Scala basic

Scala training workshop 01
Nguyen Thanh Tuan
Platform Department
@Septeni Technology

Agenda
- Summary
- Functions & Evaluations
- Higher Order Functions
- Data and Abstraction
- Exercise
- Share

Functions & Evaluations
- Conditionals and Value Definitions
- Blocks
- Tail recursion

Conditional and Value Definition
- Sample :
val <identifier>[: <type>] = <expression>
var <identifier>[: <type>] = <expression>
- If Expressions
if (<Boolean expression>) <expression>
- If-Else Expressions
if (<Boolean expression>) <expression>
else <expression>
- Match Expressions
<expression> match {
case <pattern match> => <expression>
[case...]
}
- A Pattern Alternative :
case <pattern 1> | <pattern 2> .. => <one or more expressions>
- Matching with Wildcard Patterns
case <identifier> => <one or more expressions>
- Matching with Pattern Guards
case <pattern> if <Boolean expression> => <one or more
expressions>
- Match Expressions
<expression> match {
case <pattern match> => <expression>
[case...]
}
Source : Safaribooksonline.com

Example
- If Expressions
if (10%2 == 0) println(“10 is a multiple of 2”)
- If-Else Expressions
val x = 10;
val y = 5
if (x > y) println(“Max number is :”+ x)
else println(“Max numner is :” + y)
- Match Expressions
val x = 10;
val y = 5;
val max = x > y match {
case true => x
case true => y
}
- A Pattern Alternative :
val day = “MON”
val kind = day match {
case “MON”| ”TUE” | “WED” | “THU” | “FRI”
=>
“weekday”
case “SAT” | “SUN” =>
“weekend”
}
- Matching with Wildcard Patterns
case _ => {println(s”Couldn’t parse $message”) -1}
- Matching with Pattern Guards
case 3 if 3 > 5=> println(“Something”)
Source : Safaribooksonline.com

Blocks
val x = 0
def f(y: Int) = y + 1
val result = {
val x = f(3)
x * x
} + x
- A block is delimited by braces { ...
}
- It contains a sequence of
definitions or expressions
- Blocks are themselves
expressions; a block may appear
everywhere an expression can
- The definitions inside a block are
only visible from within the block.

Tail recursion
- Recursion definition : A call B, B call A ..etc
- Reasons we don’t see alot of recursion code in Java
- is hard ( is not intuitive : you see one layer and you have to
imagine what happens when those layers stack up)
- is not designed to accommodate recursion. It’s designed to
accommodate iteration.
- But in Scala, being a function language is very much geared toward
recursion rather than iteration ( Scala in the case of tail recursive,
can eliminate the creation of a new stack frame and just re-use the
current stack frame)

Example
def pascal(c: Int, r: Int): Int = {
if(c == 0) return 1
if(c == 1 && c != r) return r
if(c == r) return 1
pascal(c-1,r-1)+pascal(c,r-1)
}

Tail&Head recursion in Scala
def listLength2(list: List[_]): Int =
{
def listLength2Helper(list: List[_],
len: Int): Int = {
if (list == Nil) len
else listLength2Helper(list.tail,
len + 1)
}
listLength2Helper(list, 0)
}
var list1 = List(1, 2, 3, 4, 5, 6, 7,
8, 9, 10)
println( listLength2( list1 ) )
def listLength1(list: List[_]): Int =
{
if (list == Nil) 0
else 1 + listLength1(list.tail)
}
var list1 = List(1, 2, 3, 4, 5, 6, 7,
8, 9, 10)
println( listLength1( list1 ) )

Higher-order function
- Higher Order Functions
- Currying

Higher-order function
Scala allows the definition of higher-order
functions.
- Functions can take other functions as
parameters
- Result is a function
- Example

Example higherOrder
object higherOrder {
def main(args: Array[String]) {
println( apply( layout, 10) )
}
def apply(f: Int => String, v: Int) = f(v)
def layout[A](x: A) = "[" + x.toString() + "]
}

Currying Functions
Currying transforms a function that takes
multiple parameters into a chain of function,
each taking a single parameter. Curried
functions are defined with multiple parameter
lists, as follows :
def strcat(s1: String)(s2: String) = s1 + s2
OR
def strcat(s1: String) = (s2: String) => s1 + s2

Example Currying functions
object Currying {
val str1:String = "Hello, "
val str2:String = "Scala!"
println( "str1 + str2 = " + strcat(str1)(str2) )
}
def strcat(s1: String)(s2: String) = {
s1 + s2
}
}

Data and Abstraction
- Class hierarchy
- Trait and Abstractclass

Class hierarchy
Source : meetfp.com

Trait and Abstract class
- Scala has traits, and a trait is more flexible than an abstract class, so you
wonder, “When should I use an abstract class?”
- Reason is :
- You want to create a base class that requires constructor arguments
- The code will be called from Java code
- We can use
abstract class Animal(name: String)
- But we can’t use
trait Animal(name: String)

Trait ( The reason we should use trait )
- One big advantage of traits is that you can extend multiple traits but only
one abstract class. Traits solve many of the problems with multiple
inheritance but allow code reuse.
- Define types by specifying the signatures of supported methods.
This is similar to how interfaces work in Java.
But trait can’t
- Traits do not have constructor parameters (but this should not be an issue in practice).
- Traits impose a slight performance overhead (but this is unlikely to impact the overall
performance of your program)
- Traits introduce compilation fragility for classes that mix them in. If a trait changes, a class that
mixes it in has to be recompiled

Exercises
- Pascal’s Triangle
The following pattern of numbers is called Pascal’s triangle.
1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
...
The numbers at the edge of the triangle are all 1, and each number inside the triangle is the sum of the two numbers above it. Write a function
that computes the elements of Pascal’s triangle by means of a recursive process.
Do this exercise by implementing the pascal function in Main.scala, which takes a column c and a row r, counting from 0 and returns the number
at that spot in the triangle. For example: pascal(0,2)=1, pascal(1,2)=2 and pascal(1,3)=3.
def pascal(c: Int, r: Int): Int
Source : coursera.org

Exercises
object Main {
println("Pascal's Triangle")
for (row <- 0 to 10) {
for (col <- 0 to row)
print(pascal(col, row) + " ")
println()
}
def pascal(c: Int, r: Int): Int = {
c match {
case 0 => 1
case 1 if c == 1 && c != r => r
case r => 1
case _ => pascal(c-1,r-1)+pascal(c,r-1)
}
}
}

Scala basic

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