CODEWITHSUNDEEP
f#
OliverVS
OliverVS

Reputation: 53

How to access the first element of a tuple in a list?

I am trying to make a collision method in a class in f#. The collision method should have one input which is time. The method should check a number of drones that should be moving with time and whether some of them will collide. A collision is defined as being within 5 "blocks" of each other.

I have so far this code for the method. 

type Airspace(drones:Drone list) = 
    let mutable drones = drones
        member x.Drones with get() = drones
    member x.Collision(time:int) =
        for d = 1 to time do 
           for i = 1 to drones.Length do
               for j = 1 to drones.Length do
                   if drones.Item(i).Position.Item1 - drones.Item(j).Position.Item1 >= -5.0 || drones.Item(i).Position.Item1 - drones.Item(j).Position.Item1 <= 5.0 
                   then if drones.Item(i).isFinished() = True && drones.Item(j).isFinished() = True then printfn "Drone: %A will collide with %A" (drones.Item(i)) (drones.Item(j))

By the way drones is a list of Drones and a drone has a current position as a tuple floatfloat, a destination also as a floatfloat and speed which is a float.

I am pretty sure this is wrong

drones.Item(i).Position.Item1

But I am not sure how I can access the first i'th element in a list and then take only the first element of the tuple and compare it with another one. How can I do this?

Upvotes: 0

Views: 377

Answers (1)

Gus
Gus

Reputation: 26194

Well, it's not 100% wrong, it might work but yes, the recommendation is not to use ItemX and instead to pattern match the tuple elements, so you can do this:

let (xi, _) = drones.Item(i).Position
let (xj, _) = drones.Item(j).Position
if xi - xj >= -5.0 || xi - xj <= 5.0

Btw, if you don't really need to use the index you can also change the loops to:

       for di in drones do
           for dj in drones do
               let (xi, _) = di.Position
               let (xj, _) = dj.Position

Also note that you can use the function List.allPairs piped to List.tryFind instead of for loops.

Upvotes: 1

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