Scala Basics Tutorial (ix): extractor, File I/o_scala

Pattern matching using extractor:

When an instance of a class is followed by parentheses using a list of 0 or more parameters, the compiler invokes the applied method on that instance. We can define both objects and classes.

As mentioned above, the purpose of the Unapply method is to extract a particular value from what we are looking for. It operates in the opposite way as apply. An extract object that compares the Unapply method in a matching statement is automatically executed, as follows:

Object Test {

def main (args:array[string]) {

Val x = Test (5)

println (x)

Match X

{

Case Test (num) => println (x+ "are bigger two times than" +num)

Unapply is invoked

Case _ => println ("I cannot calculate")

}

}

def apply (x:int) = X*2

def unapply (Z:int): option[int] = if (z%2==0) Some (Z/2) Else None

}

Let's compile and run the above program, which will produce the following results:

C:/>scalac Test.scala

C:/>scala Test

10

Bigger two times than 5

C:/>

Scala opens files using Java objects and java.io.File that can be used to read and write files in Scala programming. Here is an example of writing to a file:

Import Java.io._

Object Test {

def main (args:array[string]) {

Val writer = new PrintWriter (New File ("Test.txt"))

Writer.write ("Hello Scala")

Writer.close ()

}

}

When the above code is compiled and executed, it creates a file with "Hello Scala" content.

C:/>scalac Test.scala

C:/>scala Test

C:/>

To read a line from the screen:

Sometimes you need to read user input from the screen, and then do some further processing. The following example shows how to read input from the screen:

Object Test {

def main (args:array[string]) {

Print ("Please enter your input:")

Val line = Console.ReadLine

println ("Hi, you just typed:" + line)

}

}

When the above code is compiled and executed, it prompts for the content and continues until the ENTER key is pressed.

C:/>scalac Test.scala

C:/>scala Test

Scala Test

Please enter your Input:scala are great

Just Typed:scala is great

C:/>

Read the contents of the file:

Reading from a file is very simple. You can use Scala's source class and its companion object to read files. Here's an example of how these show how to read from a previous "test.txt" file:

Import Scala.io.Source

Object Test {

def main (args:array[string]) {

println ("Following is the content read:")

}

}

}

When the code above is compiled and executed, it reads the Test.txt file and displays the contents on the screen:

C:/>scalac Test.scala

C:/>scala Test

Scala Test

Following is the content read:

Hello Scala

C:/>

from:http://www.yiibai.com/scala/scala_basic_syntax.html