# 99 problems in Haskell, 81-84

2015 Sep 15

Vacation time is over, and thus blog posts are less frequent. BTW, if you can offer me a job that doesn’t suck, contact me.

### Problem 81: Path from one node to another one.

Note the assumption behind the implementation here, that the graph represented is directed. This means we can’t use this implementation for undirected graphs in later problems.

``````paths :: Eq a => a -> a -> [(a, a)] -> [[a]]
paths src dst g
| src == dst = [[dst]]
-- Just expand the path recursively (greed works):
| otherwise = [ [src] ++ path | edge <- g, (fst edge) == src
, path <- paths (snd edge) dst [e | e <- g, e /= edge]
]
``````

### Problem 82: Find a cycle starting from a given node.

The assumption of a directed graph still holds. We can use `paths` from previously to find cycles by finding all paths back to the initial node.

``````cycle' :: (Eq a) => a -> Friendly a -> [[a]]
cycle' v (Edge es) = (es >>= f)
where
f (a, b) = if v /= a then []
else map (v:) \$ paths b v es
``````

### Problem 83: Construct all spanning trees.

Alright! This is hard!

Even if the idea of a spanning tree can be generalised for directed multigraphs, this is probably not the intent of the question, so let’s assume our graph is undirected. Let’s start with implementing `paths` and `cycle'`:

``````paths :: (Eq a) => a -> a -> [(a, a)] -> [[a]]
paths src dst graph | src == dst = [[src]]
| otherwise = concat [map (src :) \$ paths d dst \$ [x | x <- graph, x /= (c, d)]
| (c, d) <- graph, c == src] ++
concat [map (src :) \$ paths c dst \$ [x | x <- graph, x /= (c, d)]
| (c, d) <- graph, d == src]

cycle' :: (Eq a) => a -> [(a, a)] -> [[a]]
cycle' v graph = [v : path | e <- graph, fst e == v, path <- paths (snd e) v [x | x <- graph, x /= e]]
++ [v : path | e <- graph, snd e == v, path <- paths (fst e) v [x | x <- graph, x /= e]]
``````

and now `spantree` is:

``````import Data.List

spantree :: (Eq a) => Graph a -> [Graph a]
spantree (Graph xs ys) = (filter connected) \$ (filter (not . cycles)) \$ (filter nnodes) trees
where
-- These are all the possible trees, formed by accumulating over all edges:
trees = [Graph (nodes edges) edges | edges <- foldr acc [[]] ys]
acc e es = es ++ (map (e:) es)
-- This is the list of all unique nodes in a graph:
nodes e = nub \$ concatMap (\(a, b) -> [a, b]) e
-- The conditions a tree must satisfy to be a spanning tree are:
-- * the number of nodes is the same as (Graph xs ys), and
-- * it contains no cycles, and
-- * is connected, which means there is a path between every pair of vertices
nnodes (Graph xs' ys') = length xs == length xs'
cycles (Graph xs' ys') = any ((/=) 0 . length . flip cycle' ys') xs'
-- a) Since our graph is undirected, we only need to check for paths from x'
--    to each vertex in xs'
-- b) Therefore, if there's any pair of vertices which aren't connected, then the
--    graph is also unconnected
connected (Graph (x':xs') ys') = not \$ any (null) [paths x' y' ys' | y' <- xs']
``````

### Problem 84: Construct the minimal spanning tree.

Prim’s algorithm is surprisingly easy to implement.

``````
import Data.Bool
import Data.List
import Data.Ord

data GraphW a = GraphW [a] [(a, a, Int)]
deriving (Show, Eq)

-- Initialise a graph (tree) with the first vertex, and begin:
prim (GraphW vs es) = prim' [head vs] [] (length vs - 1) (GraphW vs es)

-- Terminate when no vertices remain to be chosen:
prim' chosenV chosenE 0 _ = GraphW chosenV chosenE
prim' chosenV chosenE nE (GraphW vs es) = prim' (newV : chosenV) (newE : chosenE) (nE - 1) (GraphW vs es)
where
-- The edges we can pick from are extending outward from the tree:
edges = filter (\(a, b, _) -> (a `elem` chosenV) `xor` (b `elem` chosenV)) es
-- the new edge, newE, is the minimum weight one that connects our tree with a vertex
-- not on the tree:
newE @ (a', b', w') = minimumBy (comparing (\(_, _, w) -> w)) edges
-- the new vertex is, therefore, the end of newE which isn't part of the tree so far:
newV = if a' `elem` chosenV then b' else a'

xor :: Bool -> Bool -> Bool
xor = (/=)

-- The graph from the problem statement:
g5 = [(1,2,12),(1,3,34),(1,5,78),(2,4,55),(2,5,32),(3,4,61),(3,5,44),(4,5,93),(2,1,12),(3,1,34),(5,1,78),(4,2,55),(5,2,32),(4,3,61),(5,3,44),(5,4,93)]

main = do print \$ prim (GraphW [1,2,3,4,5] g5)
``````
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