Clear Filters
Clear Filters

How can I declare an implicit variable, y(t), after I have used it in a symbolically defined function?

7 views (last 30 days)
I'm writing a scheme that solves an ODE and in doing so I declare a symbolic function:
syms t y(t)
f = t^(-2) * (sin(2*t) - 2*t*y)
At one point I have to take a total derivative, so I need to be be implicit in the expression, but I also need to evalute at a specific value of t and y. I attempt to do this in the following lines.
y(t) = wi;
out(1) = f(ti);
However, I am prompted with the following error.
The following error occurred converting from sym to double:
Unable to convert expression containing remaining symbolic
function calls into double array. Argument must be
expression that evaluates to number.
Error in DiffEq_alt (line 13)
out(1) = f(ti);
Ultimately, I need to be able to be able to declare y explicitly, but also treat it implicitly for when I differentiate the function with respect to t.

Answers (1)

Nipun on 22 May 2024 at 5:16
Hi David,
I understand that you are trying to solve an ordinary differential equation (ODE) using symbolic mathematics in MATLAB, and you encounter an issue when attempting to evaluate a symbolic expression at specific numerical values for t and y(t). You want to keep y as a function of t for differentiation purposes but also need to substitute specific values for t and y(t).
To achieve this, you can use subs to substitute specific values into your symbolic expression before converting it to a numerical value. Here's how you can adjust your code to avoid the error:
To substitute a specific value for t and y(t), use the subs function. Let us say you want to evaluate f at t = ti and y(t) = wi. You can do this as follows:
ti = 1; % Example value for t
wi = 0.5; % Example value for y(t)
% Substitute the specific values into f
f_sub = subs(f, [t, y(t)], [ti, wi]);
After substituting the specific values, you can convert the result to a numerical value using double:
out(1) = double(f_sub);
This approach allows you to keep y as a function of t for differentiation and other symbolic operations, while also being able to evaluate f at specific numerical values for t and y(t).
Here is the combined snippet:
syms t y(t)
f = t^(-2) * (sin(2*t) - 2*t*y);
ti = 1; % Specific value for t
wi = 0.5; % Specific value for y(t)
% Substitute specific values into f and convert to numerical value
f_sub = subs(f, [t, y(t)], [ti, wi]);
out(1) = double(f_sub);
This should resolve the error and allow you to evaluate your symbolic expression at specific numerical values.
Hope this helps.


Find more on Symbolic Math Toolbox in Help Center and File Exchange


Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!