Re: Re: Apply function to parts of a list

*To*: mathgroup at smc.vnet.net*Subject*: [mg86318] Re: [mg86260] Re: [mg86224] Apply function to parts of a list*From*: Carl Woll <carlw at wolfram.com>*Date*: Sat, 8 Mar 2008 05:42:22 -0500 (EST)*References*: <200803060759.CAA29253@smc.vnet.net> <200803070727.CAA19775@smc.vnet.net>

Stern wrote: ><I sent this reply to guerom00 directly but am resending it to group so it >gets archived for posterity> > >On Thu, Mar 6, 2008 at 8:43 AM, Stern <nycstern at gmail.com> wrote: > > > >>There are probably a lot of ways to do this. The most natural for me is to >>use nested Transpose commands; this has the advantage of avoiding anonymous >>functions, so it's clearer if nested in the middle of other commands that >>use anonymous functions. >>my way: >> >>Transpose[{2*Transpose[data][[1]], Transpose[data][[2]]}] >> >> Even faster is: Transpose[{2,1} Transpose[data]] A slower alternative is to use ScalingTransform (new in 6): ScalingTransform[{2,1}][data] There is also AffineTransform, RescalingTransform, TranslationTransform and RotationTransform, providing useful tools for tranforming vectors. Carl Woll Wolfram Research >> >>This may not be memory- or compute-efficient, as it requires repeating the >>Transpose[data] command, so for large datasets you may prefer >> >> >>Module[{transp}, transp = Transpose[data]; >> Transpose[{2*transp[[1]], transp[[2]]}]]; >> >> >>Either of these is quicker than where you started -- >> >> >>In[19]:= Timing[MapAt[2*# &, data, Table[{i, 1}, {i, 1, Length[data]}]];] >>Out[19]= {0.004409, Null} >> >> >>In[20]:= Timing[Transpose[{2*Transpose[data][[1]], >>Transpose[data][[2]]}];] >>Out[20]= {0.000672, Null} >> >> >>In[21]:= Timing[Module[{transp}, transp = Transpose[data]; >> Transpose[{2*transp[[1]], transp[[2]]}]];] >>Out[21]= {0.000698, Null} >> >> >>Cheers, >> >> >>Michael >> >> >> >>On Thu, Mar 6, 2008 at 2:59 AM, guerom00 <guerom00 at gmail.com> wrote: >> >> >> >>>Hi all, >>> >>>Very often, I find myself having datas to plot of this form : >>> >>>data={{x1,y1},{x2,y2},{x3,y3}...{xn,yn}} >>> >>>So, very conventional : a list of abscissa and ordinate. Now, let's say >>>I want to multiply by 2 all my ascissa. Right now, I write : >>> >>>MapAt[2*#&,data,Table[{i,1},{i,1,Length[data]}]] >>> >>>It's OK but I isn't there another way to achieve this as I find this >>>rather "involved" for such a simple thing ? >>> >>>Thanks in advance. >>> >>> >>> >>>

**References**:**Apply function to parts of a list***From:*guerom00 <guerom00@gmail.com>

**Re: Apply function to parts of a list***From:*Stern <nycstern@gmail.com>