How do I pass a function a mathematical function of the for x.^2

Hello professionals

The question is actually already in the title.

Is there a way to pass a "function" to a function? Background is I want to write a function which works with a function and calculates values for me.

My problem is that Matlab expects a numeric value and not a char as a passing variable.

function [y] = calculatethefunction(Function)
%Funktion means somthing like "x.'2" or "x/2"
f=Function;
x=linspace(0,10,100);
y=Funktion              %my goal is now that the funtion calculates the numbers like
                        %y=x.^2
end

Thaks a lot.

Your sincerely

Patrick

Answer 1

[Star Strider]

If ‘Function’ is a string and contains only the variable ‘x’ —

Function = "x^2"

Function = "x^2"

[y,x] = calculatethefunction(Function);

figure
plot(x, y)
grid

function [y,x] = calculatethefunction(Function)
%Funktion means somthing like "x.'2" or "x/2"
f=str2func("@(x)" + vectorize(Function));
x=linspace(0,10,100);
y=f(x);              %my goal is now that the funtion calculates the numbers like
                        %y=x.^2
end

The vectorize function is lilsted as ‘Not recommended’ however it is likely the best option in situations such as this.

.

[Patrick Weiser]

WOW Thanks a lot! This is excactly what i search for.

Have a nice day.

[Star Strider]

My pleasure!

If my Answer helped you solve your problem, please Accept it!

.

Answer 2

[John D'Errico]

Time to learn about function handles! A function handle alows you to pass one function to another.

f = @(x) x.^2;

What is f?

whos f

  Name      Size            Bytes  Class              Attributes

  f         1x1                32  function_handle              

Indeed, f is a function handle. We can evaluate f.

f(2.35)

ans = 5.5225

We can pass it to ANOTHER function.

[xmin,fmin] = fminbnd(f,-5,5)

xmin = 0

fmin = 0

AND you can pass it into your own functions.

myfun(f)

Evaluating fun at pi: 9.8696

As you can see, myfun is pretty boring. It just evaluates the function I passed in, then displays the result. Such is life. :)

function myfun(fun)
  y = fun(pi);
  disp("Evaluating fun at pi: " + y)
end

[Patrick Weiser]

Hey

Thanks for the quick reply.

We are getting closer to the matter. Unfortunately, I still get an error message that he can not do anything with x...

function [y] = calculatethefunction(Function)
%Function means somthing like "x.'2" or "x/2"
f=@(x) Function;
x=linspace(0,10,100);
y=f              %my goal is now that the funtion calculates the numbers like
%y=x.^2
end

answer:

>> calculatethefunction(x.^2)
>> Unrecognized function or variable 'x'.

[Steven Lord]

Define the function handle before you call calculatethefunction and pass the function handle into it.

myfunction = @(x) x.^2;
result = calculatethefunction(myfunction)

result = 1×100

0 0.0102 0.0408 0.0918 0.1632 0.2551 0.3673 0.4999 0.6530 0.8264 1.0203 1.2346 1.4692 1.7243 1.9998 2.2957 2.6120 2.9487 3.3058 3.6833 4.0812 4.4995 4.9383 5.3974 5.8770 6.3769 6.8973 7.4380 7.9992 8.5808

Let's check that calculatethefunction did what we expected it to do.

xForCheck = linspace(0, 10, 100);
yForCheck = xForCheck.^2;
isequal(yForCheck, result) % true!

ans = logical
   1

Alternately you could define the function handle as a part of calling the function, in case you don't need it stored in a variable for later use.

result2 = calculatethefunction(@(x) x.^2);
isequal(result2, result) % also true

ans = logical
   1

function [y] = calculatethefunction(f)
%Function means somthing like "x.'2" or "x/2"
x=linspace(0,10,100);
y=f(x);          %my goal is now that the funtion calculates the numbers like
%y=x.^2
end

There are other functions you can use to build function handles like str2func or matlabFunction in Symbolic Math Toolbox, but if you know what you want your function handle to compute when you write your code it's easiest to just define it in your code.