection.immutable.List
|
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|
||||
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