20110515 scala by_example

Scala via Examples

Hu Zi Ming

[email protected]

201l-05-15

1 / 21Scala via Examples

Outline

1 What is Scala

2 GCD Algorithm

3 Grep

4 Advanced Scala

What is Scala

JVM based language

Combine OOP and FP

Scalable on architecture and performance

What is Scala

JVM based language

Combine OOP and FP

What is Scala

JVM based language

Combine OOP and FP

Outline

1 What is Scala

2 GCD Algorithm

3 Grep

4 Advanced Scala

GCD Algorithm

Greaest Common Divisor (gcd.scala)

def gcd (m: Int, n: Int): Int = (m, n) match {case (m, ‘m‘) => mcase (m, n) if n > m => gcd(n, m)case (m, 0) => mcase _ => gcd(n, m % n)

println(gcd(20, 30))

Compile and Run it

Method 1:

scala script-name

Method 2 (Need main method in object object):

scalac script-name

scala object-name

Compile and Run it

Method 1:

scala script-name

Method 2 (Need main method in object object):

scalac script-name

scala object-name

Cases for scala and scalac

Running as scala foo.scala

println("Foo")

Running as scalac foo.scala; scala Foo

object Foo {def main(args: Array[String]) = println("Foo")

java.lang.NoSuchMethodException: Foo.main is not static

class Foo {def main(args: Array[String]) = println("Foo")

Declare a method

Start with def keyword

Then function name

Followed by optional argument list

Then return type follower by a colon

Equal sign and method body

Declare a method

Then function name

Declare a method

Then function name

Declare a method

Then function name

Declare a method

Then function name

Outline

1 What is Scala

2 GCD Algorithm

3 Grep

4 Advanced Scala

Grep (grep.scala)

import scala.io.Source

class Greper(pattern: String, filename: String) {def execute = {

for (line <- Source.fromFile(filename).getLines) {if (line.trim.matches(pattern))

println(line)}

object LearnScala {def main(args: Array[String]) = {

val pattern = args(0)val filename = args(1)

val foo = new Greper(pattern, filename)foo.execute

Singleton

There is no static method in Scala

But singleton object

Singleton object can not be created by new

Singleton object can not accept arguments

Singleton object will be created when first accessed

Singleton

Command Line Arguments

Cmd line arguments will be passed into main function

The main func should be contained in singleton object

java.lang.NoSuchMethodException: Foo.main is not static

object Foo {def main(args: Array[String]) = args.foreach(println)

Immutable VS. Mutable

Variable can be declared in two types

val and var

val means value, immutable

var means variable, mutable

Immutable variable can be changed by reference

Like constant pointer in C/C++

val and var

List and Tuple

Basic containers in scala

List object and its element are immutable

List can only contains elements in same type

Combine two list means create a new one

Element in list can be accessed via [idx]

Tuple object and its element are immutable

Tuple can contains elements in different type

Tuple using 1-based index

Element in Tuple can be accessed via idx

List and Tuple

Object is Everything

Every function is value, and every value is object

Hence everything is object

1+2*3/x means 1.+(2.*(3./(x)))

1::2::3::4::Nil means 1.::(2.::(3.::(4.::(Nil))))

1+2*3/x means 1.+(2.*(3./(x)))

1::2::3::4::Nil means 1.::(2.::(3.::(4.::(Nil))))

1+2*3/x means 1.+(2.*(3./(x)))

1::2::3::4::Nil means 1.::(2.::(3.::(4.::(Nil))))

Outline

1 What is Scala

2 GCD Algorithm

3 Grep

4 Advanced Scala

Operators and Order

Operators can be defined by user

Some syntactic sugar can be implemented by this

Operator ends with ‘:’ means right associative

a * b yields a.*(b)

While a ::: b yields b.:::(a)

Operators and Order

Operators can be defined by user

Some syntactic sugar can be implemented by this

Operator ends with ‘:’ means right associative

a * b yields a.*(b)

While a ::: b yields b.:::(a)

Case Class

Scalas way to allow pattern matching on objects

Curry Example

Plain-old function

def foo(x: Int, y: Int) = x + yfoo(1, 2)

Curried function

def bar(x: Int)(y: Int) = x + ybar(1)(2)

Curry function bar means two traditional func invocation backto back

First func take Int x as parameter

And return a func value for second func

This func take the Int y as parameter

Func declared in other method

def bar2_first(x: Int) = (y: Int) => x + ydef bar2_second = bar2_first(1)bar2_second(2)

Q AND A

20110515 scala by_example

Technology

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