How to work on lists

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Given any list xs, how do I...?

  • Get the size of the list.
length xs
  • Turn a list backwards.
reverse xs
  • Get the Nth element out of a list.
xs !! n
(Related: head xs returns the first element of the list.)
(Related: last xs returns the last element of the list.)
  • Add an element to the start of a list.
new_element : xs
  • Add an element to the end of a list.
xs ++ [new_element]
  • Insert an element into the middle of a list.
Generally, you will have to split the list into two smaller lists, put the new element to in the middle, and then join everything back together. For example:
let (ys,zs) = splitAt n xs in ys ++ [new_element] ++ zs
  • Join two lists together.
list1 ++ list2
  • Delete the first N elements from a list.
drop n xs
(Related: tail xs removes just one element.)
(Related: init xs removes just the last element.)
  • Make a new list containing just the first N elements from an existing list.
take n xs
  • Split a list into two smaller lists (at the Nth position).
splitAt n xs
(Returns a tuple of two lists.)
  • Delete the just Nth element of a list.
This is tricky. AFAIK, there is no built-in function that does this. You have to split the list in two, remove the element from one list, and then join them back together, like this:
let (ys,zs) = splitAt n xs in ys ++ (tail zs)
(Related: tail xs removes the first element.)
(Related: init xs removes the last element. Slow if the list is big.)
  • Delete elements that meet some condition.
Haskell has a function called filter which will do this for you. Beware though: it should really be named 'select' instead. For example, filter odd xs returns a list of odd numbers. That is, it deletes everything that is not odd.
  • Apply a function to all list elements.
map my_function xs
  • Convert a list of foos into a list of bars.
Find or write a function to convert foo into bar, and then apply it to the whole list using map.
  • Number the elements of a list (so I can process each one differently according to its position).
zip xs [0..]
(For example, zip ['a','b','c'] [0..] gives [('a',0),('b',1),('c',2)].)
  • Total up a list of numbers.
sum xs
  • Find the highest/lowest element of a list.
minimum xs
maximum xs
(Works not just for numbers but anything that is a member of the Ord class. In particular, that includes characters and strings.)
  • Sort a list.
You'll need to import Data.List first, but then you can just do sort xs.
  • Find out if some item is in a list.
my_element `elem` xs