Transcript Haskell - Colorado School of Mines

Haskell
Chapter 1, Part II
List Comprehension
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List comprehensions are a way to filter, transform and
combine lists
Similar to mathematical set comprehensions
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{2 * x | x e N, X <= 10}
In Haskell:
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[x * 2 | x <- [1..10]]
“draw” our elements from the list [1..10]
so x takes on each value from 1 to 10
part before the pipe (|) is the output
With a predicate
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[x * 2 | x <- [50..100], x `mod` 7 == 3]
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Using a predicate in this way is called filtering
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Can separate predicates with a comma
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[x | x <- [10..20], x /= 13, x /= 15, x /= 19]
Can draw from several lists
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[x+y| x<-[1,2,3], y <- [10,100, 1000]]
result: [11,101,1001,12,102,1002,13,103,1003]
More list comprehensions
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Can use a temporary variable
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Can be used with strings (they’re lists too)
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length' xs = sum [1 | _ <- xs]
removeNonUppercase st = [c | c <- st, c `elem` ['A'..'Z']]
Nested list comprehensions
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-- let xxs =
[[1,3,5,2,3,1,2,4,5],[1,2,3,4,5,6,7,8,9],[1,2,4,2,1,6,3,1,3,2,3,6]]
removeOdd xxs = [[x | x <- xs, even x] | xs <- xxs]
Function definitions – must load, not just interpret. Use let to bind.
Tuples
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Used to store several heterogeneous elements as a single
value
Tuples have a fixed size
Elements surrounded by parentheses
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(1,3)
(3, ‘a’, “hello”)
(50, 50.4, “hello”, ‘b’)
tuple of size 2 is a different type from tuple of size 3
tuples with different member elements are different types
More tuples
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Storing pairs is common in Haskell
Useful functions to manipulate:
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fst
snd
zip [1,2,3] [4,5,6] => [(1,4),(2,5),(3,6)]
zip [1..] ["apple", "orange", "banana"] =>
[(1,"apple"),(2,"orange"),(3,"banana")]
Tuples in list comprehensions
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Generate tuples
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triples = [(a,b,c) | c <- [1..10], a<-[1..10], b<-[1..10]]
Generate tuples with filter
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rightTriangle = [(a,b,c) | c <- [1..10], a<-[1..c], b<-[1..a],
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a^2 + b^2 == c^2]
Play and Share
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evenCubes [1..20]
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onlyBig [200,30,50,20,120] 100
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[(2,17),(4,17),(2,23),(4,23)]
removeDigits "abc1d23A.98"
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15
evenOddPairs [1..4][20, 17, 23, 42]
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[(0,0),(1,1),(2,2),(3,3),(4,4),(5,5),(6,6),(7,7),(8,8),(9,9),(10,10)]
countOdd [1..30]
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[(1,2),(1,3),(1,4),(2,1),(2,3),(2,4),(3,1),(3,2),(3,4),(4,1),(4,2),(4,3)]
diagonal 10
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[200,120]
noDiagonal [1..4]
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[8,64,216,512,1000,1728,2744,4096,5832,8000]
"abcdA.“
ends [[4,5,6],[1,2],[7,1,0]]
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[6,2,0]
* These are parameters