Re: Flatten alternative
- To: mathgroup at smc.vnet.net
- Subject: [mg105498] Re: [mg105469] Flatten alternative
- From: Leonid Shifrin <lshifr at gmail.com>
- Date: Sat, 5 Dec 2009 05:33:54 -0500 (EST)
- References: <200912040931.EAA06957@smc.vnet.net>
Hi,
How about this:
Clear[myFlatten];
myFlatten[x_List] :=
x //. y_ :> Replace[y, {a___, {b___}, c___} :> {a, b, c}, {0}]
My first attempt by the way was a simpler one:
Clear[myFlattenNaive]
myFlattenNaive[x_List] := x //. {a___, {b___}, c___} :> {a, b, c};
This looks ok, but fails in the following case, for example:
In[1]:= {a,b,{c,{d,{e,f,g[h,{i,j,{k}}]}}}}//myFlattenNaive
Out[1]= {a,b,c,d,e,f,g[h,{i,j,k}]}
The wrong thing is that it flattens lists also inside ofther heads, and this
is not what Flatten does. The more complex version does not do it:
In[2]:= {a,b,{c,{d,{e,f,g[h,{i,j,{k}}]}}}}//myFlatten
Out[2]= {a,b,c,d,e,f,g[h,{i,j,{k}}]}
The added advantage of the myFlatten solution is that it is easy to add to
it a level specification as an optional parameter to make myFlatten flatten
on a specific level, if this is needed.
Needless to say, for deeply nested / complex lists with many sublists on the
same level, myFlattenwill be grossly inefficient.
Regards,
Leonid
On Fri, Dec 4, 2009 at 12:31 PM, Jezzybear <jezzybear19 at hotmail.co.uk>wrote:
> I am trying to create a function myflat[list] to mimic the behavior of
> Mathematica's Flatten[]
> function.Example:
> In[1]:= myflat[{{{a}}, {b, c}, {d}}]
> Out[1]= {a, b, c, d}
> In[2]:= myflat[{{}, 1, {{2}}}]
> Out[2]= {1, 2}
> However in writing this function I want to use only Mathematica's
> pattern matching features, the functions
> First[], Rest[], Prepend[], ListQ[]. I am trying to do this using some
> subfunctions like creating one called myjoin
>
>
>
- References:
- Flatten alternative
- From: Jezzybear <jezzybear19@hotmail.co.uk>
- Flatten alternative