Creating a symbolic variable, then using it as a real variable

I need my program to calculate the derivative of a function, and as far as I understand that means I need to make a symbolic variable. How then can I continue to use that variable as a real vector?

This is what I have tried:

syms x

y = input('Please enter a valid function of x')

deriv = diff(y)

syms clear

x = 0:pi/30:pi;

plot(x,y)

Answer 1

[Matt Fig]

syms x
y = input('Please enter a valid function of x: ')
deriv = diff(y)  % This will be symbolic
x = 0:pi/30:pi;  % Now x is a double.
y = matlabFunction(y)  % y is a function handle.
plot(x,y(x))

[Tom]

Thanks a lot, Matt! Can I ask, why does that only work with y(x) in the plot argument, instead of just y?

[Matt Fig]

y is a function handle (as I say in the comment!), not a vector. If you want to make y a vector instead:

syms x
y = input('Please enter a valid function of x: ')
x = 0:pi/30:pi;  % Now x is a double.
y = subs(y);  % y is now a double.
plot(x,y)

Or without the symbolic toolbox at all:

x = 0:pi/30:pi;
y = 'Please enter a valid function of x: ';
y = feval(vectorize(inline(input(y,'s'))),x);
plot(x,y)

Answer 2

[Walter Roberson]

Use matlabFunction()

[Tom]

I couldn't work out how to use that. Can you see why this doesn't work?

syms x

y = sin(x)

deriv = diff(y)

subs(y,[x],[0:pi/30:pi])

plot(x,y)

[Tom]

Or indeed...

syms x

fx = input(':Please enter a valid function of x:')

dydx = diff(fx)

y = matlabFunction(fx)

plot(x,y)