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22.02.16 - List Comprehensions

Set Comprehensions​

Comprehension notation can be used to construct new sets from old sets x2∣x∈1..5{x^2 | x \in {1..5}}

In haskell can be done with list comprehension

[x^2 | x <- [1..5]]
  • The expression x <- [1..5] is a generator, states how to generate values for x. Can be used multiple times separated by commas.
  • Changing the order of the generators changes the order of the elements in the final list.
  • Multiple generators are like nested loops, with later generators being more deeply nested loops

Dependant generators​

Later generators can depend on the variables that are introduced by earlier generators Using a dependent generator we can define the library function that concatenates a lists of lists

Guards​

Use guards to restrict the values produced by earlier generators [x | x <- [1..10], even x] Using a guard we can define a function that maps a positive integer to its list of factors

The zip function​

zip :: [a] -> [b] -> [(a,b)]
  • Maps two lists to a lists of pairs of their corresponding elements.
  • Define a function that returns the list of app pairs of adjacent elements from a list
  • Using pairs can define a function that decides if the elements in a list are sorted
  • Using zip can define a function that returns the list of all positions of a value in a list

String Comprehensions-​

  • A string is a sequence of characters enclosed in double quotes. Internally, strings are represented as a lists of characters.
  • Any polymorphic function that operates on lists can also be applied to strings
  • List comprehensions can also work them