hey I have a function defined as,
drift = #(x1,x2,u)[x2,(a*sin(x1) - b*x2 + u)]*.05;
then I attempt to call it while in a triple for loop (iterating over
x1= 1:length(n1),
x2 = 1:length(n2)
x3 = 1:length(nu)
)
next = [n1(x1), n2(x2)] + drift([n1(x1), n2(x2), nu(k)]);
and I get Error using myMDP>#(x1,x2,u)[x2,(a*sin(x1)-b*x2+u)]*.05 (line 148)
Not enough input arguments.
that error. Why?
f([1,2,3]) %function call with a single parameter, which is a 3 element vector
f(1,2,3) %function call with three parameters
f([1,2,3],[1,2,3],[1,2,3]) %function call with three parameters, each a 3 element vector.
Related
Hi I am new to matlab so not familiar with its grammar.
I want to write a function to solve some functions using specific algo.
What I want to do is to write a function using another function which I want to slove as input.
For example, if I want to get the root of x^2 - 1 = 0 ,I need to plug in this function as in input.
my code is like
function [y] = brent(f, x0, x1, max_iter, tolerance)
fx0 = f(x0)
fx1 = f(x1)
......
end
f is the function I want to solve. My question is how should I write the code so the function 'brent' can use the function 'f' to calculate the values at specific points.
ex. In the second line, I need to get the value of f(x0) (x0 is a point).
Matlab talks about function handles. Those can be input parameter as anything:
Write your main function:
function a = func(f,x)
a = f(x) + 7;
Define your function to be input, and call 'normally'
>> myfun = #(x) x^2-1;
>> func(myfun,3)
ans =
15
>> func(#sin,0)
ans =
7
see:
https://se.mathworks.com/help/matlab/matlab_prog/creating-a-function-handle.html
I have a use case as follows:
Inside F.m I have a function F that takes as its argument a 2 x 1 matrix x. F needs to matrix multiply the matrix kmat by x. kmat is a variable that is generated by a script.
So, what I did was set kmat to be global in the script:
global kmat;
kmat = rand(2);
In F.m:
function result = F(x)
global kmat;
result = kmat*x;
end
Then finally, in the script I have (x_0 has already been defined as an appropriate 2 x 1 matrix, and tstart and tend are positive integers):
xs = ode45(F, [tstart, tend], x_0);
However, this is causing the error:
Error using F (line 3)
Not enough input arguments.
Error in script (line 12)
xs = ode45(F, [tstart, tend], x_0);
What is going on here, and what can I do to fix it? Alternatively, what is the right way to pass kmat to F?
Firstly, the proper way to handle kmat is to make it an input argument to F.m
function result = F(x,kmat)
result = kmat*x;
end
Secondly, the input function to ode45 must be a function with inputs t and x (possibly vectors, t is the dependent variable and x is the dependent). Since your F function doesn't have t as an input argument, and you have an extra parameter kmat, you have to make a small anonymous function when you call ode45
ode45(#(t,x) F(x,kmat),[tstart tend],x_0)
If your derivative function was function result=derivative(t,x), then you simply do ode45(#derivative,[tstart tend],x_0) as Erik said.
I believe F in ode45(F,...) should be a function handle, i.e. #F. Also, you can have a look at this page of the MATLAB documentation for different methods to pass extra parameters to functions.
When I type help gmres in MATLAB I get the following example:
n = 21; A = gallery('wilk',n); b = sum(A,2);
tol = 1e-12; maxit = 15;
x1 = gmres(#(x)afun(x,n),b,10,tol,maxit,#(x)mfun(x,n));
where the two functions are:
function y = afun(x,n)
y = [0; x(1:n-1)] + [((n-1)/2:-1:0)'; (1:(n-1)/2)'].*x+[x(2:n); 0];
end
and
function y = mfun(r,n)
y = r ./ [((n-1)/2:-1:1)'; 1; (1:(n-1)/2)'];
end
I tested it and it works great. My question is in both those functions what is the value for x since we never give it one?
Also shouldn't the call to gmres be written like this: (y in the #handle)
x1 = gmres(#(y)afun(x,n),b,10,tol,maxit,#(y)mfun(x,n));
Function handles are one way to parametrize functions in MATLAB. From the documentation page, we find the following example:
b = 2;
c = 3.5;
cubicpoly = #(x) x^3 + b*x + c;
x = fzero(cubicpoly,0)
which results in:
x =
-1.0945
So what's happening here? fzero is a so-called function function, that takes function handles as inputs, and performs operations on them -- in this case, finds the root of the given function. Practically, this means that fzero decides which values for the input argument x to cubicpoly to try in order to find the root. This means the user just provides a function - no need to give the inputs - and fzero will query the function with different values for x to eventually find the root.
The function you ask about, gmres, operates in a similar manner. What this means is that you merely need to provide a function that takes an appropriate number of input arguments, and gmres will take care of calling it with appropriate inputs to produce its output.
