Table of function derivative evaluation

Question

I want to create a table with numerical values of a function derivative by using the following code:

f[x_, a_] := f[x, a] = Tanh[a (1 - Cos[x])];
g[x_, a_] := g[x, a] = D[f[x, a], x];
pderivative = Table[{x, Evaluate[g[x, 5]]}, {x, - π,  π, 2 π/500.}]

When I look at the result, it's not what I expect. Any trick to fix this?

Answer 1

Method I

Define your g by Derivative instead of D:

g[x_, a_] := g[x, a] = Derivative[1, 0][f][x, a];

Note that Evaluate in pderivative is not a necessity.

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Method II

Alternatively it also works in this way:

g[x_, a_] = D[f[x, a], x]

FYI, please check this link concerning the difference between Set (=) and SetDelayed (:=).