Why can I not pass a maths function into another function

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

[Stephen23]

@Campbell Gray: please show us the complete error message. This means all of the red text.

[Adam]

Which line is actually causing the problem? You should be able to use the debugger to isolate it better.

The only function handle I see in your code is 'fb' and judging from your reply to Walter (which should be included in the original question, by the way, since it is fundamental to the problem), this becomes f in your heun function, which is then given what appear to always be two scalars as arguments. This in itself seems odd given your definition of fb:

fb = @(t,y)(5*y(t));

y(t) is rarely going to make sense when y and t are scalars (only when t is 1 when it is a degenerate statement anyway).

I wouldn't necessarily have expected all this to yield the error message you show, but I may be losing some of the subtleties in exactly what point will throw the first error.

[Campbell Gray]

This is the code:

 fa = @(t,y)(sin(t));                     
 heun(1) = heun( fa, pi/10, 10, 0, 0 );

The error message I am getting:

Function 'subsindex' is not defined for values of class 'function_handle'.

Error in Tables (line 37)
heun(1) = heun( fa, pi/10, 10, 0, 0 );

[Adam]

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]

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.

Answer 1

[Walter Roberson]

At that point in your code, heun is a variable (other than a function handle) rather than a function.

[Campbell Gray]

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]

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.