IF not working with vector

Hello, sorry for the trivial question. 

I wrote this function:

function f = g(m)
%piecewise function evaluation

if m <= 0 
    f(n) = -2*m;
else
    f(n) = 2*m;
end
 
end

Which actually works when g is given numbers individually

g(2) = 2, g(-2) = 2

However when I try to plot it by evaluting the function g in an array of value in x created with linspace it seems to "forget" about if condition in the function statement.

x = linspace(-1,1);
g(x) = "gives a straight line 2x, instead of behaving like abs(2x)"

[darova]

What is this?

What about for loop?

[Alessandro Cabodi]

Since it was not working, I actually tried to implement a while loop. I forgot to remove them when i posted the question.

Answer 1

[Ameer Hamza]

IIf you want to write your own function with if-else block look to implement abs function, then you will need to write a loop

function f = g(m)
%piecewise function evaluation
    for i=1:numel(m)
        if m(i) <= 0 
            f(i) = -2*m(i);
        else
            f(i) = 2*m(i);
        end
    end
end

Or a one-liner if your question is specifically related to abs(2*m);

function f = g(m)
    f = 2*m.*(m>0) - 2*m.*(m<0);
end

[Stephen23]

"...then you will need to write a loop"

Logical indexing would be simpler.

[Ameer Hamza]

Yes, but I think that the example of abs(2*m) is just an MWE. The OP might be trying to solve some other problem where directly using logical indexing might not work.

[Alessandro Cabodi]

I just started using matlab, what's the meaning of the second code section? It reminds me to a "?" operator in C language but I don't understand what is the purpose of ".*2" being it a scalar.

I tried the first of your suggestions and it actually works, so thank you, but I hoped it was not necessary to use a for loop: it is one of the first exercises i was given in matlab and loops have not been mentioned yet, so i think it could be done without them. However I don't know how to manipulate indexes in such a way to make the function works... are your suggestions sort of the only easy ways to do it?

[Ameer Hamza]

Alessandro, the ternary operator (?), is not available in MATLAB. But you can kind of get similar semantic with the indexing as mentioned in my answer. I agree that the way i wrote the second equation is a bit confusing. Please check the updated answer.

[Ameer Hamza]

"are your suggestions sort of the only easy ways to do it"

easy is a subjective term. You can say that it requires the least number of statements. Someone might say that following is the "easiest"

function f = g(m)
    f = 2*m;
    f(f<0) = -f(f<0);
end

[Alessandro Cabodi]

Ok thank you, it's a bit more clear now. I think i'm gonna use the for loop, at my level it is more immediate, thank you however for having clarified the second code.