generating random vector in r with particular sum

Question

My aim is to create a vector, with sum 0, in which there are the same number of entries -x and the same number of entry equals x, the length of the vector is even, so it sums up to 0. I created a function, that has x as an input. there i insert a sample of the vectorlength but i the end it doesn't work out.

vector<-function(x){ 
    for(i in length(sample)){
      if(i %% 2!=0){
     output[sample[i]]<-(-x)
   }
    if(i %% 2 ==0){
      output[sample[i]]<-x
    }
  }
return(output)
  }

please add your expected input and output to your question. – Roman Apr 11 '18 at 14:26 — Roman, Apr 11 '18 at 14:26
expected input: any natural number – Kasimir Apr 11 '18 at 14:28 — Kasimir, Apr 11 '18 at 14:28
expected output: a vektor of length sample with sum 0 – Kasimir Apr 11 '18 at 14:29 — Kasimir, Apr 11 '18 at 14:29

ebeneditos · Answer 1 · 2018-04-11T14:40:21.200

0

Try this:

vector <- function(x, sample){
  c(rep(x, sample/2), rep(-x, sample/2))
}
print(vector(x = 1, sample = 4))
# [1]  1  1 -1 -1

Edit

If alterning is required:

vector <- function(x, sample){
    c(rbind(rep(-x, sample/2), rep(x, sample/2)))
}
print(vector(x = 1, sample = 4))
# [1] -1  1 -1  1

edited Apr 11 '18 at 14:40

answered Apr 11 '18 at 14:30

ebeneditos

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why this `rbind`? – Roman Apr 11 '18 at 14:34
To alternate x and -x, see this answer: https://stackoverflow.com/a/25961969/7224320 – ebeneditos Apr 11 '18 at 14:36
why alternating? Is it required here? – Roman Apr 11 '18 at 14:37
In the example function it seems like it is trying to alternate, although it may not be required. – ebeneditos Apr 11 '18 at 14:39

Roman · Answer 2 · 2018-04-11T14:49:33.160

0

You can try

foo <- function(x, sample){
  a <- sample(sample, x/2, replace = T)
  c(a,-a)
  # or alternating
  # c(rbind(a,-a))
}
set.seed(123)
foo(4, 1:10)
[1]  3  8 -3 -8

edited Apr 11 '18 at 14:49

answered Apr 11 '18 at 14:36

Roman

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Ralf Stubner · Answer 3 · 2018-04-11T15:18:16.107

0

According to the title you are looking for a random vector. In that case you can simply first generate an ordered vector with the desired properties and then use sample to shuffle it:

f <- function(x, size){
  sample(c(rep(x, size/2), rep(-x, size/2), if(size %% 2 != 0) 0))
}
f(x = 1, size = 6)
#> [1]  1 -1 -1  1 -1  1
f(x = 1, size = 7)
#> [1]  0 -1 -1  1 -1  1  1

Edit: Now the function even allows for an odd size.

edited Apr 11 '18 at 15:18

answered Apr 11 '18 at 14:52

Ralf Stubner

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that looks simple and sweet but what to do with size odd? – Kasimir Apr 11 '18 at 15:06
@Kasimir You wrote "length of the vector is even", but see the edit ... – Ralf Stubner Apr 11 '18 at 15:18

generating random vector in r with particular sum

3 Answers3

Edit