Re: Elegant syntax for multiple conditional assignment?

*To*: mathgroup at smc.vnet.net*Subject*: [mg54444] Re: Elegant syntax for multiple conditional assignment?*From*: Peter Pein <petsie at arcor.de>*Date*: Sun, 20 Feb 2005 00:10:39 -0500 (EST)*References*: <cv6q9a$65c$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Scott Hemphill wrote: > Dear Mathematica aficionados: > > I have several state variables, each of which is a vector of "n" elements. > I would like to assign each element in each vector based on a condition > which is in the corresponding position in a condition vector. To be more > specific: > > Each of a, c, x and y is a List of Length n. > cond is a List of Length n, containing True and False values > > What I have is > > old = {a, c, x, y}; > > code which changes a, c, x, and y > > new = {a, c, x, y}; > > What I want to do is assign to each element of a, c, x and y the old value > of a, c, x or y if the corresponding element of cond is False, and the new > value of a, c, x or y if the corresponding element of cond is True. > > I've got a method that works: > > t=Transpose; > {a, c, x, y} = t[If[#[[1]],#[[2]],#[[3]]]& /@ t[{cond,t[new],t[old]}]]; > > All the transpositions and array indexing make me wonder if there's a more > elegant way of expressing this. > > Any takers? Thanks in advance. > > Scott Something like the following? In[1]:= n=5; old=Table[10i+j,{i,4},{j,n}]; new=-old; cond=Table[Random[]>.5,{n}] Out[4]= {True,False,True,True,True} In[5]:= If[Or@@cond, ReplacePart[old,new,#,#]&@Position[Table[cond,{4}],True], old] Out[5]= {{-11,12,-13,-14,-15}, {-21,22,-23,-24,-25}, {-31,32,-33,-34,-35}, {-41,42,-43,-44,-45}} -- Peter Pein Berlin