CS205: Scalable Software Systems

Transcription

1 CS205: Scalable Software Systems Lecture 4 September 14, 2016 Lecture 4 CS205: Scalable Software Systems September 14, / 16

2 Quick Recap Things covered so far Problem solving by recursive decomposition of a problem into sub problems Type of recursive solutions: Tail recursive- Non tail recursive Introduction to Scala Converting mathematical algorithms to scala programs Lecture 4 CS205: Scalable Software Systems September 14, / 16

3 Translating our solutions to programs Translating our solutions to a program What all do we need? 1 how to define a function? 2 how to call a function? 3 how to write boolean expressions using variables and constants? 4 how to represent lists? 5 how to return a value from a function? 6 how to do a case analysis? Lecture 4 CS205: Scalable Software Systems September 14, / 16

4 Translating our solutions to programs Translating our solutions to programs: Count odd even def count ( lst : List [ Int ]): Int = { lst match { case Nil => 0 case l:: tail => ( count ( tail )+1)%2 } } Lecture 4 CS205: Scalable Software Systems September 14, / 16

5 Translating our solutions to programs Translating our solutions to programs: List reversal def reverse ( lst : List [ Int ]): List [ Int ] = { lst match { case Nil = > Nil case l:: tail => reverse ( tail ) ::: List (l) } } Lecture 4 CS205: Scalable Software Systems September 14, / 16

6 Translating our solutions to programs Combined program with the main function o b j e c t r e v e r s a l { d e f main ( a r g s : A r r a y [ S t r i n g ] ) : U n i t = { d e f r e v e r s e ( l s t : L i s t [ I n t ] ) : L i s t [ I n t ] = { l s t match{ c a s e N i l => N i l c a s e l : : t a i l => r e v e r s e ( t a i l ) : : : L i s t ( l ) } } v a l l s t = L i s t ( 1, 2, 3, 4 ) C o n s o l e. p r i n t l n ( R e v e r s a l o f + l s t + i s + r e v e r s e ( l s t ) ) } } Lecture 4 CS205: Scalable Software Systems September 14, / 16

7 Translating our solutions to programs Running the program How to compile/run a program? $ scalac filename.scala [compiles the code present in the file filename.scala] $ scala reversal [Executes the program by invoking the main function present in the object reversal] Lecture 4 CS205: Scalable Software Systems September 14, / 16

8 Scala hands-on Scala hands-on 1 Scala installation: 2 Scala book- Programming in Scala, by By Martin Odersky, Lex Spoon, Bill Venners 3 Scala doc: 4 scalac: compiler 5 scala: runtime comamnd 6 sbt: scala build tool Lecture 4 CS205: Scalable Software Systems September 14, / 16

9 Scala hands-on Higher order functions and currying Functions are variables and they can be passed around like variables Functions can be partially constructed by passing only a subset of arguments: currying Lecture 4 CS205: Scalable Software Systems September 14, / 16

10 Some exercises Some exercises Sum of integers in a list Sum of square of integers in a list Sum of cubes of integers in a list Lecture 4 CS205: Scalable Software Systems September 14, / 16

11 Some exercises Some exercises Sum of integers in a list Sum of square of integers in a list Sum of cubes of integers in a list Multiplication of integers in a list Lecture 4 CS205: Scalable Software Systems September 14, / 16

12 Some exercises Some exercises Sum of integers in a list Sum of square of integers in a list Sum of cubes of integers in a list Multiplication of integers in a list Get a sublist with all numbers greater than 2 Get a sublist with all numbers greater than 5 Get a sublist with all numbers greater than 10 Lecture 4 CS205: Scalable Software Systems September 14, / 16

13 Some exercises Some exercises Sum of integers in a list Sum of square of integers in a list Sum of cubes of integers in a list Multiplication of integers in a list Get a sublist with all numbers greater than 2 Get a sublist with all numbers greater than 5 Get a sublist with all numbers greater than 10 Get a sublist with all numbers less than 3 Lecture 4 CS205: Scalable Software Systems September 14, / 16

14 Some exercises Function Currying Functions are variables and they can be passed around like variables Functions can be partially constructed by passing only a subset of arguments: currying def mult (x:int, y: Int ) = x * y def double ( z: Int ) = 2 * z def triple ( w: Int ) = 3 * z Lecture 4 CS205: Scalable Software Systems September 14, / 16

15 Some exercises Some salient points about scala Basic building blocks are expressions and every expression has a type Last expression in a function definition becomes the return expression of that function There are more than one ways to write things, some are more readable than others Variables can be defined using var, val, or def var: its content can be changed later (mutable) val: its content can not be changed def: its content is evaluated at the time of use, not at the time of definition Lecture 4 CS205: Scalable Software Systems September 14, / 16

16 Some exercises Functional features in Scala Pattern matching Higher order functions Function currying Nested function definitions Lecture 4 CS205: Scalable Software Systems September 14, / 16

17 Some exercises Object oriented features in Scala Class and object definition Inheritance and overriding the default behaviours Traits Case classes Lecture 4 CS205: Scalable Software Systems September 14, / 16

18 Further readings and practice questions Readings scala: examples/index.html SBT: More questions to solve using recursive thinking: Lecture 4 CS205: Scalable Software Systems September 14, / 16

19 Further readings and practice questions Project discussion How to choose? Look around for intersting problems (within the campus, outside the campus) Talk to your friends, professors (from other areas) in terms of the end user requirements Can you use some data from the existing services (websites) to do more meaningful computation? Can you make your system in such a way that people can easily develop intersting plugins (different apps) on top of your system? Lecture 4 CS205: Scalable Software Systems September 14, / 16