Re: Modify a list of pure functions
- To: mathgroup at smc.vnet.net
- Subject: [mg70843] Re: Modify a list of pure functions
- From: David Bailey <dave at Remove_Thisdbailey.co.uk>
- Date: Sat, 28 Oct 2006 23:39:07 -0400 (EDT)
- References: <ehv8ts$g5i$1@smc.vnet.net>
. wrote:
> Hi all,
>
> I have a problem where I need to build up a list of pure functions that
> have been modified from a base pure function. I'm not having much luck
> in getting it to work correctly, and was wondering if anyone had some
> good ideas. A toy version of the problem is:
>
> f = Sin[#]&
> listF = Table[(i f[##])&, {i, 5}]
>
> What I'd hope to get is listF containing:
> {Sin[##]&, (2 Sin[##])&, (3 Sin[##])&, (4 Sin[##])&, (5 Sin[##])&}
>
> But instead, I get something where neither i nor f have been evaluated.
> As I say, this is a toy example. One thing that does partially work
> for the toy case, but does not seem to work for more complicated
> situations is to use ReplaceAll:
>
> listF2 = Table[{j f[##])& /. j->i, {i,5}]
>
> This does give:
> {1 f[##1] &, 2 f[##1] &, 3 f[##1]&, 4 f[##1] &, 5 f[##1] &}
>
> However, all that has been done is the index has been inserted; no
> significant evaluation seems to have occurred. So this method doesn't
> work in more complicated situations. For example, suppose insted of
> using the index i directly, it actually referred to a list:
>
> coeffs = Reverse[ Range[5] ];
> listF3 = Table[ (coeffs[[j]] f[##]) & /. j->i, {i, 5}]
>
> This just gives:
> {coeffs[[1]]*f[##1] & , coeffs[[2]]*f[##1] & , coeffs[[3]]*f[##1] & ,
> coeffs[[4]]*f[##1] & , coeffs[[5]]*f[##1] & }
>
> Which is no good if coeffs is a temporary variable.
>
> Any suggestions would be most appreciated. Thanks!
>
Hello,
Your problem is caused by the fact that pure functions are represented
(in FullForm) as objects with head Function, which has the attribute
HoldAll. I am not quite sure if this solution is general enough for your
real problem, but one approach is to inject things inside the function
using With:
coeffs = Reverse[Range[5]];
listF3 = Table[With[{tmp = coeffs[[j]]}, (tmp f[##]) &], {j, 1, 5}]
David Bailey
http://www.dbaileyconsultancy.co.uk