How to work on lists

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