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Defining functions

Asked by Richard on 8 Jan 2012
Latest activity Commented on by Walter Roberson
on 26 Jul 2017

How might I define a function handle?

For example, I want to define a function f(x)=2*x^3+7*x^2+x

I want MATLAB to evaluate f(x) at random values x. I have heard of feval and fhandles, but I don't know how to do it.

Thanks.

  2 Comments

This was the code I was trying to write:

a= 0.0009
a= convtime([1],'samples','seconds')

This code gave an error saying that my function (convtime) was undefined. How you define a function in MATLAB?

Leia Sofia Mendez:

convtime appears to be from the File Exchange https://www.mathworks.com/matlabcentral/fileexchange/28204-time---rate-unit-conversion-function .

You should go to that link, and click the download button, and download the .zip file. You should unzip to a directory that is not under your MATLAB installation directory. You would then use pathtool in MATLAB to add that directory to your MATLAB path.

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6 Answers

Answer by Chandra Kurniawan on 8 Jan 2012
 Accepted Answer

Hi, Richard.

First, you need to create a new m-file then type this code

function y = f(x)
y = 2 * (x^3) + 7 * (x^2) + x;

Save with filename 'f.m'

Then create a new m-file and type this code

x =  randi(7);
y = f(x)

randi will generate random number

  3 Comments

to evaluate this function just type :
y = feval(@f,x)

Thanks, Chandra! :)

The problem or lets say the dificulty of this code is that this couple of lines should be saved as a function and I personally don't wanna do that for a simple function like this! or even complicated ones!

Not my favorite way of defining a function

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Answer by Walter Roberson
on 8 Jan 2012

Function handle version:

f = @(x) 2*x^3+7*x^2+x;

Then f is already the function handle, and you can call f(3.7) (for example)

There is no need to use feval() for this, but you could.

  1 Comment

I rather this anonymous way of defining a function! It's way easier. I also know another way of doing this, surprisingly nobudy mentioned that so far! lol I'm gonna put it in the answers.

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Answer by Junaid
on 8 Jan 2012

Dear Richard,

To define a function in matlab you can do following syntax of given function:

function n = F(x)
n= 2*x^3+7*x^2+x;

that is it. You can put end at the end of function. But it is also acceptable not to put to various matlab versions. If you put end for one function then you have to put for all function in single m file.

then you can generate random numbers, either integer or double, and can get the values of this function.

  1 Comment

Thanks, Junaid! :)

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Answer by cyril
on 21 Mar 2014
Edited by cyril
on 21 Mar 2014

> f = @(x) 2*x^3+7*x^2+x;
> f(0)
0

surprising no one mentioned anonymous functions...

  1 Comment

@ Cyril

Walter did, just make sure you checked the other answers and comments!

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Answer by samy youssef on 11 Mar 2015
Edited by Walter Roberson
on 26 Sep 2016

here is a function i developed to calculate the log of any number with different base:

function d =log_for_diff_base(myNumber,myBase)
x=log(myNumber);
y=log(myBase);
d=x/y;
end

  1 Comment

Okay... but irrelevant to the original question.

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Answer by Nikitha Challa on 26 Sep 2016

x = x + a/x 2 in matlab code

  2 Comments

Not a function at all. This is not even valid MATLAB code as written.

That does not appear to be a question, and it is not an Answer to what was asked here?

If the question is to solve the equation

x == x + a/(x^2)

then for finite a values, the solutions are -inf and +inf as a/(x^2) would be 0 for those values, leading to the equality -inf == -inf and +inf == +inf

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