CS 330 Lecture 26 – List Comprehensions and Map
Agenda
- what ?s
- list comprehensions
- translating between list comprehensions and loops (js2coffee)
- a special case of list comprehensions: map
TODO
- Start the Funfun homework.
Note
Last time we spent most of the lecture discussing our Program This problem. So, it’s today that we jump into discussing list comprehensions. We start by looking at the template of imperative code that they replace. We introduce the notion of a predicate. We’ll solve a handful of list comprehension problems together.
As we have time, we’ll examine a special case of a list comprehension, one with no predicates and involving only one list. This pattern is called a map, not to be confused with the abstract data type (though the underlying idea of mapping one value to another is the same). We’ll write our own map' function that accepts a transformation function and a list as input.
Code
comprehensions.hs
import Data.List
preditor :: String -> String
preditor s = [pred c | c <- s]
uwecify :: [String] -> [String]
uwecify ids = [id ++ "@uwec.edu" | id <- ids]
addies :: [String]
addies = [col : show row | col <- "ABCD", row <- [0..9]]
pairings :: [String] -> [(String, String)]
pairings items = [(x, y) | x <- items, y <- items, x /= y]
alpha = ['a'..'z']
foowords :: [String]
foowords = [[a, b, c, d, e] | a <- alpha, b <- alpha, c <- alpha, d <- alpha, e <- alpha, "foo" `isInfixOf` [a, b, c, d, e]]