Multiplying every secound element in a vector with -1

Hello I am looking for how to multiply every secound element in vector 1xn with -1, but I want to write this as a function. I can do this manually like this for example with an 1x3 vector;

function vny = byt_tecken(v)
        vny=[v(1,1),-v(1,2),v(1,3)]
end

But how do I do this for any given 1xn vector? Thanks!

Answer 1

[John D'Errico]

You need to start learning MATLAB. That is, actually learn to use vectors, matrices, not just work with single numbers, and loops. Until you do, you will continue to have these problems.

Here is one way to do what you asked for. There are surely a zillion others.

v = 1:10;
vny = v.*(mod(1:numel(v),2)*2 - 1)

vny =
     1    -2     3    -4     5    -6     7    -8     9   -10

The point is to start thinking in terms of a vector and vector operations, tools that operate on every element of a vector in one call.

Answer 2

[Daniel M]

Here is another way:

function vny = byt_tecken(v)
    vny = v.*-(-1).^(1:numel(v));
end

Answer 3

[James Tursa]

Or yet another of the zillion ways

vny = v;
vny(2:2:end) = -vny(2:2:end);