Undefined function 'eval' for input arguments of type 'double'.

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Ana Luisa
Ana Luisa 2012-11-8
When using the code below:
function [Df]=difc(f,u,h)
syms x
x=u+h; lp=eval(f);
x=u-h; ll=eval(f);
Df=(lp-ll)/(2*h);
end
, the same error keeps appearing ''Undefined function 'eval' for input arguments of type 'double'.''
I would like to know what´s the problem and what can I do to solve it, in order to have a working code.
Thankyou very much
  2 个评论
Walter Roberson
Walter Roberson 2012-11-8
What are you passing in to the function?
Note: your assignments to "x" are undoing the effect of "syms x".
Ana Luisa
Ana Luisa 2012-11-8
I want to pass the values lp=u+h and ll=u-h, being that u and h are real numbers given by the user

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采纳的回答

C.J. Harris
C.J. Harris 2012-11-8
You are getting that error because you are trying to evaluate a double. A double is just a double - there is nothing there to evaluate.
What are you expecting the 'eval' function to do?
  2 个评论
Ana Luisa
Ana Luisa 2012-11-8
I'm trying to calculate the numerical first derivative of a function f using the central differences method with an error of O(h^2).

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