Strange Behaviour of ReplaceAll with Subscript (A bug or what?)

*To*: mathgroup at smc.vnet.net*Subject*: [mg113317] Strange Behaviour of ReplaceAll with Subscript (A bug or what?)*From*: Thomas Dowling <thomasgdowling at gmail.com>*Date*: Sat, 23 Oct 2010 07:06:36 -0400 (EDT)

Hello, I have come across some strange behaviour with ReplaceAll when creating subscripted variables, which I am not able to explain. The Problem: I have lists like the following: list1 = Partition[Range[6], {3}] Out[5]={{1,2,3},{4,5,6}} list2 = Partition[Range[9], {3}] list3 = Partition[Range[12], {3}] I wish to convert to a list of indexed variables which I do as follows: replist={Subscript[a,x],Subscript[a,y],Subscript[a,z]}; list1/.{x_,y_,z_}-> replist This produces the expected output {{Subscript[a, 1],Subscript[a, 2],Subscript[a, 3]},{Subscript[a, 4],Subscript[a, 5],Subscript[a, 6]}} The same applies to list3 list3/.{x_,y_,z_}-> replist However, **list2** produces the following list2/.{x_,y_,z_}-> replist {Subscript[a, {1,2,3}],Subscript[a, {4,5,6}],Subscript[a, {7,8,9}]} No such problem arises with Power or with Times (but the same problem occurs with Superscript) list2/.{x_,y_,z_}-> {Power[a,x],Power[a,y],Power[a,z]} gives as output {{a,a^2,a^3},{a^4,a^5,a^6},{a^7,a^8,a^9}} A Workaround: There is an easy solution using Replace and specifying the level Replace[list2, {x_,y_,z_}-> replist, {1}] giving the expected output: {{Subscript[a, 1],Subscript[a, 2],Subscript[a, 3]},{Subscript[a, 4],Subscript[a, 5],Subscript[a, 6]},{Subscript[a, 7],Subscript[a, 8],Subscript[a, 9]}} The problem arises when the number of entries in the ReplaceAll list (ie,{x_,y_,z_}) is the same as the number of sublists in the target list (The same problem occurs with the following) Partition[Range[16], {4}]/.{w_,x_,y_,z_}-> {Subscript[a,w],Subscript[a,x],Subscript[a,y],Subscript[a,z]} Is this a bug or is such behaviour expected (and if so, what am I missing?) Is thre a way of telling ReplaceAll what level to use (so that it ALWAYS produces the expected result)? The more general problem I have is that lists of this type are generated by a program, and everything is well unless the generated list happens to have 3 sublists. I can fix things using Replace, but should I have predicted such a problem occurring? I would be interested in your thoughts, and in better workarounds (and in better approaches). Thanks Thomas Dowling