Re: How can this MapAt application be done more efficiently

*To*: mathgroup at smc.vnet.net*Subject*: [mg4215] Re: How can this MapAt application be done more efficiently*From*: rubin at msu.edu (Paul A. Rubin)*Date*: Tue, 18 Jun 1996 03:25:48 -0400*Organization*: Michigan State University*Sender*: owner-wri-mathgroup at wolfram.com

In article <4pis80$iuo at dragonfly.wolfram.com>, Joel Cannon <cannon at alpha.centenary.edu> wrote: ->I have written a function to Drop any zero's from a table of numbers. ->Here is an example: -> -> In[161]:= dropzero[ {{5, 0}, {4, 1}, {3, 2, 0}, {3, 1, 1}} ] -> -> Out[161]= {{5}, {4, 1}, {3, 2}, {3, 1, 1}} -> ->I would like to learn how to do it more efficiently. In what I am ->doing, the 0's will always occur as the last element of the inner ->lists (although it would be nice to have it work in general). -> ->Here is my function, with another function that it calls: -> -> -> dropzero[a_] := MapAt[droplast,a, Partition[ Map[First,Position[a,0]],1] ] -> -> droplast[a_] := Drop[a, -1] -> ->It feels to me like two things could be improved. -> ->1. I had to define the function droplast because I couldn't figure out ->how to use Drop[#,-1] with in the MapAt expression and get it to work. -> ->2. Of less importance, ->the argument "Partition[Map[First,Position[a,0]],1]" seems a bit clumsy. Substitution proves faster. I pasted in your functions, and added a singleton zero to your example for test purposes. In[]:= Timing[ dropzero[ {{5, 0}, {4, 1}, {3, 2, 0}, {3, 1, 1}, {0}} ] ] Out[]= {0.329 Second, {{5}, {4, 1}, {3, 2}, {3, 1, 1}, {}}} In[]:= dropzero2[ a_ ] := a /. {x___, 0} -> {x} (* substitution *) In[]:= Timing[ dropzero2[ {{5, 0}, {4, 1}, {3, 2, 0}, {3, 1, 1}, {0}} ] ] Out[]= {0.055 Second, {{5}, {4, 1}, {3, 2}, {3, 1, 1}, {}}} BTW, that's three underscores ("_") after "x" in the definition of dropzero2. Paul ************************************************************************** * Paul A. Rubin Phone: (517) 432-3509 * * Department of Management Fax: (517) 432-1111 * * Eli Broad Graduate School of Management Net: RUBIN at MSU.EDU * * Michigan State University * * East Lansing, MI 48824-1122 (USA) * ************************************************************************** Mathematicians are like Frenchmen: whenever you say something to them, they translate it into their own language, and at once it is something entirely different. J. W. v. GOETHE ==== [MESSAGE SEPARATOR] ====