Excel 365 SUM dynamic array at row level

Lorenzo 

Since you now have Lambda functions, the solution simplifies.  First, assuming the dummy array has been given a name

= BYROW(Dummy_arr, Sumλ)

where

Sumλ = LAMBDA(x,SUM(x))

or including the array definition within a LET

= LET(
      dummyArr, {1;10;100}*SEQUENCE(1,3),
      BYROW(dummyArr, Sumλ)
  )

I have attached a file, but it contains little by way of additional information.

Lorenzo 

Some neat tricks, which address one of the weaknesses of dynamic arrays which still cause developers to revert to the flawed concepts of copying formulas with relative references!

In normal use, array formulas largely replace the need for helper ranges, so reducing the risk of error.  In the attached, I have taken a different strategy and allowed the array formulas to write to dynamic ranges to a hidden helper sheet so that the xxxIFS family of functions may be used.

p.s. Lorenzo Matrix equations are not that scary!  Because of the nature of my former employment, I have probably written more matrix equations than I have English language paragraphs!

where 'mask' is defined to by

= IF(dummyArr#,k)

Lorenzo 

Perhaps for an array of max values:

=LET(
i,SEQUENCE(ROWS(array)),
m,COLUMNS(array),
arrayRank,i+PERCENTRANK.EXC(array,array,15),
rowRank,SMALL(arrayRank,i*m),
rowTot,MMULT(array*(arrayRank=rowRank),1^i),
rowFreq,MMULT(N(arrayRank=rowRank),1^i),
rowTot/rowFreq
)

Change i*m to (i-1)*m+1 for array of min values

In the given example:

array ={1,2,3;10,20,30;100,200,300}
arrayRank ={1.1,1.2,1.3;2.4,2.5,2.6;3.7,3.8,3.9}
rowRank ={1.3;2.6;3.9}
rowTot ={3;30;300}

The result is divided by the number of occurences of the max value if there is more than one.