How to evaluate a function that has been saved to a variable

I have a function:

f = @(x) 2*tanh(x/2)

I'm taking the 1,2,3 derivatives of

d1 = diff(f(x))
d2 = diff(f(x),2)
d3 = diff(f(x),3)

I want to plot them now but not sure how to do this. If I try the following it gives me an error:

x = -1:0.01:1;
plot(x, d1(x))

How do you plot them if they are stored in a variable? or evaluate them?

[Adam Danz]

> How do you plot them if they are stored in a variable? or evaluate them?

Your example uses values stored in a variable so I'm not sure what you're asking.

Don't forget that d1 will have 1 less value than x since you're differentiating, d2 will have 2 less values, etc.

[Eth]

Hi @Adam Danz I can plot the function with no issues as:

plot(x,f(x))

But when I try to plot the the derivatives as:

plot(x,d1(x))

I get the error:

Array indices must be positive integers or logical values.
Error in sym/subsref (line 909)
            R_tilde = builtin('subsref',L_tilde,Idx);

Answer 1

[Walter Roberson]

%provided that x is symbolic
d1(x) = diff(f(x))
d2(x) = diff(f(x),2)
d3(x) = diff(f(x),3)

[Eth]

Thank You!

[Walter Roberson]

f = @(x) 2*tanh(x/2)

f = function_handle with value:
    @(x)2*tanh(x/2)

syms x
d1(x) = diff(f(x))

d1(x) = 

d2(x) = diff(f(x),2)

d2(x) = 

d3(x) = diff(f(x),3)

d3(x) = 

simplify(d2(2))

ans = 

Looks fine to me.

If you are expecting decimal values, then use vpa()

Answer 2

[David Hill]

syms x;
f = 2*tanh(x/2);
d1 = diff(f);
d2 = diff(f,2);
d3 = diff(f,3);
fplot(f,[-1,1]);
hold on;
fplot(d1,[-1,1]);
fplot(d2,[-1,1]);
fplot(d3,[-1,1]);