What does this error mean? Undefined operator '*'

Hello,

I am trying to multiply a function to later on integrate and I keep getting this error:

Undefined operator '*' for input arguments of type 'function_handle'.

Error in HW11_RaquelTerrerVanGool_1 (line 19)
    F=2*pi*r*f

And this is what I have so far:

r=[0 1 2 3 4 5 6 7 8]; % Radius in inches
vf=[3.00 2.92 2.78 2.61 2.36 1.78 1.40 0.67 0.00]; % Velocity in ft/s
v=vf*12; % Velocity converted to in/s
R=8 % Radius of the pipe in inches
 
f=polyfit(r,v,4);
 
f=@(r)(f(5)+f(4)*r+f(3)*r.^2+f(2)*r.^3+f(1)*r.^4);
 
for r=0:length(r)
    F=2*pi*r*f
end
 
Q=int(F,0,R)

Can someone help me please? Thank you very much.

[John D'Errico]

By the way, this will get you in deep trouble one day, when you try to debug what is happening.

f=polyfit(r,v,4);
f=@(r)(f(5)+f(4)*r+f(3)*r.^2+f(2)*r.^3+f(1)*r.^4);

So you create the vector f as a vector if coefficients of a function. Then you create a function handle named f, using the vector f itself. It is just pleading for errors to arise. And then, what if you decide to execute that line again, but without recreating f as a vector?

f=@(r)(f(5)+f(4)*r+f(3)*r.^2+f(2)*r.^3+f(1)*r.^4);

oops! f(5) is now a call to that function handle that you just created.

It is code that just begs for a massive headache one day for you.

Likewise, things like

r=[0 1 2 3 4 5 6 7 8];
for r = 0:length(r )

is another case where you will encounter massive problems. you create r as a vector, then use the same variable name r as a loop construct, based on r itself.

In both cases, f and r become completely different things, based on where in your code you happen to be looking. Doing things like this is asking for trouble. Far better is to just use a new variable name, when you have something different. Variable names are completely free. No charge to use a new one.

But why does your code fail? That is the immediate problem.

f as a function handle is not a function itself. Instead of f, we could have called it Fred, or Barney, so just a name. Think of it as a name for the function you just created. So when you try to multiply f times something, MATLAB gets confused. It does not understand how to multiply the name of something times 2*pi.

However, you could do this:

F=2*pi*r*f(r)

Of course, the next step will then also fail, because that loop just keeps on overwriting F. You have created a scalar F each time through the loop. So the secondary problem will be you wither need to create a FUNCTION F, or a vector F.

And THAT depends on whether you really want to use int, or integrate, or trapz.

INT cannot integrate a vector of values. INT is a symbolic tool.

TRAPZ cannot integrate a function. It works on vectors of numbers.

INTEGRATE is a numerical integration tool, that works on a function defined in numerical form.

So whatever you are trying to do, you need to decide which tool you will use.

[Raquel Terrer van Gool]

Noted! I will start doig that, thank you very much for your time!

Answer 1

[Stephen23]

On this line

f=@(r)...

you defined f to be a function handle of an anonymous function with one input argument. Multiplication is not defined for function handles, only for numeric arrays**. If you want to multiply the output of that function (which will be a numeric array then you will need to call the function so that it returns that array.

Inside the loop you will need to call that function with one input argument, e.g.:

F=2*pi*r*f(r)
%         ^^^ call the function with one input argument

Note that your loop simply overwrites F on each iteration, only keeping F from the final iteration. I doubt that that is very useful.

** Well, also for logical and char.