MATLAB: Error while mutliplying functions.

multiply espressions

I'm trying to multiply these functions:

a = x^2

b = x

Obviously, It should give me: x^3

But when I write the code:

a = @(x)x^2;

b = @(x)x;

c = a*b;

I got the error: Undefined operator '*' for input arguments of type 'function_handle'.

What am I doing wrong?

Best Answer

You cannot multiply function handles. You can either:

1) evaluate the function handles for some value/s, and multiply their outputs:

 >> a(2).*b(2)
 ans =
     8

2) create a new function (which can be evaluated later):

>> c = @(x)a(x).*b(x);
>> c(2)
ans =
     8

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MATLAB: I got the error “Undefined operator ‘*’ for input arguments of type ‘function_handle”

You need to call the function f, like this:

g=@(x)x*f(x)+4

f is a function handle, not a numeric value. Function handles cannot be multiplied. Only when you call the function handle with its input does it return a numeric value, which can then be multiplied.

This is just like any other function in MATLAB:

5*sin     % error, because sin is a function not a value
5*sin(pi) % okay, the function sin returns a numeric value

MATLAB: The value that makes the derivative zero

Why not see what MATLAB thinks you are giving it? When you get an error, think about what it is telling you. Then use MATLAB to see what your code does. GET YOUR HANDS DIRTY.

a(0)
ans =
2*x*exp(-x) - x^2*exp(-x)

See that when you created the function handle a, you never told MATLAB that k was actually a function of x. You might have tried this instead:

a = @(x) double(subs(k,x));
a(0)
ans =
     0

So now MATLAB can evaluate a as a function of x.

fzero(a,[1,10])
ans =
     2
fzero(a,.5)
ans =
    7.844e-23

It seems to work now. Of course, simpler was just:

solve(k==0)
ans =
 0
 2

Best Answer

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MATLAB: I got the error “Undefined operator ‘*’ for input arguments of type ‘function_handle”

MATLAB: The value that makes the derivative zero

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