# [R] how to select an element from a vector based on a probability

Nordlund, Dan (DSHS/RDA) NordlDJ at dshs.wa.gov
Thu Apr 10 22:04:03 CEST 2014

```> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Simone Gabbriellini
> Sent: Thursday, April 10, 2014 11:59 AM
> Cc: r-help at r-project.org
> Subject: Re: [R] how to select an element from a vector based on a
> probability
>
> Hello, Rui,
>
> it does, indeed!
>
> thanks,
> Simone
>
> > Hello,
> >
> > Use ?sample.
> >
> > sample(x, 1, prob = x)
> >

Just be aware that, in using this method, the probability of selection of a particular value will also be a function of how frequent the value is.  For example,

set.seed(7632)
x <- c(2,2,6,2,1,1,1,3)
table(sample(x, 10000, prob=x, replace=TRUE))

1    2    3    6
1664 3340 1696 3300

The probability that a vector position with a value of 1 will be selected is 1/18 (in this particular example).  However, the probability that a value of 1 will be selected is 1/6 since there are three 1's.  The probability of selecting the position with a value of 3 is 3/18.  But since there is only one position with a value of 3, the probability of getting the value 1 on any given sample is equal to the probability of getting the value 3.

> > Hope this helps,
> >
> >
> > Em 10-04-2014 19:49, Simone Gabbriellini escreveu:
> >
> >> Hello List,
> >>
> >> I have an array like:
> >>
> >> c(4, 3, 5, 4, 2, 2, 2, 4, 2, 6, 6, 7, 5, 5, 5, 10, 10, 11, 10,
> >> 12, 10, 11, 9, 12, 10, 36, 35, 36, 36, 36, 35, 35, 36, 37, 35,
> >> 35, 38, 35, 38, 36, 37, 36, 36, 37, 36, 35, 35, 36, 36, 35, 35,
> >> 36, 35, 38, 35, 35, 35, 36, 35, 35, 35, 6, 5, 8, 6, 6, 7, 1,
> >> 7, 7, 8, 9, 7, 8, 7, 7, 13, 13, 13, 14, 13, 13, 13, 14, 14, 15,
> >> 15, 14, 13, 14, 39, 39, 39, 39, 39, 39, 41, 40, 39, 39, 39, 39,
> >> 40, 39, 39, 41, 41, 40, 39, 40, 41, 40, 41, 40, 40, 40, 39, 41,
> >> 39, 39, 39, 39, 40, 39, 39, 40, 40, 39, 39, 39, 1, 4, 3, 4)
> >>
> >> I would like to pick up an element with a probability proportional
> to
> >> the element value, thus higher values should be picked up more often
> >> than small values (i.e., picking up 38 should be more probable than
> >> picking up 3)
> >>
> >> Do you have any idea on how to code such a rich-get-richer
> mechanism?
> >>
> >> Best regards,
> >> Simone
> >>
> >
>
>
>

Dan

Daniel J. Nordlund, PhD
Research and Data Analysis Division