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Functional Programming Lecture 7 - TreesPowerPoint Presentation

Functional Programming Lecture 7 - Trees

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### Functional ProgrammingLecture 7 - Trees

Binary Trees of Numbers

42

data NTree = NilT

| Node Int NTree NTree

Node 42

(Node 13 NilT NilT)

(Node 19 NilT (Node 12 NilT NilT))

13

19

12

Operations on Binary Trees of Numbers

Most operations can be defined using primitive recursion and pattern matching.

depth :: Ntree -> Int

-- calculate the depth of a tree

depth NilT = 0

depth (Node n t1 t2) = 1 + max (depth t1) (depth t2)

sumtree :: Ntree -> Int

-- sum all the nodes of a tree

sumtree NilT = 0

sumtree (Node n t1 t2)

= n + (sumtree t1) + (sumtree t2)

Evaluate on tree 42

13 19

12

Operations on Binary Trees of Numbers

occurs :: Int -> Ntree -> Int

-- occurrences of a number in a tree

occurs x NilT = 0

occurs x (Node n t1 t2)

| (x == n) = 1 + (occurs x t1) + (occurs x t2)

| otherwise = (occurs x t1) + (occurs x t2)

occurs on lists?

occurs :: Int -> [Int] -> Int

occurs x [] = 0

occurs x (y:ys)

| (x == y) = 1+(occurs x ys)

| otherwise = occurs x ys

-- left subtree

left NilT = NilT

left (Node n t1 t2) = t1

right :: Ntree -> Ntree

-- right subtree

right NilT = NilT

right (Node n t1 t2) = t2

Operations on Binary Trees of Numbers

Conjoin two trees

6

2 3 => 2 3

4 6 1 5 4 6 1 5

conjoin :: Int -> Ntree -> Ntree -> Ntree

-- conjoin two trees

conjoin x t1 t2 = Node x t1 t2

Operations on Binary Trees of Numbers

maxt :: Ntree -> Int

-- find max value in a tree

maxt NilT = ??

maxt(Node n NilT NilT) = n

maxt(Node n NilT t2) = max n (maxt t2)

maxt(Node n t1 NilT) = max n (maxt t1)

maxt (Node n t1 t2) = max3 n (maxt t1) (maxt t2)

or

maxt t = maxlist preorder t

where maxlist [x] = x

maxlist (x:xs) = max x (maxlist xs)

Operations on Binary Trees of Numbers

or

maxt :: Ntree -> Maybe Int

maxt NilT = Nothing

maxt(Node n NilT NilT) = Just n

maxt(Node n NilT t2) = max n y

maxt(Node n t1 NilT) = max n x

maxt (Node n t1 t2) = Just max3 n x y

where Just x = maxt t1

Just y = maxt t2

Traversing Binary Trees of Numbers

preorder :: Ntree -> [Int]

preorder NilT = []

preorder (Node n t1 t2) = n:(preorder t1 ++ preorder t2)

Example: 42

3 8

16 5

preorder (Node 42 (Node 3 Nil Nil) (Node 8 (Node 16 Nil Nil) (Node 5 Nil Nil)))

Traversing Binary Trees of Numbers

inorder :: Ntree -> [Int]

inorder NilT = []

inorder (Node n t1 t2)

= inorder t1 ++ [n] ++ inorder t2

Example: 42

3 8

16 5

inorder (Node 42 (Node 3 Nil Nil) (Node 8 (Node 16 Nil Nil) (Node 5 Nil Nil)))

Traversing Binary Trees of Numbers

postorder :: Ntree -> [Int]

postorder NilT = []

postorder (Node n t1 t2)

= postorder t1 ++ postorder t2 ++ [n]

Example: 42

3 8

16 5

Sorting Binary Trees of Numbers

sorttree :: Ntree -> [Int]

sorttree t = sort (preorder t)

where sort is your favourite sorting algorithm.

Insertion sort

sort xs = inssort(xs,[])

a common programming paradigm

inssort :: ([Int],[Int]) -> [Int]

-- insertion sort

-- first component is list to be sorted

-- second component is sorted list

inssort ([],ys) = ys

inssort (x:xs, ys) = inssort(xs, ins x ys)

ins :: Int ->[Int] -> [Int]

-- insert integer at correct position in sorted list

ins x [] = [x]

ins x (y:ys) = if (x<=y) then (x:y:ys) else y: (ins x ys)

sort( [3,2,1]

Map

maptree :: (Int -> Int) -> Ntree -> Ntree

maptree f NilT = NilT

maptree f (Node n t1 t2) =

Node (f n) (maptree f t1) (maptree f t2)

Example: t= 42

3 8

16 5

maptree times2 t

Polymorphic Binary Trees

We can define trees with an arbitrary type at the nodes:

data Tree a = Nil

| Node a (Tree a) (Tree a)

deriving Show

So define:

maptree :: (a -> b) -> (Tree a) -> (Tree b)

maptree Nil = Nil

maptree f (Node n t1 t2) =

Node f n (maptree f t1) (maptree f t2)

preorder :: Tree a -> [a]

Etc.

What is type of maptree times2?

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