Invalid index (4) for list: valid range is 1..3

Question

I have the below mentioned throwing the mismatch error. pls guide 
 
 a!forEach( items: ri!Addresslist, expression: reduce( a!update, fv!item, merge( { "AddressId", "ReferenceId", "Type", "CreateBy", "CreateDate" }, { if( fv!item.AddressId = ri!primarykey, null, fv!item.AddressId ), ri!Dependent[fv!index].AppianUserId, "Permanent Home Address", loggedInUser(), a!defaultValue(fv!item.CreateDate, now()) } ) ) ) 
 Expression evaluation error at function a!forEach: Error in a!forEach() expression during iteration 4: Expression evaluation error at function 'merge' parameter 2 [line 14]: Invalid index (4) for list: valid range is 1..3

Stefan Helzle · Answer

Seems like that your rule inputs AddressList and Dependent are not of the same length. a!forEach(
 items: ri!Addresslist,
 expression: reduce(
 a!update,
 fv!item,
 merge(
 {
 "AddressId",
 "ReferenceId",
 "Type",
 "CreateBy",
 "CreateDate"
 },
 {
 if(
 fv!item.AddressId = ri!primarykey,
 null,
 fv!item.AddressId
 ),
 ri!Dependent[fv!index].AppianUserId,
 "Permanent Home Address",
 loggedInUser(),
 a!defaultValue(fv!item.CreateDate, now())
 }
 )
 )
)

Mike Schmitt · Answer

Your use of "reduce()" here does not make sense. What are you hoping to accomplish / why aren't you just using a!update() on its own?

Mike Schmitt · Answer

om786 said: reduce provide the utitlity to perform specific action to be run for all listed items in address table. 
 I'm almost completely sure reduce() is not needed here and will actively mess you up. Unless each item in ri!address somehow also contains a list (array) of items. Can you share what the data structure of ri!address is, in general?