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

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Eth
Eth 2021-5-13
评论： Walter Roberson 2021-5-13
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?
  2 个评论
Adam Danz
Adam Danz 2021-5-13
> 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
Eth 2021-5-13
编辑：Eth 2021-5-13
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);

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采纳的回答

Walter Roberson
Walter Roberson 2021-5-13
%provided that x is symbolic
d1(x) = diff(f(x))
d2(x) = diff(f(x),2)
d3(x) = diff(f(x),3)
  2 个评论
Walter Roberson
Walter Roberson 2021-5-13
Ran in:
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()

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更多回答（1 个）

David Hill
David Hill 2021-5-13
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]);

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