How to multiply -1 with _n+1 value of a variable?

[Attaullah Shah]

I am facing a problem that seems simple, but I cannot figure it out to solve. Let us say I have the following data:
input id date Ri
1 1 .3
1 2 .4
1 3 .6
1 4 .7
end

what I want is to multiply -1 with secon Ri , fourth Ri, sixth Ri and so. So, for the above data, I should get the data in following result
input id date Ri
1 1 .3
1 2 -.4
1 3 .6
1 4 -.7
end

[Aljar Meesters]

replace Ri = -Ri if mod(_n, 2) == 0 will do the trick.
Best,

Aljar

[Nick Cox]

Multiplication by -1 is otherwise known as negation.

I wouldn't overwrite the original, not least because you would mess it up if you make a mistake. Some possibilities are

gen new_Ri = cond(mod(_n,2), Ri, -Ri)

or

gen new_Ri = cond(!mod(_n,2), -Ri, Ri)

or

gen new_Ri = cond(mod(_n,2) == 0, -Ri, Ri)

or

gen new_Ri = Ri
replace new_Ri = -Ri if !mod(_n, 2)

or

gen new_Ri = Ri * (mod(_n,2) - mod(_n-1,2))

although the last is a kind of feeble joke.

The mod() function (systematically misnamed since FORTRAN in the 1950s) yields the remainder on division. If you divide an integer by 2, there are two possible remainders, 1 if an integer is odd and 0 if it is even.

Thought or experiment shows that with observation numbers _n (manifestly positive integers) (mod(_n,2) == 1) and mod(_n,2) both yield 1 or 0; they can also be negated logically.

See http://www.stata-journal.com/sjpdf.h...iclenum=pr0031 To my chagrin, I forgot to mention applications to rotation problems. Thus mod(270 + 180, 360) yields a new orientation in degrees after a rotation of 180 degrees clockwise.

[Attaullah Shah]

Thanks Aljar and Nick for your replies. Nick! your comments and solutions are always like poetry.

[Jeph Herrin]

Or

gen new_Ri=Ri*((-1)^_n)

[Nick Cox]

Jeph's solution is neat. I was queasy on whether there might be a problem on e.g. powering with powers like 1e6, but every seems to work out as mathematics implies.

[Jeph Herrin]

Jeph's solution is neat.

...except that it changes the odd numbers, not the evens. Should have been

gen new_RI=Ri*((-1)^(_n+1))