Re: PlotPoints Question

*To*: mathgroup at smc.vnet.net*Subject*: [mg13487] Re: PlotPoints Question*From*: "Allan Hayes" <hay at haystack.demon.cc.uk>*Date*: Sun, 26 Jul 1998 02:33:45 -0400*References*: <6p98ec$b82@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Kevin J. McCann wrote in message <6p98ec$b82 at smc.vnet.net>... >I wrote a routine the other day to display the points that Plot actually >uses. Much to my surprise, when I set PlotPoints to some number like 5, >I found that more than this number were actually used. > >For example, try this > >p1 = Plot[x^2, {x, 0, 2}, PlotPoints ->5, PlotRange ->All]; > >and then to > >p1//FullForm > >You will see that 19 points are actually used. Given all that, what does >the PlotPoints option really do? > >Kevin > > Kevin, Plot[expr,{x,a,b}] and ParametricPlot[expr,{x,a,b}] functions are both adaptive and have the following control options for the user PlotPoints -> (pp, say)number of initial sample points MaxBend -> the max "normalized bend" in degrees between segments to allow before considering whether to add extra sample points PlotDivision -> (pd,say)prevents extra sample points between consecutive sample points closer than about (b-a)/((pp pd) It looks as if the following, if not something simpler, should give only 5 points in the output. p1 = Plot[x^2, {x, 0, 2}, PlotPoints ->5, PlotRange ->All, PlotDivision->1, MaxBend -> 200] but in fact it gives 6 points: Cases[%, Line[pts_]:> pts, Infinity][[1]] -7 -13 {{5. 10 , 2.5 10 }, {0.4868038988749894, 0.2369780359598908}, {1.017705598312484, 1.035724684836571}, {1.516312396062462, 2.299203282452687}, {1.995820893249937, 3.983301037932975}, {1.999999499999999, 3.999998000000248}} Length[%] 6 Allan ------------------------------------------------------------- Allan Hayes Training and Consulting Leicester UK http://www.haystack.demon.co.uk hay at haystack.demon.co.uk voice: +44 (0)116 271 4198 fax: +44(0)116 271 8642