[R] Sign of specific elements of a vector

[Frank S.]

Hi everyone,
 
I have an "x" vector and I would want to change the sign every 20 elements. For this puspose,
I wrote the following code:
 
set.seed(1)
x <- rnorm(100)
x
x[seq(20,100, by=20)] <- -x[seq(20,100, by=20)]
x
 
However, I'm afraid  it is a rudimentary form to get the desired result. II wonder wether there is a cool way to do so, that is, for example with apply or sign function.
 
Thans in advanced for your help!

Frank S.
 		 	   		  
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