Index of minimum element using J

user_986905 0
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In J it is trivial to find the minimum element of an array:

   <./ 5 6 4 9
4

But how to find the index of the minimum element using J?

(In case this is an XY problem, I should say that I want the index rather than the value so that I can look up the corresponding elements in other arrays.)

j
2 Answers

I think that I would use this

ind=: I. @: (= <./)

It starts with the <./ which returns your lowest value and it uses = as the other half of a monadic hook. Monadic hooks take the y argument and apply the rightmost verb to it and then the left verb uses that as its right argument with the original y as its left argument. So, (= <./) 5 6 4 9 is the same as 5 6 4 9 = 4 and this returns 0 0 1 0. From there it is simple to use the monadic I. (indices) to return the index of the value 1 which is of course 2.

This also will return multiple indices if there is more than one least value.

   ind 5 6 4 9 4
2 4

If you only want the first or the last value you can use the simpler monadic hooks that incorporate i. (index of) or i: (index of last):

   indfirst =: i. <./
   indfirst 5 6 4 9 4
2
   indlast =: i: <./
   indlast 5 6 4 9 4
4
user_24121780

Another option is to take the first element of the permutation that would sort the list: {.@/:. This has the advantage of working not only for list but for arbitrary arrays and gives you the index of the lexicographically least item.

user_8166460
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