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LBROWN7's avatar
LBROWN7
Brass Contributor
Oct 09, 2023

Recursive LAMBDA implementation of Excel's REDUCE function.

Not a question -- just sharing some interesting code. Out of curiosity, I decided to develop an implementation of Excel's REDUCE function as a recursive Excel LAMDA function.  I was truly amazed how...
excel
Formulas and Functions
SergeiBaklan's avatar
SergeiBaklan
Diamond Contributor
Oct 13, 2023

lori_m 

That could be great workaround. However, performance with SCAN is still not good. Tried poor thunking

SCAN3 = LAMBDA(initial_value, array, function,
    LET(
        sc, SCAN(LAMBDA(initial_value), array, LAMBDA(a, v, LAMBDA(function(a(), v)))),
        n, COLUMNS(sc),
        res, LAMBDA(ME, k,
            IF( k = 1,
                INDEX(sc, 1, 1)(),
                HSTACK(ME(ME, k - 1), INDEX(sc, 1, k)() ))),
        res(res, n)
    )
)

but that gives nothing.

Native function is required. Not sure Microsoft will do it, it looks like all efforts now are concentrated on Python.

 

LBROWN7's avatar
LBROWN7
Brass Contributor
Oct 13, 2023

HI SergeiBaklan

I added an explicit stack parameter to the recursive scan and that seems to do the trick.

 

=LAMBDA(initial_value, array, CLAMBDA,
    LET(
        _00, "Implementation of SCAN in Excel Lambda",
        _01, "accumulator must be a scalar or  column vector"
        vec, TOROW(array),
        rec_L, LAMBDA(stack, acc, cindex, rec_LL,
            LET(
                cvalue, INDEX(vec, 0, cindex),
                new_acc, CLAMBDA(acc, cvalue),
                new_stack, HSTACK(stack, new_acc),
                IF(
                    cindex = COLUMNS(vec),
                    new_stack,
                    rec_LL(new_stack, new_acc, cindex + 1, rec_LL)
                )
            )
        ),
        tresult, (rec_L(initial_value, initial_value, 1, rec_L)),
        MAP(tresult, LAMBDA(v, IF(ISERROR(v), "", v)))
    )
)({1}, {2, 3, 4}, LAMBDA(a, b, VSTACK(a, b)))

 


Probably need to add code to drop the 1st column - but above seems to be moving in the right direction.

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  • LBROWN7's avatar
    LBROWN7
    Brass Contributor
    Oct 14, 2023

    Here is a correct version of the above, with the correct stack initialization.  I think this is a valid scan function.

    =LAMBDA(initial_value, array, CLAMBDA,
    LET(
        _00, "Implementation of SCAN in Excel Lambda",
        vec, TOROW(array),
        rec_L, LAMBDA(stack, acc, cindex, rec_LL,
            LET(
                cvalue, INDEX(vec, 0, cindex),
                new_acc, CLAMBDA(acc, cvalue),
                new_stack, IF(cindex = 1, new_acc, HSTACK(stack, new_acc)),
                IF(cindex = COLUMNS(vec), new_stack, rec_LL(new_stack, new_acc, cindex + 1, rec_LL))
            )
        ),
        tresult, (rec_L("", initial_value, 1, rec_L)),
        MAP(tresult, LAMBDA(v, IF(ISERROR(v), "", v)))
    )
)({1, 2}, {2, 3, 4}, LAMBDA(a, b, VSTACK(a, b)))

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