Re: Elegant syntax for multiple conditional assignment?
- To: mathgroup at smc.vnet.net
- Subject: [mg54409] Re: [mg54337] Elegant syntax for multiple conditional assignment?
- From: Bob Hanlon <hanlonr at cox.net>
- Date: Sun, 20 Feb 2005 00:08:18 -0500 (EST)
- Reply-to: hanlonr at cox.net
- Sender: owner-wri-mathgroup at wolfram.com
old={a1,c1,x1,y1};
new={a2,c2,x2,y2};
cond={True,False,True,False};
{a,c,x,y} = If[#[[1]],#[[2]],#[[3]]]&/@
Transpose[{cond,new,old}]
{a2,c1,x2,y1}
{a,c,x,y} = If[#[[1]],#[[2]],#[[3]]]&/@
Thread[{cond,new,old}]
{a2,c1,x2,y1}
Bob Hanlon
>
> From: Scott Hemphill <hemphill at hemphills.net>
To: mathgroup at smc.vnet.net
> Date: 2005/02/19 Sat AM 02:31:38 EST
> To: mathgroup at smc.vnet.net
> Subject: [mg54409] [mg54337] Elegant syntax for multiple conditional assignment?
>
> 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
> --
> Scott Hemphill hemphill at alumni.caltech.edu
> "This isn't flying. This is falling, with style." -- Buzz Lightyear
>
>