feval() with fun as a vector
3 views (last 30 days)
Show older comments
Hi, I am looking how to use feval with the "fun" string input as a vecor
F=[f1,f2,...,fn]
This is in order tu compute the Jacobian
---------------------------------
function y = test(x)
y=[x(1)^3+x(2)^3 , x(1)];
---------------------------------
------------------------------------
function g = jacobian(fun, x0)
delta = 0.001;
for i = 1 : min(size(x0))
for j = 1 : max(size(x0))
x=x0;
x(j) = x0(j);
aux1 = arrayfun ( fun(i), x );
x(j) = x0(j) + delta;
aux2 = feval ( fun(i), x );
g(i,j)=(aux2-aux1)/(delta);
end
end
end
----------------------------------
And I call the function like this
jacobian('test',x)
with x=[1,1]
The problem is in fun(i), that gives a "t" from "test", I would like to take only the "i" element of the vector.
Do you have a clue?
arrayfun and eval do the same
Regards
0 Comments
Accepted Answer
Walter Roberson
on 6 Feb 2011
Provided that I have understood your requirements...
fbase = str2fun(fun);
ith = @(v,i) v(i);
for i = 1 : min(size(x0))
fi = @(x) ith(fbase(x), i);
for j = 1 : max(size(x0))
[...]
aux1 = fi(x);
[...]
aux2 = fi(x);
[...]
end
end
More Answers (1)
Julio Cesar
on 7 Feb 2011
2 Comments
Walter Roberson
on 7 Feb 2011
test = @(x) [4*x(1)+x(2), 6*x(1)*x(2)];
If you were to use that, then you would not need to use str2func.
You could vectorize without much trouble, at least for the test functions you showed. For example,
test = @(x) [4*x(:,1)+x(:,2), 6*x(:,1)*x(:,2)];
Then a single invocation of fi on an N by 2 array of x values would calculate them all. You would, however, need to modify to
ith = @(v,i) v(:,i);
Your statement
x(j) = x0(j);
is seemingly redundant, coming right after
x = x0;
if x has been assigned all of x0, then copying the j'th position of x0 to the j'th position of x is just going to be copying something that is already there.
x1 = x0;
x1(j) = x1(j) + delta;
aux = fi([x0;x1]);
g(i,j) = (aux(2)-aux(1))/delta;
And you can proceed from there to vectorize over the entire j loop. All provided that your test function is vectorizable.
See Also
Categories
Find more on Startup and Shutdown in Help Center and File Exchange
Products
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!