CODEWITHSUNDEEP
arraysalgorithmlisthaskell
HopefulSplash
HopefulSplash

Reputation: 31

Accessing Values In Matrix (Haskell Newbie)

i am trying to do the equvilant of this in haskell bascially 

for (int i = 0; i < city_Permutation_Route.length - 1; i++) {
        route_Distance = route_Distance + city_Distance_Matrix[city_Permutation_Route[i]][city_Permutation_Route[i + 1]];

    }

there i get the weight of each route, compare it against each of the others so that i print out the route with the lowest weighted route as follows

Route Weight = 453.4 Route = 0,1,2,3,4,5,6,7,8

i have functions to get the total route and all the other data but do not understand how to get values from the matrix

Question: How do i do this in haskell

i want to be able to get the values from my distance matrix using the permutation values as the index to it

Upvotes: 0

Views: 179

Answers (1)

Cirdec
Cirdec

Reputation: 24156

Given a permutation, for example [3, 2, 7, 5, 4, 6, 0, 1] you can compute all of the legs by zipping it with its own tail.

zip [3, 2, 7, 5, 4, 6, 0, 1]
    (tail [3, 2, 7, 5, 4, 6, 0, 1])

zip [3, 2, 7, 5, 4, 6, 0, 1]
    [2, 7, 5, 4, 6, 0, 1]

[(3,2),(2,7),(7,5),(5,4),(4,6),(6,0),(0,1)]

These are the indexes into the distance matrix for the cost of traveling between two points. If we look these up in the city_Distance_Matrix using the list indexing function !! we get the cost for each leg

map (\(c0, c1) -> city_Distance_Matrix !! c0 !! c1)
    [(3,2),(2,7),(7,5),(5,4),(4,6),(6,0),(0,1)]

    [97.4, 71.6, 111.0,138.0,85.2 ,86.3 ,129.0]

If we total these, we get the total cost for traveling the legs of this permutation.

sum [97.4, 71.6, 111.0,138.0,85.2 ,86.3 ,129.0] = 718.5

Putting this all together, we can define a function that computes the total length of all the legs of a permutation of the cities. We can simplify the function by using zipWith which is a combination of zip and map.

totalLength :: [Int] -> Double
totalLength cities = sum $ zipWith (\c0 c1 -> city_Distance_Matrix !! c0 !! c1) cities (tail cities)

You should be able to use this to find the permutation whose totalLength is minimal.

Upvotes: 4

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