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PeterBartholomew1's avatar
PeterBartholomew1
Silver Contributor
Aug 10, 2021
Solved

Array of arrays using Lambda helper functions

Am I missing something?  What I want to do is create an array of filtered lists with a single dynamic array formula.  The output should be in the form of a 2D array with the shorter lists padded with...
Formulas and Functions
office 365
  • tboulden's avatar
    tboulden
    Aug 11, 2021

    PeterBartholomew1Just to get rid of that pesky blank row:

     

    =REDUCE(
    "",
    ListCountry#,
    LAMBDA(acc,val,
        IF(
            TYPE(acc)<>64,TRANSPOSE(FILTER(StartList[ATHLETE],StartList[COUNTRY]=val)),
             VSTACKλ(,acc,TRANSPOSE(FILTER(StartList[ATHLETE],StartList[COUNTRY]=val)))
        )
    )
)

     

    I think since MAP will iterate over an array of any shape, the results have to fit back into the same shape; I'd be interested to know if this is a mistaken assumption on my part.

PeterBartholomew1's avatar
PeterBartholomew1
Silver Contributor
Jan 07, 2022

tboulden 

I was looking for a later post but, since this one turned up, your more recent strategy using Thunks appears to be relevant for the sorted list of lists problem.

= LET(
      THUNKλ,  LAMBDA(x, LAMBDA(x)),
      EXPANDλ, LAMBDA(ϑ, LAMBDA(r,c, INDEX(INDEX(ϑ,r,1)(),c))),
      m, ROWS(ListCountry#),
      n, MAX(COUNTIFS(StartList[COUNTRY],ListCountry#)),
      resultϑ, MAP(ListCountry#,
         LAMBDA(country, THUNKλ(FILTER(StartList[ATHLETE], StartList[COUNTRY]=country)))),
      display, MAKEARRAY(m,n, EXPANDλ(resultϑ)),
      IFERROR(display,""))

I have a nasty feeling that such techniques are not for the fainthearted!  Teaching the approach could be challenging as well.

  • tboulden's avatar
    tboulden
    Iron Contributor
    Jan 15, 2022
    Sorry for the delay! I like this approach, had to refresh my memory on what territory we traversed in this thread.

    I agree about teaching to others, have been conversing with someone on a forum asking "why use thunks?" and asking for simple examples. I may have to show them this as it may be more accessible way of demonstrating. Hope 2022 has started off well for you!
    • PeterBartholomew1's avatar
      PeterBartholomew1
      Silver Contributor
      Jan 15, 2022

      tboulden 

      It was the fact that I do not find picking up on some of our past discussions to be a trivial exercise that made me think of how one might train others.  The challenge might be almost as significant as setting out to move an Excel user to M or DAX.  Whilst one may argue that Lambda functions are simple extensions of the traditional spreadsheet methods to include user-defined functions, the process of solution development shifts from the ad-hoc to being a programming exercise (by way of contrast, traditional methods allow users to interact with their numbers whilst remaining in denial that they are actually programming).

       

      I am happy that you should modify or use the example as you see fit and I would be interested to know whether you convince your forum member.

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