How to find max's index in a Seq, List or Array in F#


How to find max's index in a Seq, List or Array in F#



Seq.max finds the max number. I'd like to have something like Seq.findIndex

Seq.maxIndex returns the index of the maximum element.




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I believe you are looking for something like:.
referencing my facebook application with a querystring
let maxIndex seq =      fst (Seq.maxBy snd (Seq.mapi (fun i x -> i, x) seq)) 
Note that giving this function an empty sequence will result in an ArgumentException..
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(Alternatively, written in pipelining style:.
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let maxIndex seq =       seq     |> Seq.mapi (fun i x -> i, x)     |> Seq.maxBy snd      |> fst 
).
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Why not simply use .
let l=[1;2;5;3];; Seq.findIndex  (fun x -> x= Seq.max l) l ;; 
?. Or maybe as Johan Kullbom suggest in a comment:.
"let m = Seq.max l in Seq.findIndex (fun x -> x = m) l" 
if you what a little better O(n). However, the need to get the index looks to me like a imperative "code smell" .. In FP it's usually better to use existing functions before you roll your own.

I now this in the eyes of a C programmer seems like a for(i (for(j construct but I bet that you probably really don't need to know the index if you start think in FP.. More or less a duplicate of http://stackoverflow.com/questions/1496980/finding-index-of-element-in-a-list-in-haskell. PS.

I can't resist.

In Haskell (ghc) the way should probably be something like.
let cmpSnd (_, y1) (_, y2) = compare y1  y2  let maxIndex l= fst $ maximumBy cmpSnd $ zip [0..] l 
However, since zip in F# doesn't seem to allow zip with unequal lengths of the list(?) the use of mapi is probably the way to go (my haskell version in F#).
let cmpSnd xs=  snd xs ;;  let zipIndex a= Seq.mapi (fun i x -> i,x) a;;  let maxIndex seq=fst (Seq.maxBy cmpSnd (zipIndex seq));; 
and the reason is only so that I can make a list .
let l= [[0;199;1];[4;4];[0;0;399]] 
test with makeIndex l;; and decide that what I really want is a.
let cmpSnd' (a,(xs: int list))  = Seq.sum  xs;; let maxIndex' seq=fst (Seq.maxBy cmpSnd' (zipIndex seq));; 
Now time to decomposite and make makeIndex take a function .
let maxIndexF seq maxF=fst (Seq.maxBy maxF (zipIndex seq));;  val l : int list list = [[1; 2; 199]; [3; 3]; [4; 1]; [0; 299]]  > maxIndexF l cmpSnd' ;; val it : int = 3 > maxIndexF l cmpSnd ;; val it : int = 2 
Finish it up.
let maxIndexF'  maxF=fst << Seq.maxBy maxF << zipIndex ;;  maxIndexF' cmpSnd' l;; maxIndexF' cmpSnd l;; 



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