teaching machines

CS 330 Lecture 30 – Pattern Matching, Partial Function Evaluation, and Map

April 20, 2015 by Chris Johnson. Filed under cs330, lectures, spring 2015.

Agenda

• what ?s
• more on pattern matching
  •  capitalize
  •  proportionalize
  •  at, a subscript operation
• partial function evaluation
  •  kthSmallest
  •  winner
  •  runnerUp
• the map pattern

Intentions

• I can organize my solution to a problem by matching my data against patterns in case statements or function parameters.
• I can generate functions from other functions by partially applying a subset of their parameters.
• I can transform one list to another without writing the same old traversal code again and again.

Program This

Write proportionalize, a function that accepts a list of percentages in [0, 100] and converts them to proportions in [0, 1].

Code

Hello.hs

import Data.Char
import Data.List

capitalize :: [Char] -> [Char]
capitalize s =
  if s == [] then
    []
  else 
    toUpper (head s) : capitalize (tail s)

capitalize2 s
  | s == [] = []
  | otherwise = toUpper (head s) : capitalize2 (tail s)

capitalize3 s =
  case s of 
    [] -> []
    (first:rest) -> toUpper first : capitalize3 rest

-- --------------------------------------------------------------------------- 

capitalize4 [] = []
capitalize4 (first:rest) = toUpper first : capitalize4 rest

-- --------------------------------------------------------------------------- 

proportionalize [] = []
proportionalize (first:rest) = first / 100 : proportionalize rest

-- --------------------------------------------------------------------------- 

at :: Int -> [t] -> t
-- at 0 [] = error "index oob" 
at 0 (first:_) = first
at n [] = error "index oob"
at n (_:rest) = at (n - 1) rest

-- --------------------------------------------------------------------------- 

kthSmallest :: (Ord t) => Int -> [t] -> t
kthSmallest k items = at k (sort items)

-- winner items = kthSmallest 0 items 
winner :: (Ord t) => [t] -> t
winner = kthSmallest 0

runnerUp :: (Ord t) => [t] -> t
runnerUp = kthSmallest 1

-- --------------------------------------------------------------------------- 

map' xform [] = []
map' xform (first:rest) = xform first : map' xform rest

-- proportionalize [] = [] 
-- proportionalize (first:rest) = first / 100 : proportionalize rest 

Haiku