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How to plot derivative directly?

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  How to plot derivative directly? Šerých Jakub <Serych@panska.cz> - 2011-04-11 11:05 +0000
    Re: How to plot derivative directly? Peter Pein <petsie@dordos.net> - 2011-04-12 09:53 +0000
    Re: How to plot derivative directly? Helen Read <readhpr@gmail.com> - 2011-04-12 09:54 +0000
    Re: How to plot derivative directly? Stefan <wutchamacallit27@gmail.com> - 2011-04-12 09:56 +0000
    Re: How to plot derivative directly? Wolfgang Windsteiger <Wolfgang.Windsteiger@risc.jku.at> - 2011-04-12 09:57 +0000
    Re: How to plot derivative directly? Peter Breitfeld <phbrf@t-online.de> - 2011-04-12 09:58 +0000

#1604 — How to plot derivative directly?

Dear mathgroup,

it seems to me, that response to my question shall be very simple,
but I cannot find it. :-(

I want to plot the derivative of the function. I would like to do it
directly, something like:

Plot[D[x^3 - 6 (x + 1)^2 + x - 7, x],{x,-3,8}]

It returns: General::ivar: "-2.99978 is not a valid variable."

I can understand that it is because local variable x from Plot command
interferes with the x variable from the D[].

Yes I can bypass the problem by:
deriv = D[x^3 - 6 (x + 1)^2 + x - 7, x]
Plot[deriv, {x, -3, 8}]

which is fully functional, but as far as I know Mathematica, there must
be some simple solution how to do it directly inside the Plot[].

Thanks in advance for kick-off

Jakub 

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#1609

Am 11.04.2011 13:05, schrieb Śerých Jakub:
> Dear mathgroup,
>
> it seems to me, that response to my question shall be very simple,
> but I cannot find it. :-(
>
> I want to plot the derivative of the function. I would like to do it
> directly, something like:
>
> Plot[D[x^3 - 6 (x + 1)^2 + x - 7, x],{x,-3,8}]
>
> It returns: General::ivar: "-2.99978 is not a valid variable."
>
> I can understand that it is because local variable x from Plot command
> interferes with the x variable from the D[].
>
> Yes I can bypass the problem by:
> deriv = D[x^3 - 6 (x + 1)^2 + x - 7, x]
> Plot[deriv, {x, -3, 8}]
>
> which is fully functional, but as far as I know Mathematica, there must
> be some simple solution how to do it directly inside the Plot[].
>
> Thanks in advance for kick-off
>
> Jakub
>

Hi Jakub,

have a look at 
http://reference.wolfram.com/mathematica/ref/HoldFirst.html (an 
attribute being set for Plot) and use

Plot[D[f[x],x]//Evaluate,{x,x0,x1}]

Peter

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#1615

On 4/11/2011 7:05 AM, =8Aer=FDch Jakub wrote:
> Dear mathgroup,
>
> it seems to me, that response to my question shall be very simple,
> but I cannot find it. :-(
>
> I want to plot the derivative of the function. I would like to do it
> directly, something like:
>
> Plot[D[x^3 - 6 (x + 1)^2 + x - 7, x],{x,-3,8}]
>
> It returns: General::ivar: "-2.99978 is not a valid variable."
>
> I can understand that it is because local variable x from Plot command
> interferes with the x variable from the D[].
>
> Yes I can bypass the problem by:
> deriv = D[x^3 - 6 (x + 1)^2 + x - 7, x]
> Plot[deriv, {x, -3, 8}]
>
> which is fully functional, but as far as I know Mathematica, there must
> be some simple solution how to do it directly inside the Plot[].
>
> Thanks in advance for kick-off

Well, you can do this:

Plot[Evaluate[D[x^3 - 6 (x + 1) 2 + x - 7, x]], {x, -3, 8}]


But I think it's worth defining your function as a function.

g[x_] := x^3 - 6 (x + 1) 2 + x - 7

Plot[g'[x], {x, -3, 8}]

--
Helen Read
University of Vermont

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#1623

On Apr 11, 7:05 am, =A9er=FDch Jakub <Ser...@panska.cz> wrote:
> Dear mathgroup,
>
> it seems to me, that response to my question shall be very simple,
> but I cannot find it. :-(
>
> I want to plot the derivative of the function. I would like to do it
> directly, something like:
>
> Plot[D[x^3 - 6 (x + 1)^2 + x - 7, x],{x,-3,8}]
>
> It returns: General::ivar: "-2.99978 is not a valid variable."
>
> I can understand that it is because local variable x from Plot command
> interferes with the x variable from the D[].
>
> Yes I can bypass the problem by:
> deriv = D[x^3 - 6 (x + 1)^2 + x - 7, x]
> Plot[deriv, {x, -3, 8}]
>
> which is fully functional, but as far as I know Mathematica, there must
> be some simple solution how to do it directly inside the Plot[].
>
> Thanks in advance for kick-off
>
> Jakub

Jakub, the Plot[] function picks a set of discrete x values (within
the range specified, very closely spaced to create the smooth graph)
and tries to evaluate your function with those numbers in place of the
x's. This creates a problem when it then tries to evaluate
D[-3^3-6(-3+1)^2+(-3)-7, -3], since you cant differentiate with
respect to a number like -3, hence the 'General::ivar: "-2.99978 is
not a valid variable." ' error message. You can get it to evaluate the
derivative before substituting in numbers for x by simply enclosing
your derivative inside an Evaluate[]

Plot[Evaluate[D[x^3 - 6 (x + 1)^2 + x - 7, x]], {x, -3, 8}]

Hope that helps!

-Stefan

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#1628

Dear Jakub,

this is due to

In[3]:= Attributes[Plot]
Out[3]= {HoldAll, Protected}

You must force Mathematica to evaluate the first parameter of Plot 
before Plot goes on processing. The easiest way to accomplish this is an 
explicit "Evaluate":

Plot[Evaluate[D[x^3 - 6 (x + 1) 2 + x - 7, x]], {x, -3, 8}]

Hope this helps,
ciao,
WW.

On 04/11/2011 01:05 PM, Šerých Jakub wrote:
> Plot[D[x3  - 6 (x + 1)2  + x - 7, x],{x,-3,8}]

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#1633

Šerých Jakub wrote:

> Plot[D[x^3 - 6 (x + 1)^2 + x - 7, x],{x,-3,8}]

Use Evaluate:

Plot[D[x^3 - 6 (x + 1)^2 + x - 7, x] // Evaluate, {x, -3, 8}]

-- 
_________________________________________________________________
Peter Breitfeld, Bad Saulgau, Germany -- http://www.pBreitfeld.de

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