BTree

The BTree type represents a tree using the b-tree algorithm.

A BTree is a self-balancing tree as its nodes are sorted in the inorder traversal. The node is a set of elements pointing to its children, and a leaf has no children and nothing in itself.

This implementation uses an order 3 BTree, this means:

• {one, two} elements per node and {two, three} subtrees.
• A leaf contains nothing.
• Every leaf is equidistant from the root.
• Subtrees must have the same height.
• Data are ordered left to right.

           4               <-- (3) root
        /     \
       2       6           <-- (2) internal nodes
      / \     / \
     1   3   5   7         <-- (1) internal nodes
    / \ / \ / \ / \
   .  ..  ..  ..   .       <-- (0) leafs

The complexity of the main operations:

Instalation

$> cabal install btree-algorithm

then you import the module:

import Data.BTree

Constructors

empty :: BTree a

An empty BTree. It consists of a single leaf.

It may be used to construct a new BTree by inserting elements into it.

>>> let t = empty
>>> t
fromList []

>>> insert 1 t
fromList [1]

>>> insert 2 empty
fromList [2]

singleton :: Ord a => a -> BTree a

Creates a singleton BTree from a single element.

>>> let t = singleton 1
>>> t
fromList [1]

fromList :: Ord a => [a] -> BTree a

Creates a BTree from a list.

>>> let t = fromList [1,2,3,4,5,6,7]
>>> t
fromList [1,2,3,4,5,6,7]

>>> draw t
"(((. 1 .) 2 (. 3 .)) 4 ((. 5 .) 6 (. 7 .)))"

Operations

insert :: forall a. Ord a => a -> BTree a -> BTree a

Inserts an element into a BTree. Inserts the element into the BTree in inorder fashion.

>>> let t = insert 1 empty
>>> t
fromList [1]

>>> draw t
"(. 1 .)"

>>> let n = insert 2 t
>>> n
fromList [1,2]

>>> draw n
"(. 1 . 2 .)"

delete :: forall a. Ord a => a -> BTree a -> BTree a

Delete an element from a BTree.

>>> let t = fromList [1,2,3,4,5,6,7]
>>> let n = delete 3 t
>>> draw n
"((. 1 . 2 .) 4 (. 5 .) 6 (. 7 .))"

>>> n
fromList [1,2,4,5,6,7]

search :: forall a. Ord a => a -> BTree a -> Maybe a

Search an element in a BTree. It will return a Maybe.

>>> let t = fromList [1,2,3,4,5]
>>> search 3 t
Just 3

>>> search 6 t
Nothing

height :: forall a. BTree a -> Int

The height of the BTree.

Represents the number of levels in the BTree.

            4             <-- height 3
        /      \
       2       6          <-- height 2
      / \     / \
     1   3   5   7        <-- height 1
    / \ / \ / \ / \
   .  ..  ..  ..   .      <-- height 0

>>> height (fromList [1,2,3,4,5,6,7])
3

Traversal

preorder :: forall a. BTree a -> [a]

Return the BTree as a list in a pre-order traversal.

The pre-order traversal runs through the elements of the BTree in the following order:

1. The root element.
2. The left subtree.
3. The right subtree.

A BTree of [1, 2, 3, 4, 5, 6, 7] is represented as:

            4
        /      \
       2       6
      / \     / \
     1   3   5   7
    / \ / \ / \ / \
   .  ..  ..  ..   .

>>> preorder (fromList [1,2,3,4,5,6,7])
[4,2,1,3,6,5,7]

inorder :: forall a. BTree a -> [a]

Return the BTree as a list in an in-order traversal.

The in-order traversal runs through the elements of the BTree in the following order:

1. The left subtree.
2. The root element.
3. The right subtree.

Note: The in-order traversal is the default traversal for the BTree, and is used to return as list.

A BTree of [1, 2, 3, 4, 5, 6, 7] is represented as:

            4
        /      \
       2       6
      / \     / \
     1   3   5   7
    / \ / \ / \ / \
   .  ..  ..  ..   .

>>> inorder (fromList [1,2,3,4,5,6,7])
[1,2,3,4,5,6,7]

postorder :: forall a. BTree a -> [a]

Return the BTree as a list in a post-order traversal.

The post-order traversal runs through the elements of the BTree in the following order:

1. The left subtree.
2. The right subtree.
3. The root element.

A BTree of [1, 2, 3, 4, 5, 6, 7] is represented as:

            4
        /      \
       2       6
      / \     / \
     1   3   5   7
    / \ / \ / \ / \
.  ..  ..  ..   .

>>> postorder (fromList [1,2,3,4,5,6,7])
[1,3,2,5,7,6,4]

levels :: forall a. Ord a => BTree a -> [[a]]

Return the BTree as a list of lists, grouped by hights.

  a
 / \    => [[a], [b,c]]
b   c

>>> levels (fromList [1,2,3,4,5,6,7])
[[4],[2,6],[1,3,5,7]]

Ascii Drawing

draw :: forall a. Show a => BTree a -> String

Draws the BTree.

The output is a string of the form:

• Each Leaf is represented by a ' . '
• Each Node is represented as (left, value, right) where left and right are the left and right subtrees.

So, the current list [1, 2, 3, 4, 5, 6, 7] will be represented as:

"(((. 1 .) 2 (. 3 .)) 4 ((. 5 .) 6 (. 7 .)))"

Which is an inline representation of the BTree.

>>> draw (fromList [1,2,3,4,5,6,7])
"((((. 1 .) 2 (. 3 .)) 4 ((. 5 .) 6 (. 7 .))))"