Why can I not pass a maths function into another function

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Campbell Gray
Campbell Gray 2018-1-30
评论： Walter Roberson 2018-1-30
I have a function 'heun' which I am trying to pass a mathematical function into, I have managed to do this before but can not do it consistently.
fb = @(t,y)(5*y(t));
y = heun( fb, 0.1/10, 10, 0, 1 );
this is my error message:
Function 'subsindex' is not defined for values of class 'function_handle'
Please help
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Adam
Adam 2018-1-30
编辑：Adam 2018-1-30
This is fundamentally different code to what you showed in the question. You are trying to use heun as both the function name and the name of the variable to which you are trying to assign the result. This clearly won't work. Use different names for variables and for functions!!
Walter Roberson
Walter Roberson 2018-1-30
The above would work the first time, because the first time through the variable heun has not been assigned to so heun refers to the function. The second time through, though, heun is now a scalar value rather than a reference to the function.

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Walter Roberson
Walter Roberson 2018-1-30
At that point in your code, heun is a variable (other than a function handle) rather than a function.
  2 个评论
Campbell Gray
Campbell Gray 2018-1-30
Sorry I don't quite understand.
I have defined the function in another script as such:
function y = heun( f, h, n, t0, y0 )
t(1) = t0;
z(1) = y0;
for i = 1:1:n
k1 = h*f(t(i),z(i));
k2 = h*f(t(i)+(h/3),z(i)+(k1/3));
k3 = h*f(t(i)+((2*h)/3),z(i)+((2*k2)/3));
z(i+1) = z(i) + ((k1/4) + ((3*k3)/4));
t(i+1) = t(i) + h;
i = i+1;
end
y = z(n+1);
end
Walter Roberson
Walter Roberson 2018-1-30
The error says that you tried to use a function handle as a subscript. That would happen if you tried to do something like this:
heun = 17;
fb = @(t,y)(5*y(t));
y = heun( fb, 0.1/10, 10, 0, 1 );
In this code, when the third line is executed, heun refers to the array named heun, rather than to the available function named heun, so this would be a request to subscript the array named heun with first index fb (a function handle), second index 0.01, third index 10, fourth index 0, fifth index 1.
The priority for resolving names always goes to local variables first. Otherwise code such as
sum = 0;
for K = 1 : 5
sum(K+1) = sum(K) + K;
end
would not be able to work, if MATLAB gave priority to function calls over local variables.

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