MATLAB: Format coefficients of the derivative of a function

derivativedifferential equationsdifferentiationformatspecifiersymbolic

The following code is meant to output the derivative of a function but the coefficients are displayed as fractions even when I try to change the format to short or long.

syms x
e = 2.718282; 
f = x * e^x^2; 
format short
diff(f)

The output is a function whose coefficients are the ratios of large integers. How can their format be changed?

Best Answer

Convert everthing to symbolic to prevent the loss of precision

syms x
e = exp(sym(1));
f = x * e^x^2; 
format short
df = diff(f)

Result

>> df
df =
exp(2*x) + 2*x*exp(2*x)

Related Solutions

MATLAB: Symbolic differentiation with fminunc


function [f,g] = rosenbrockwithgrad_s(x)
X = sym('x', [1 2]);
% Calculate objective f

F = 100*(X(2) - X(1)^2)^2 + (1-X(1))^2;
f = double( subs(F, X, x) );
if nargout > 1 % gradient required
   G = diff(F);
   g = double( subs(G, X, x) );
end

However, this would involve repeated calculation of diff(F) symbolically, which is pointless. Instead you would do something like,

    function [f, g] = f_and_g(x, F, G)
        f = F(x);
        if nargout > 1
            g = G(x);
        end
    end

together with (outside)

X = sym('x', [1 2]);
% Calculate objective f
f = 100*(X(2) - X(1)^2)^2 + (1-X(1))^2;
g = diff(f);
F = matlabFunction(f, 'vars', {X});
G = matlabFunction(g, 'vars', {X});
x = fminunc( @(x) f_and_g(x, F, G), x0 )

MATLAB: Derivative of a Function

If you have the Symbolic Toolbox:

>> g = @(x) exp(x^2-2)
g = 
    @(x)exp(x^2-2)
>> syms x
>> diff(g(x))
ans =
2*x*exp(x^2 - 2)

Best Answer

Related Solutions

MATLAB: Symbolic differentiation with fminunc

MATLAB: Derivative of a Function

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