• Post Reply Bookmark Topic Watch Topic
  • New Topic
programming forums Java Mobile Certification Databases Caching Books Engineering Micro Controllers OS Languages Paradigms IDEs Build Tools Frameworks Application Servers Open Source This Site Careers Other Pie Elite all forums
this forum made possible by our volunteer staff, including ...
Marshals:
  • Campbell Ritchie
  • Jeanne Boyarsky
  • Ron McLeod
  • Paul Clapham
  • Liutauras Vilda
Sheriffs:
  • paul wheaton
  • Rob Spoor
  • Devaka Cooray
Saloon Keepers:
  • Stephan van Hulst
  • Tim Holloway
  • Carey Brown
  • Frits Walraven
  • Tim Moores
Bartenders:
  • Mikalai Zaikin

Scala exercises for beginners (in FP)

 
Ranch Hand
Posts: 376
Scala Monad
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Scala exercises for beginners
I had a lot of fun solving them, particularly trying to use a minimal set of operations (pattern matching, recursion,::, Nil).
Solving them with fold/reduce helped to clarify some details, though
 
pie sneak
Posts: 4727
Mac VI Editor Ruby
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Thanks for the link - this is great fun, Gabriel!

Here's my first pass. I haven't learned folding of lists yet, so that's my next priority and then I'll give another crack at it. Any helpful opinions welcome!


results:
 
Ranch Hand
Posts: 1296
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
The seminal treatise on folds in functional programming is the very readable: A tutorial on the universality and expressiveness of fold.

All the functions in the exercise save the add function can be solved using left and right folds.



I find the symbollic /: and :\ difficult to mentally translate, but I guess if I used them more I'd get used to them.
[ July 21, 2008: Message edited by: Garrett Rowe ]
 
Garrett Rowe
Ranch Hand
Posts: 1296
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Oh, and by the rules of the exercise, ::: is a prohibited function.
 
Gabriel Claramunt
Ranch Hand
Posts: 376
Scala Monad
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator

Originally posted by Garrett Rowe:
Oh, and by the rules of the exercise, ::: is a prohibited function.


Yes, ':::' is prohibited, but is the list concatenation operator. In Marc's case, it can be easily replaced by the '::' ("cons") operator.
 
Gabriel Claramunt
Ranch Hand
Posts: 376
Scala Monad
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator

Originally posted by Marc Peabody:


I did the list recursion using Nil and '::'

Nil is the empty list and the operator '::' appends an element to the beginning.
I found quite interesting the fact the pattern matching can match and split the pattern 'element::list'
Of course, with a fold is a lot easier:

def map[A, B](x: List[A], f: A => B): List[B] =
x.foldRight(Nil: List[B])((x,xs)=>f(x)::xs)


My fold/reduce 'cheat sheet' is something like:

For the list (a,b,c,d) ,

reduceLeft(f) => f(f(f(a,b),c),d)

foldLeft(Z)(f) => f(f(f(f(Z,a),b),c),d)

("starts from the left")


reduceRight(f) => f(a,f(b,f(c,d)))

foldRight(Z)(f) => f(a,f(b,f(c,f(d,Z))))

("starts from the right")

[ July 21, 2008: Message edited by: Gabriel Claramunt ]
[ July 21, 2008: Message edited by: Gabriel Claramunt ]
 
Marc Peabody
pie sneak
Posts: 4727
Mac VI Editor Ruby
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator

Originally posted by Gabriel Claramunt:

Yes, ':::' is prohibited, but is the list concatenation operator. In Marc's case, it can be easily replaced by the '::' ("cons") operator.


Totally missed that ::: was in the illegal list. Oops!

I suppose if I were to take it out of my append() then I could just use my append method everywhere else...

Fold is starting to make sense a bit (basically seems to be a tiny DSL for recursion) but I get the feeling that most programmers would rather put their brain through a meat grinder than try to wrap their head around this.
[ July 21, 2008: Message edited by: Marc Peabody ]
 
Gabriel Claramunt
Ranch Hand
Posts: 376
Scala Monad
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Here are my solutions with recursion and with fold/reduce
reply
    Bookmark Topic Watch Topic
  • New Topic