CODEWITHSUNDEEP
haskell
randomdude
randomdude

Reputation: 267

How to get the the greatest number and its index from a list?

I'm trying to find the greatest number from my list and also its index. For example this is my list:

list = [4.3,0.0,6.2214,9.1,1.023990433763,3.839]

I can find the greatest number with this line:

greatest = maximum list

In this example, the greatest number is 9.1, but how can I display its index (in the example, 3)?

Upvotes: 2

Views: 132

Answers (1)

leftaroundabout
leftaroundabout

Reputation: 120711

The easiest way is to explicitly annotate the list with indices, then do the search there.

Prelude> zip [4.3,0.0,6.2214,9.1,1.023990433763,3.839] [0..]
[(4.3,0),(0.0,1),(6.2214,2),(9.1,3),(1.023990433763,4),(3.839,5)]

Because tuples Ord lexicographically, you can actually simply use maximum on that list:

Prelude> maximum $ zip [4.3,0.0,6.2214,9.1,1.023990433763,3.839] [0..]
(9.1,3)

This is a bit hackish though because you still have at least in principle also a comparison on the index, which you don't really want. So a cleaner way is

> :m +Data.List Data.Ord
> maximumBy (comparing fst) $ zip [4.3,0.0,6.2214,9.1,1.023990433763,3.839] [0..]
(9.1,3)

Of course, if in the end you want only the index, you can easily throw away the value:

> snd . maximumBy (comparing fst) $ zip [4.3,0.0,6.2214,9.1,1.023990433763,3.839] [0..]
3

Upvotes: 10

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