Re: Manipulate and ParametricPlot Control?

*To*: mathgroup at smc.vnet.net*Subject*: [mg114385] Re: Manipulate and ParametricPlot Control?*From*: Albert Retey <awnl at gmx-topmail.de>*Date*: Thu, 2 Dec 2010 05:43:29 -0500 (EST)*References*: <id4sf3$cbg$1@smc.vnet.net>

Hi, > > As a simple test file, I created a notebook (which I don't have any > more) containing the one cell > > Manipulate[ Plot[x^a, {x,0,1}], {a,0,5}] > > (May have had a PlotRange option also; don't recall for sure.) > > Executed it; worked as expected. > > Decided to set an initial value for a; as best I recall (details are > fuzzy), I edited the cell to change {a,0,5} to {{a,1},0,5} (that's what > I intended to do, anyway). > > When I re-executed the cell, I got a live Manipulate that displayed a > ***two-dimensional*** (2D) Control -- something I had never seen before, > but which looked exactly like the one I more recently found in the > ParametricPlot example in the Manipulate Help. > > Exercising this Manipulate displayed **two** x^a curves, with values > corresponding to the H and V axes of the 2D Control. Functioned fine; > no error msgs at any time. > > I made a (presumably erroneous) guess that the edited cell must have > somehow been influenced by the earlier execution of the initial, > un-edited version of the same cell (I had two implementations of the "a" > variable floating around, maybe??). So, I tried adding Removes before > the cell; tried Saving it, Quiting Mathematica, and re-opening it. The 2D > Control obstinately remained (and worked). This then led to the > re-virginating query. from the documentation: "Manipulate is a scoping construct that implements lexical scoping." So the two a will (should) not interact. I have never seen a case that would suggest that the scoping would break. > I may have made a typo in the editing I noted above, but I sure as hell > did NOT change Plot to ParametricPlot. And when I try some tests now > using Plot[ {f1, f2}, ...] inside a Manipulate, I get two curves -- but > never a 2D Control. > > Any ideas what went wrong here? My guess is that you erroneously entered this: Manipulate[Plot[x^a, {x, 0, 1}], {a, {0, 1}, 5}] the x^a will then evaluate to {x^a[[1]],x^a[[2]} and thus two lines are plotted. From the fact that the lines have the same color you can also guess that Plot recognizes x^a as two lines only after evaluating the first argument. hth, albert