My diff function won't work and I'm not sure why
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My goal is to give an initial x, and return the function values at that x
function [f, g, h] = myfunction(x)
syms x
q = exp(2*sin(x)) - x;
f = q(x);
dq = diff(q);
g = dq(x);
dq2 = diff(dq);
h = dq2(x);
end
So what I've been struggling with is that if I assign my equation as above, then I can find "f = q(x)" but there is an error for the "diff". If I change the syntax to
q = @(x) exp(2*sin(x)) - x;
then I'm able to calculate using "diff" but then the "f = q(x)" doesn't work. I'm pretty new to MATLAB and not great at it so any advice would be great!
Update: This is the error message I am recieving for the first code
Error using sym/subsindex (line 855)
Invalid indexing or function definition. Indexing must follow MATLAB indexing. Function arguments must be symbolic variables, and function body must be sym expression.
Error in sym/subsref (line 900)
R_tilde = builtin('subsref',L_tilde,Idx);
Error in myfunction (line 4)
f = q(x);
And for the other code it is:
Undefined function 'diff' for input arguments of type 'function_handle'.
Error in myfunction2 (line 4)
dq = diff(q);
1 commentaire
Steven Lord
le 12 Fév 2020
What is the full and exact text of the error messages you receive in each of the circumstances? Please show us all the text displayed in red and/or orange in the Command Window when you try to run this code.
Réponses (3)
Dimitris Kalogiros
le 12 Fév 2020
Try this :
clearvars; close all; clc;
syms x
q(x) = exp(2*sin(x)) - x;
f(x) = q(x)
dq(x) = diff(q(x));
g(x) = dq(x)
dq2(x) = diff(dq(x));
h(x) = dq2(x)
or this one:
clearvars; close all; clc;
syms x
q(x) = exp(2*sin(x)) - x;
f = q(x)
dq(x) = diff(q(x));
g = dq(x)
dq2(x) = diff(dq(x));
h = dq2(x)
David Goodmanson
le 12 Fév 2020
Modifié(e) : David Goodmanson
le 12 Fév 2020
Hi Julia,
if you want to use this for a variety of functions, then
fun = @(x) exp(2*sin(x)) - x; % one example
[f g h] = newfuns(fun)
function [a b c] = newfuns(fun)
syms x
a = fun(x);
b = diff(a);
c = diff(b);
end
f =
exp(2*sin(x)) - x
g =
2*exp(2*sin(x))*cos(x) - 1
h =
4*exp(2*sin(x))*cos(x)^2 - 2*exp(2*sin(x))*sin(x)
Dimitris Kalogiros
le 13 Fév 2020
Modifié(e) : Dimitris Kalogiros
le 13 Fév 2020
I think , the answer to your problem is the following piece of code:
clearvars; clc; close all;
% call of the function
myVal=5;
[f, g, h] = myfunction(myVal);
fprintf('f = %d g = %d h = %d', f, g, h);
% definition of the function
function [f, g, h] = myfunction(y)
syms x
q = exp(2*sin(x)) - x;
f = double( vpa( subs( q, y) ) );
dq = diff(q);
g = double( vpa(subs( dq , y)) );
dq2 = diff(dq);
h = double( vpa(subs( dq2, y)) );
end
A much more usuful version , is the following. Where you can pass your mathematical function as an argument:
clearvars; clc; close all;
% define your mathematical function
syms x
myF=exp(2*sin(x)) - x;
% define the evaluation point
myVal=5;
% call your function calculator
[f, g, h] = myFunctionCalculator(myVal, myF, x);
% print results
fprintf('f = %d g = %d h = %d', f, g, h);
function [f, g, h] = myFunctionCalculator(xo, q, x)
f = double( vpa( subs( q, x, xo) ) );
dq = diff(q);
g = double( vpa(subs( dq , x, xo)) );
dq2 = diff(dq);
h = double( vpa(subs( dq2, x, xo)) );
end
.
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