Re: avoiding non-machine numbers

*To*: mathgroup at smc.vnet.net*Subject*: [mg115593] Re: avoiding non-machine numbers*From*: "Rolf.Mertig at gmail.com" <rolf.mertig at gmail.com>*Date*: Sun, 16 Jan 2011 05:50:33 -0500 (EST)*References*: <igpbd4$fae$1@smc.vnet.net>

Compile seems sufficiently fast: In[7]:= fun = Compile[{{n, _Integer}}, Module[{t}, t = RandomReal[NormalDistribution[0, 20], n]; Exp[-t^2]]]; Timing[re = fun[400000];Min[re] ] Out[8]= {0.094,0.} Rolf -- GluonVision GmbH http://www.gluonvision.com Berlin, Germany On Jan 14, 12:17 pm, wpb <wicher.berg... at gmail.com> wrote: > By default, Mathematica switches to non-machine numbers when needed, > eg, > > In[4334]:= Exp[-50.^2] > > Out[4334]= 1.835672669162*10^-1086 > > But this has a severe possible drawback in terms of computational > speed. For example, the following is very quick > > In[4420]:= t = RandomReal[NormalDistribution[0, .1], 400000]; > Exp[-t^2]; // Timing > > Out[4421]= {0., Null} > > but in the following computation non-machine numbers are generated, > and we get an enormous decrease in speed: > > In[4422]:= t = RandomReal[NormalDistribution[0, 20], 400000]; > Exp[-t^2]; // Timing > > Out[4423]= {1.25, Null} > > So is it possible to get a computation such as Exp[-50.^2] to evaluate > to zero in a very fast way? That is, that small numbers outside > machine-precision range evaluate to zero automatically? > > Thanks, Wicher