CODEWITHSUNDEEP
matlabsyntax-errorsymbolic-math
Bonnie
Bonnie

Reputation: 37

MATLAB Unable to Store Symbolic Function with Multiple Variables

I'm trying to take the definite integral from -1 to 1 of a function with respect to x. The function has variables a, b, c, d, and x, all of which I've defined as syms variables. I'm trying to keep a, b, c, d in my final integral because I'll later be differentiating with respect to each one for an optimization problem. Here's the current code that I have: 

syms f(x);
syms a b c d;
f(x)= (exp(x)-a*(1/sqrt(2))-b*(sqrt(3/2)*x)-c((sqrt(45/8))*(x^2-(1/3)))+d((sqrt(175/8))*((x^3)-(3/5)*(x))))^2;
integral = int(f, x, [-1 1]);
disp(integral);

Similar code worked when I tried it using only variables x and y for a smaller function. However, when I try this code, I get:

Error using sym/subsindex (line 825) 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 870)
R_tilde = builtin('subsref',L_tilde,Idx);

Error in HW11 (line 4)
f(x)= (exp(x)-a*(1/sqrt(2))-b*(sqrt(3/2)x)-c((sqrt(45/8))(x^2-(1/3)))+d((sqrt(175/8))((x^3)-(3/5)(x))))^2;

I'm pretty new to symbolic functions and syms variables in MATLAB, why is MATLAB rejecting this code? The similar code that I tried that worked was: 

syms f(x);
syms y;
f(x) = (x^2) + y;
integral = int(f, x, [0 3]);
disp(integral);

Upvotes: 1

Views: 1344

Answers (1)

Eliahu Aaron
Eliahu Aaron

Reputation: 4582

As mentioned in the comment by Adam, you probably forgot to add a multiplication operator * after the the c and d, so when you write c(...) and d(...) MATLAB treats these as indexing of an array but you cannot index arrays with symbolic variables or expressions. You need to change it to c*(...) and d*(...).

Replace:

f(x)= (exp(x)-a*(1/sqrt(2))-b*(sqrt(3/2)*x)-c((sqrt(45/8))*(x^2-(1/3)))+d((sqrt(175/8))*((x^3)-(3/5)*(x))))^2;

With:

f(x)= (exp(x)-a*(1/sqrt(2))-b*(sqrt(3/2)*x)-c*((sqrt(45/8))*(x^2-(1/3)))+d*((sqrt(175/8))*((x^3)-(3/5)*(x))))^2;

Upvotes: 2

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