How to take a symbolic derivative, and then change that symbolic variable to one assigned with linspace

I've been working on a problem, and it needs four derivatives. The equation itself is very large.

It looks much like this

y = const*(x^4 - 2*L*x^3 + x^2) ----> Where const is a collection of constants.

To take the first derivative, I did the following.

syms x, const, L y = const*(x^4 - 2*L*x^3 + x^2)

dy = diff(y,x); d2y = diff(dy,x)*const; d3y = diff(dy2,x)*const;

Following this, I then assigned values to the constants, and x = linspace(0:L), where L is already assigned -- however, when I try to use this in a for-loop to see its graph, it simply prints out the symbolic equation. I could go back and manually place the assigned variables in there, but that would completely ruin the purpose of this program.

Answer 1

[Star Strider]

I’m not sure what you’re doing. If you want to do the plotting in the Symbolic Math Toolbox, use the fplot (or ezplot) functions:

figure(1)
fplot(dy, [0 L])
hold on
fplot(d2y, [0 L])
fplot(d3y, [0 L])
hold off
grid

Everything except ‘x’ has to be defined numerically before the fplot calls.

[Noah Koger]

I wanted to solve symbolically the derivative, and then assign those symbolic characters a value to then be evaluated.

[Star Strider]

To do that, use the matlabFunction function.

For example:

syms x const L
const = 3;
L = 10;
y = const*(x^4 - 2*L*x^3 + x^2)
dy = diff(y,x);
d2y = diff(dy,x)*const;
d3y = diff(d2y,x)*const;

y_fcn = matlabFunction(y)
dy_fcn = matlabFunction(dy)
d2y_fcn = matlabFunction(d2y)
d3y_fcn = matlabFunction(d3y)

x = linspace(0, L);

figure(1)
plot(x, y_fcn(x))
hold on
plot(x, dy_fcn(x))
plot(x, d2y_fcn(x))
plot(x, d3y_fcn(x))
hold off
grid