Re: Re: Plotting a function and its derivative

*To*: mathgroup at smc.vnet.net*Subject*: [mg49555] Re: [mg49529] Re: Plotting a function and its derivative*From*: DrBob <drbob at bigfoot.com>*Date*: Fri, 23 Jul 2004 05:59:52 -0400 (EDT)*References*: <200407220647.CAA21144@smc.vnet.net>*Reply-to*: drbob at bigfoot.com*Sender*: owner-wri-mathgroup at wolfram.com

Where Bill says DelayedSet, he means SetDelayed. Bobby On Thu, 22 Jul 2004 02:47:10 -0400 (EDT), Bill Rowe <readnewsciv at earthlink.net> wrote: > On 7/21/04 at 6:40 AM, skaai at earthlink.net (adrian sky) wrote: > >> anyways, what i'm trying to do is illustrate the simplest of >> derivatives in the form of a graph but i am getting errors. > >> the function i'm trying to derive is x^2 its derivative i'm trying >> to illustrate is 2x > >> i would like to set up a graph which allows me to determine the >> argument (be it x^2 or x^4+1), calculate its derivative, and graph >> them so as to illustrate their relationship. > >> i know that the derivative can be graphed at any x value, and as >> far as this is concerned, the value can be anything, probably a >> small value to keep the graph manageable. > >> anyways, i'm inputting this: f[x_] := x^2; d[x_] := Ã¢Ë?â??_x f[x] which >> supposedly should assign the f(x) a value of ^2 and d(x) a value of >> the derivative of f(x) > >> i can even type f(x) and get back the value x^2 and d(x) gives me >> 2x, so it seems the system understands and assigns the requested >> values > >> but when I try to plot this with the command: Plot[{f[x], d[x]}, >> {x, -5, 5}] i get numerous errors and only get the graph of x^2 the >> errors are: General::ivar: -5. is not a valid variable. Plot::plnr: >> d[x] is not a machine-size real number at x = -4.9999995833333335` >> Plot::plnr: d[x] is not a machine-size real number at x = >> -4.594330084270842` and a few more similar plot errors > > The problem you are having is due to the way Plot evaluates its arguements. What is happening is Plot sends your function d a numeric value and d attempts to take the derivative of a numeric value with respect to x. This is clearly not what you want. > > There are two ways around this. First, you can define d using Set rather than DelayedSet. That will cause the derviative to be evaluated before Plot tries to substitute a value for x. > > The second way would be to use Evaluate to force evaluation of the derivative before Plot substitutes values for x, i.e., with your d defined as is do: > > Plot[Evaluate@{f[x],d[x]}, {x, -5, 5}] > > Look at the discussion of Plot, Set and DelayedSet in the Mathematica Book for a more complete discussion. > -- > To reply via email subtract one hundred and four > > > -- DrBob at bigfoot.com www.eclecticdreams.net

**References**:**Re: Plotting a function and its derivative***From:*Bill Rowe <readnewsciv@earthlink.net>

**Re: Plotting 2D functions with discontinuities**

**Re: Plotting a function and its derivative**

**Re: Plotting a function and its derivative**

**Re: Plotting a function and its derivative**