how to make the inline function can identify the matab coded function

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David Zhang
David Zhang 2012-1-31
编辑： dpb 2013-9-26
I am using the sybolic toolbox of matlab but find the inline function cant identify the matlab coded function. For example,
if i use
syms x y
FT=@(x, y)min(x, y);
ans=dblquad(FT, -1, 1, -1, 1);
the answer is correct.
but if i use it alternatively as
syms x y
FT=inline(min(x, y));
ans=dblquad(FT, -1, 1, -1, 1);
it is failed.
So how to make the inline function to identify the matlab's own function?
Thanks so much

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Walter Roberson
Walter Roberson 2012-1-31
syms is irrelevant to what you are doing. You should not be using syms.
Your first approach passes a function handle of two arguments as the first argument to dblquad(). dlbquad() will pass the function a vector for the first argument, and a scalar for the second argument. min() is happy to work with that, and will compare each value in x to the scalar in y.
Your second approach tries to evaluate min(x,y) and pass the result to inline(). In your code, because you defined x and y as syms, min(x,y) is going to be a symbolic expression. inline() requires, however, that it be passed a character expression. Correct coding would be to leave out the syms command and use
FT = inline('min(x,y)');
By the way, your first code version, with the function handle FT, could have been coded as simply
ans = dblquad(@min, -1, 1, -1, 1);
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Walter Roberson
Walter Roberson 2012-2-3
Ah, the silly routine produces a DOM_poly object that has to be converted to an expression.
Try:
syms v w
for K = 1 : N1
tlpg = feval(symengine, 'orthpoly::legendre', sym(K), w);
lgp{K} = matlabFunction( tlpg(v), v);
end
And if that doesn't work, the more verbose
syms v w
for K = 1 : N1
tlpg = feval(symengine, 'orthpoly::legendre', sym(K), w);
lgp{K} = matlabFunction( feval(symengine, 'evalp', tlpg, sym('w=v') ), v);
end
The legendre polynomials do not normally have two variables. Are you just looking for consistency in invocation, looking ahead to a time when you might be using a function with more than one variable? If so then,
for i = 1 : N1
fx = @(x,y) lgp{i}(x);
for j = 1 : N1
fy = @(x,y) lgp{j}(y);
Ans(i,j) = dlbquad(@(x,y) min(x,y).*fx(x,y).*fy(x,y),a, b, a, b);
end
end
Note: I do not have the symbolic toolbox, so the evalp code has not been tested. (I use Maple, which handles the legendre polynomials differently.)

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