Hey all, I'm interested in writing a simple expression rule which will

I just had the same requirment. I did the following. I create one rule with:
where(ri!array=value,-1) // value being a single value
and another one that is using apply:
=apply(rule!GetIndexOfArrayWithValue,ri!values,ri!array) //values and array are both multiple

not sure wether this is the most efficient way, but I didn't come up with anything better.

that appears to work, thanks a bunch.

Maybe this one

=apply(fn!displayvalue, {1, 5, 12}, {1,3,5,8}, {"one", "three", "five", "eight"}, "no")

Yep. Something like this:
=apply(fn!displayvalue, ri!values, ri!array, enumerate(count(ri!array))+1, "no")
would also work. Not sure which solution is better performance wise.

Can also go for wherecontains(ri!smallArray,ri!bigArray). it is not an documented function. So you will not be able to search in appin expression editor. But using the function in exp editor will give you the desired output.

That's definetely the nicest approach.

I guess though that it's possible that non documented functions can get deprecated in any release, right? Or are there maybe any other risks in using undocumented functions?

In 6.7 and greater there is a where() function: forum.appian.com/.../Array_Functions

where() doesn't have a way to replicate the functionality I was originally asking about, though (the ability to find the "where" indexes of multiple objects within an array). wherecontains() certainly does, thanks to subhankarb for mentioning that.

I would discourage the use of undocumented functions.

Instead, you could use reduce() and 2 expression rules to achieve the desired result:

Rule 1
--------
Name: appendIndexIfContains
Inputs: indices (Integer, Multiple), bigArrayValue (Integer), bigArrayIndex (Integer), smallArray (Integer, Multiple)
Expression: =if(contains(ri!smallArray,bigArrayValue),append(ri!indices,ri!bigArrayIndex),ri!indices)

Rule 2
--------
Name: getIndicesWhereBigContainsSmall
Inputs: bigArray (Integer, Multiple), smallArray (Integer, Multiple)
Expression: =reduce(rule!appendIndexIfContains, {},merge(ri!bigArray,enumerate(length(ri!bigArray))+1),ri!smallArray)