Function for Script. I need to write a Function that selects the positive and negative numbers from a matrix. How to write this?

I need to write a Function that selects the positive and negative numbers from a matrix. How to write this?

Answer 1

[dpb]

function [p,n]=splitsigns(x)
% return positive/negative values from array x in vectors p/n, respectively

p=x(x>0);
n=x(x<0);

This one excludes 0; pick where you want those if do...

[ricsf]

Thank you!!!

[ricsf]

But, if I have a matrix with positive numbers and have to change it to negative and vice-versa?

[dpb]

Simple enough but need a better definition of what you need/want...

x=-x;      % switch all signs
x=abs(x);  % all negative are now positive

Many other choices as well, depending...

[ricsf]

I'm new in Matlab... I'm doing exercices...

Thanks!

[ricsf]

And if i switch the input and output values, like this:

function (x)=splitsigns(p,n)
  x=p(p>=0)
  x=n(x<0)

Then the second assignment overwrites the first and the result of the second yields an empty set, irregardless since you guaranteed no negatives are in x with the first.

Might as well as written

function (x)=splitsigns(p,n)
  x=[];

which, overall, is pretty useless... :)

[ricsf]

Ok! So, like you have writen in the first one, just to have no doubts in this.

I've the declaration of the function, with the: Output Arg (n,p) and the Input Arg (x)

And the Variables of the funtion are: p=x(x>0); n=x(x<0);

Am I thinking correctly?

[dpb]

Read the help files on functions, but in general yes. Other than I'd say that p=x(x>0); and n=x(x<0); are expressions, not variables. There are no strictly local variables in those functions, only the input/output arguments (which are, of course variables just not local).

Answer 2

[John Petersen]

a = 0.5 - rand(3,3);

%Positive numbers
a(a>=0)

% negative numbers
a(a>0)

[ricsf]

Thank You!

But, if I have a matrix with positive numbers and have to change it to negative and vice-versa?