dsolve using a string

Team,

I am trying to figure out why the code below won't work. This is taken out of a larger program where it asks at the beginning using input "What equation would you like solved?" That is why dfeq = '2*x*y' at this particular point.

>> initcondx=1

initcondx =

1

>> initcondy=1

initcondy =

1

>> dfeq='2*x*y'

dfeq =

'2*x*y'

>> syms y x y(x)

>> ode=diff(y,x)==str2sym(dfeq)

ode(x) =

diff(y(x), x) == 2*x*y

>> cond=y(initcondx)==initcondy

cond =

y(1) == 1

>> dsolve(ode,cond)

Error using mupadengine/feval (line 187)

Expecting an ODE in the specified variable.

Error in dsolve>mupadDsolve (line 340)

T = feval(symengine,'symobj::dsolve',sys,x,options);

Error in dsolve (line 194)

sol = mupadDsolve(args, options);

[Matt Baron]

I've got the problem narrowed down to this/

If I input an equation into the variable dfeq using input

dfeq then equals '2*x*y'

If I then use str2sym(dfeq), it outputs 2*x*y

however when I use that with diff(y,x)==str2ym(dfeq), it comes out without the y(x)

See below;

>> ode=diff(y,x)==str2sym(dfeq)

ode(x) =

diff(y(x), x) == 2*x*y

>> ode=diff(y,x)==2*x*y

ode(x) =

diff(y(x), x) == 2*x*y(x)

How do I get it keep the y(x) with str2sym?

[Matt Baron]

I think I have figured out this is just not possible to do.

Answer 1

[Amal Raj]

When using str2sym to convert a string to a symbolic expression, it treats the variables as independent symbols and does not associate them with any specific function. That's why when you use str2sym(dfeq) in the diff function, it does not include the (x) notation for the function y(x).

To keep the y(x) notation, you can explicitly define y as a function of x using the symfun function. Here's an example:

dfeq = '2*x*y';
syms y(x) x; % Define symbolic variables
y = symfun(y(x), x); % Define y as a function of x
ode = diff(y, x) == str2sym(dfeq); % Define the ODE

Now, when you use diff(y, x), it will include the (x) notation for the function y(x).