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General::ivar is not a valid variable when plotting - what actually causes this and how to avoid it?
Beginner question:
Why can't I use D[] like that?
Plot[{Sin[x], D[Sin[x], x]}, {x, -2 Pi, 2 Pi}]
If I assign the result of D[] to a variable and put that variable in the list it works.
Plot has attribute HoldAll which means in this case that D[Sin[x], x] isn't evaluated until after x is replaced with some number, so you end up with something like D[Sin[-6.28], -6.28] etc. which causes the errors since you can't take a derivative with respect to a number.
One way to get around this is to use Evaluate to evaluate the derivative before the numbers are plugged in, i.e. to do something like
Plot[Evaluate[{Sin[x], D[Sin[x], x]}], {x, -2 Pi, 2 Pi}]
Plot[{Sin[x], Evaluate[D[Sin[x], x]]}, {x, -2 Pi, 2 Pi}] won't work. You should be able to evaluate just the D[..] part, shouldn't you?
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– 0sh
Apr 5 '12 at 13:38
Evaluate only works on the first level of the held expression. So for example Hold[Evaluate[f[1+2]]] works, but Hold[f[Evaluate[1+2]]] doesn't.
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– Heike
Apr 5 '12 at 13:49
Quick answer:
The help page for plot states that "Plot has attribute HoldAll, and evaluates f only after assigning specific numerical values to x." Therefore what D[] effectively sees is something like D[Sin[0.1],0.1] (where I chose 0.1 as an arbitrary numerical value) and issues an error message that 0.1 is no valid variable. To work around that you can use Evaluate[] like this
Plot[Evaluate@{Sin[x],D[Sin[x], x]}, {x, -2 Pi, 2 Pi}]
You needn't use Evaluate, consider using of Derivative instead of D if you don't like to use Evaluate. Derivative operates on built-in or user-defined functions and returns pure functions, e.g.
Derivative[#][Sin] & /@ Range[4]
{Cos[#1] &, -Sin[#1] &, -Cos[#1] &, Sin[#1] &}
There are shortcuts for Derivative, e.g. f' or f'' are equivalent respectively to Derivative[1][f] and Derivative[2][f]
and in your specific examples it works like this :
Plot[{Sin[x], Derivative[1][Sin][x]}, {x, -2 Pi, 2 Pi}]
Plot[{Sin[x], Sin'[x]}, {x, -2 Pi, 2 Pi}]? For any function of typef[x]you can usef'[x]in place of theD[f[x],x]. $\endgroup$ – PlatoManiac Apr 5 '12 at 13:22