MATLAB: Hello, I face a error in the following program.

feval funtionmatlab function

clear all
syms x y 
f1=x^3+y
f2=y^2+x
df1dx=diff(f1,x)
x=0.5
y=1.5
A=[feval(df1dx,x,y)]

This is the program.

HERE THE DIFFERENTIATED EQUATION SAY OF F1=X^3+Y IS 3X^2+1. WE CANT ABLE TO EVALUATE THE FUNCTION AT Y BECAUSE THERE IS NO Y VARIABLE IN THE ABOVE EQUATION. HOW WILL WE LOGICALLY APPROACH TO THESE KIND OF PROBLEMS. BUT USAGE OF Y IN FEVAL FUNCTION IS NECESSARY, SINCE WE DONT KNOW THE DIFFERENTIATED EQU. CONTAIN HOW MANY NO. OF VARIABLES PLS SOLVE THIS PROBLEM.

Best Answer

dF1 = matlabFunction(df1dx, 'vars', [x, y]);
dF1(0.5, 1.5)

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MATLAB: What’s wrong with the folow expression

diff(f) would invoke f with no arguments, fetch its return value, and apply the numeric diff() function to it, where the numeric diff() function is (X(2:end) - X(1:end-1)).

If you have the symbolic toolbox, you could try

syms t
df = diff(f(t),t);
f2 = diff(df,t);
f1 = inline(char(f(t)));
df1 = inline(char(df));
df2 = inline(char(f2));

Or more directly,

syms t
ft = f(t);
df = diff(ft,t);
f2 = diff(df, t);
f1 = @f;
df1 = matlabFunction(df, 'vars', t);
df2 = matlabFunction(f2, 'vars', t);
f

MATLAB: How to build a symbolic function based on other symbolic functions？

When you use a symbolic function (by name, with no parameters) with a binary operator, and the other operand does not involve symbolic functions without parameters, then MATLAB can unambiguously construct a new symbolic function that takes the same arguments of the existing symbolic function and returns the appropriate value.

When you use a symbolic function (by name, with no parameters) with a binary operator, and the other operand involves a symbolic function with exactly the same number and order of parameters, then again MATLAB can unambiguously construct a new symbolic function that takes the same arguments of the existing symbolic function and returns the appropriate value.

However, when you use a symbolic function (by name, with no parameters) with a binary operator and the other operand involves a symbolic function with a different number or order of parameters, then MATLAB cannot know what the "right" order of parameters for the function to construct would be. When you add f1 (a function of just x) and f2 (a function of just y), then should the function that is created on the fly be a function of (x,y) or of (y,x) ?

It is important to understand for this purpose that this attempted automatic function construction is done first, without looking at the left hand side of the assignment.

The assignment statement

f3(x,y) = f1 + f2

is not treated like

function result = f3(x,y)
   result = f1 + f2
end

and is not treated like

f3 = @(x,y) f1 + f2

The f3(x,y) on the left hand side is not keyword abbreviation for constructing a true function.

Instead,

f3(x,y) = f1 + f2

is treated as

temporary = f1 + f2
f3 = symfun(temporary, [x, y])

to emphasize, the right hand side of the f3(x,y) = f1 + f2 is fully evaluated (probably returning a symbolic expression or symbolic function), and then turned into a symbolic function with the variable names given on the left hand side.

I do not mean this as an approximation of how MATLAB works internally: I mean that this is how MATLAB processes the line.

So, when f1 + f2 is trying to figure out whether to construct a temporary function of (x,y) or of (y,x), it does not have access to the context that eventually the output is going into a symfun that will have (x,y) as the inputs: the expression has to fully evaluate on its own, and that fails because it cannot reasonably decide the parameter order for the temporary function to be emitted.

Now if you had coded

f3(x,y) = f1(x) + f2(y)

then the result of f1(x) and of f2(y) would be expressions rather than symfun, and there is no problem adding expressions. Then, the expression fully constructed and inside an internal temporary variable,

f3 = symfun(temporary, [x,y])

would have no problem.

The error only occurs when you use symfun without parameters in expressions, and you encounter a situation in which the parameters do not match between two symfun.

Best Answer

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MATLAB: What’s wrong with the folow expression

MATLAB: How to build a symbolic function based on other symbolic functions？

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