How to replace every other element in an array with a different number

I have a vector that I am trying to turn every other entry in the array to be negative.

So it would look like x = [200 -198 196 -194 ... -2 0]

This is what I have. Is there a different approach I should be taking?

x = [200:-2:0]
x(2:2:end) = ()

[Stephen23]

"I have a vector that I am trying to turn every other entry in the array to be negative."

What should happen if a value is already negative? Is the intent to simply negate values regardless of their signs, or to actually negate absolute values (which matches what you described). So far it is not clear.

Answer 1

[Stephen23]

>> V = 200:-2:0;
>> V(2:2:end) = -V(2:2:end)

Answer 2

[David Hill]

a=yourVector.*(-ones(size(yourVector))).^(0:length(yourVector)-1);