Re: Listable functions with two brackets f[][] (SubValues)

*To*: mathgroup at smc.vnet.net*Subject*: [mg64931] Re: Listable functions with two brackets f[][] (SubValues)*From*: dh <dh at metrohm.ch>*Date*: Wed, 8 Mar 2006 00:59:43 -0500 (EST)*References*: <dujrti$923$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Hi, I can not see the benefit of using f[a_][b_] instead of f[a_,b_]. But if you insists on using f[][]: Consider f[a][b]: f[a] returns a function that takes [b] as argument. Only a symbol can have the attribute "Listable". Therefore, f[a] can not have the arttribute "Listable". It followes, that the definition has to be done in 2 stages. First we define the "inner" function and make it listable: Remove[f1,f2]; f1[fun_,a_Line]:=Line[fun /@ a[[1]]]; SetAttributes[f1, Listable] now we define the final function f2[][] using f1: f2[vec_][a_]:= f1[#+vec &, a] this will now work as you want: f2[{1, 3}][c] f2[{1, 3}][b] Daniel Goyder Dr HGD wrote: > I would like to use a double bracket function f[][] as, I believe, > originally promoted by David Park. The second bracket is called the > SubValues of f. My problem is that I would like the second bracket to > be Listable. Is this possible.? > > The following is a toy example by way of illustration. Suppose I wish > to make a function that translates the graphics primitive Line. Then I > can make a function as follows and it works for one Line[]. > > In[1]:= > ClearAll[f]; > f[vec_][a_Line] := Line[(#1 + vec & ) /@ a[[1]]] > > In[3]:= > b = Line[{{1, 2}, {3, 4}, {5, 6}}]; > > In[4]:= > f[{1, 3}][b] > > Out[4]= > Line[{{2,5},{4,7},{6,9}}] > > However, if I have a list of Lines[] it does not work because the > second bracket does not have the Attribute Listable. > > In[5]:= > c = {b, Line[{{2, 3}, {5, 6}, {9, 3}}]}; > > In[6]:= > f[{1, 3}][c] > > Out[6]= > f[{1,3}][{Line[{{1,2},{3,4},{5,6}}],Line[{{2,3},{5,6},{9,3}}]}] > > Attempts to make the function f[][] Listable fail because it makes the > first and not the second bracket Listable. > > In[7]:= > SetAttributes[f, Listable] > > In[8]:= > f[{1, 3}][c] > > Out[8]= > {f[1],f[3]}[{Line[{{1,2},{3,4},{5,6}}],Line[{{2,3},{5,6},{9,3}}]}] > > Is there a solution? > > Thanks > Hugh Goyder >