Finally, let's consider your suggestion of calling gmres as follows:
x1 = gmres(#(y)afun(x,n),b,10,tol,maxit,#(y)mfun(x,n));
This might work, or then again it might not -- it depends whether you have a variable called x in the workspace of the function eventually calling either afun or mfun. Notice that now the function handles take one input, y, but its value is nowhere used in the expression of the function defined. This means it will not have any effect on the output.
Consider the following example to illustrate what happens:
f = #(y)2*x+1; % define a function handle
f(1) % error! Undefined function or variable 'x'!
% the following this works, and g will now use x from the workspace
x = 42;
g = #(y)2*x+1; % define a function handle that knows about x
g(1)
g(2)
g(3) % ...but the result will be independent of y as it's not used.
In the following boiled down example illustrating the error, the function f_what(..) should return the values of input argument y at indices in the array ts:
function Y = f_what(y, ts)
function get_out = get(t)
get_out = y(t);
end
Y = arrayfun(get, ts);
end
Calling it:
>> f_what(1:10, 1:5)
Error using f_what/get (line 4)
Not enough input arguments.
Error in f_what (line 7)
Y = arrayfun(get, ts);
Also, for some reason, the following, where get(..) should be the same as the one above, works:
function Y = f_what(y, ts)
get = #(t) y(t);
Y = arrayfun(get, ts);
end
Calling it:
>> f_what(1:10, 1:5)
ans =
1 2 3 4 5
"Not enough input arguments" ... arrayfun(..) should call its first argument with, in this case, one argument. And get(..) has one input argument. I don't get why it's not enough.
Edit: even more boiled down:
function Y = f_what
function get_out = get_(t)
get_out = t;
end
Y = arrayfun(get_, 1:5);
end
Still the same error.
Edit 2: It works if I supply #get to the first argument of arrayfun(..), instead of get. But I still don't get why it doesn't work without the #.
Looking at the arrayfun documentation
func
Handle to a function that accepts n input arguments and returns m output arguments.
A handle in matlab is denoted using #, so you need to pass #get as first parameter. Otherwise matlab tries to evaluate function get instead of obtaining its handle, which results in "not enough parameters" error.
In the example that works you defined get to be a handle to an anonymous function, that's why it worked.
I have one file with the following code:
function fx=ff(x)
fx=x;
I have another file with the following code:
function g = LaplaceTransform(s,N)
g = ff(x)*exp(-s*x);
a=0;
b=1;
If=0;
h=(b-a)/N;
If=If+g(a)*h/2+g(b)*h/2;
for i=1:(N-1)
If=If+g(a+h*i)*h;
end;
If
Whenever I run the second file, I get the following error:
Undefined function or variable 'x'.
What I am trying to do is integrate the function g between 0 and 1 using trapezoidal approximations. However, I am unsure how to deal with x and that is clearly causing problems as can be seen with the error.
Any help would be great. Thanks.
Looks like what you're trying to do is create a function in the variable g. That is, you want the first line to mean,
"Let g(x) be a function that is calculated like this: ff(x)*exp(-s*x)",
rather than
"calculate the value of ff(x)*exp(-s*x) and put the result in g".
Solution
You can create a subfunction for this
function result = g(x)
result = ff(x) * exp(-s * x);
end
Or you can create an anonymous function
g = #(x) ff(x) * exp(-s * x);
Then you can use g(a), g(b), etc to calculate what you want.
You can also use the TRAPZ function to perform trapezoidal numerical integration. Here is an example:
%# parameters
a = 0; b = 1;
N = 100; s = 1;
f = #(x) x;
%# integration
X = linspace(a,b,N);
Y = f(X).*exp(-s*X);
If = trapz(X,Y) %# value returned: 0.26423
%# plot
area(X,Y, 'FaceColor',[.5 .8 .9], 'EdgeColor','b', 'LineWidth',2)
grid on, set(gca, 'Layer','top', 'XLim',[a-0.5 b+0.5])
title('$\int_0^1 f(x) e^{-sx} \,dx$', 'Interpreter','latex', 'FontSize',14)
The error message here is about as self-explanatory as it gets. You aren't defining a variable called x, so when you reference it on the first line of your function, MATLAB doesn't know what to use. You need to either define it in the function before referencing it, pass it into the function, or define it somewhere further up the stack so that it will be accessible when you call LaplaceTransform.
Since you're trying to numerically integrate with respect to x, I'm guessing you want x to take on values evenly spaced on your domain [0,1]. You could accomplish this using e.g.
x = linspace(a,b,N);
EDIT: There are a couple of other problems here: first, when you define g, you need to use .* instead of * to multiply the elements in the arrays (by default MATLAB interprets multiplication as matrix multiplication). Second, your calls g(a) and g(b) are treating g as a function instead of as an array of function values. This is something that takes some getting used to in MATLAB; instead of g(a), you really want the first element of the vector g, which is given by g(1). Similarly, instead of g(b), you want the last element of g, which is given by g(length(g)) or g(end). If this doesn't make sense, I'd suggest looking at a basic MATLAB tutorial to get a handle on how vectors and functions are used.