RE: Appending an element to a list
- To: mathgroup at smc.vnet.net
- Subject: [mg23943] RE: [mg23880] Appending an element to a list
- From: "David Park" <djmp at earthlink.net>
- Date: Fri, 16 Jun 2000 00:57:30 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
> -----Original Message----- > From: Felix Farkas [mailto:farkas at icamail.epfl.ch] To: mathgroup at smc.vnet.net > > Hello, > i do have a problem, which is probably very easy, but i couldn't find > a reasonable way to solve it. I would appreciate very much any hint. > > I have a list of lists. My purpose is to append a new elements to this > list. These elements are generated by some function. Sometimes the > output of the function is an empty list meaning no element. In this > situation Append includes the new element in the list. i would like to > avoid this behaviour. > > ex: > > In[1]:= d={} > Out[1]= {} > > In[2]:= Append[{{2,2},{3,3}},d] > Out[2]:= {{2,2},{3,3},{}} > > > Thanks in advance. Felix Farkas > Felix, Your could do this (Sequence[] disappears from a list of items): Append[{{2, 2}, {3, 3}}, {}] /. {} -> Sequence[] or define your own Append function: myAppend[l_, element_] := If[Length[element] > 0, Append[l, element], l] myAppend[{{2, 2}, {3, 3}}, {}] {{2, 2}, {3, 3}} myAppend[{{2, 2}, {3, 3}}, {4, 4}] {{2, 2}, {3, 3}, {4, 4}} David Park djmp at earthlink.net http://home.earthlink.net/~djmp/