Re: Incredible slow Plot

*To*: mathgroup at smc.vnet.net*Subject*: [mg120150] Re: Incredible slow Plot*From*: DrMajorBob <btreat1 at austin.rr.com>*Date*: Sun, 10 Jul 2011 05:00:44 -0400 (EDT)*References*: <201107091135.HAA13652@smc.vnet.net>*Reply-to*: drmajorbob at yahoo.com

We need details, so this is just a guess... but make sure NDSolve isn't being solved all over again for each plot point. That is, you compute an Interpolation function, you Set (not SetDelayed) a function equal to it, and you use that function in the Plot. Here's one way to do it right (for the first example in Help for NDSolve): f = y /. First@ NDSolve[{y'[x] == y[x] Cos[x + y[x]], y[0] == 1}, y, {x, 0, 30}]; t1 = First@Timing[Plot[f@x, {x, 0, 30}, PlotRange -> All];] 0.012584 and here's a simple way to do it wrong: f := y /. First@NDSolve[{y'[x] == y[x] Cos[x + y[x]], y[0] == 1}, y, {x, 0, 30}]; t2 = First@Timing[Plot[f@z, {z, 0, 30}, PlotRange -> All];] 2.29799 t2/t1 182.612 That's a big difference in speed, you'll have to agree. Bobby On Sat, 09 Jul 2011 06:35:00 -0500, Iván Lazaro <gaminster at gmail.com> wrote: > Hi! > > I'm trying to Plot a result from NDSolve. The problem is that when I > pass this solution to Plot it takes an incredible long time (four > hours for times between 0 and 1200) to plot. I tried generating an > image file (.jpg, .eps, etc.) but that doesn't change anything. I > tried also giving to Plot the option PerformanceGoal -> "Speed", but > it gives me only half an hour less of waiting time. The other options > (like PlotPoints -> Number, MaxRecursion -> 0 ) give an improvement > but the image is ugly unless PlotPoints is high, which at the end is > the same. > > I tried also asking for Plot only the last 200 units of time, trying > to avoid the most oscillating part of the graphic, but that really > doesn't help (and really it's not what I need). > > Finally, the resulting InterpolatingFunction, when evaluated outside > Plot, evaluates fast, and a ListPlot using points returned by a Table > works well, but the resulting graphic is insanely big (35, 36 Mb) if I > put the points necessary to give account of the features of the > dynamic. > > Unfortunately I can't give you an example because the equations I'm > solving are too long, but I wonder if there is someway to generate the > graphic, either on the interface or in a file, to not be so slow. > > Thanks in advance, and sorry for this vague question (which, by the > way, without examples, is useless... by I had to give it a try). > -- DrMajorBob at yahoo.com

**References**:**Incredible slow Plot***From:*Iván Lazaro <gaminster@gmail.com>