[R] A vector of normal distributed values with a sum-to-zero constraint

[Boris Steipe]

Make a copy with opposite sign. This is Normal, symmetric, but no longer random.

  set.seed(112358)
  x <- rnorm(5000, 0, 0.5)
  x <- c(x, -x)
  sum(x)
  hist(x)

B.

On 2014-04-01, at 8:56 AM, Marc Marí Dell'Olmo wrote:

> Dear all,
> 
> Anyone knows how to generate a vector of Normal distributed values
> (for example N(0,0.5)), but with a sum-to-zero constraint??
> 
> The sum would be exactly zero, without decimals.
> 
> I made some attempts:
> 
>> l <- 1000000
>> aux <- rnorm(l,0,0.5)
>> s <- sum(aux)/l
>> aux2 <- aux-s
>> sum(aux2)
> [1] -0.000000000006131392
>> 
>> aux[1]<- -sum(aux[2:l])
>> sum(aux)
> [1] -0.00000000000003530422
> 
> 
> but the sum is not exactly zero and not all parameters are N(0,0.5)
> distributed...
> 
> Perhaps is obvious but I can't find the way to do it..
> 
> Thank you very much!
> 
> Marc
> 
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