# 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`