sample {base}  R Documentation 
sample
takes a sample of the specified size from the elements
of x
using either with or without replacement.
sample(x, size, replace = FALSE, prob = NULL) sample.int(n, size = n, replace = FALSE, prob = NULL, useHash = (!replace && is.null(prob) && size <= n/2 && n > 1e7))
x 
either a vector of one or more elements from which to choose, or a positive integer. See ‘Details.’ 
n 
a positive number, the number of items to choose from. See ‘Details.’ 
size 
a nonnegative integer giving the number of items to choose. 
replace 
should sampling be with replacement? 
prob 
a vector of probability weights for obtaining the elements of the vector being sampled. 
useHash 

If x
has length 1, is numeric (in the sense of
is.numeric
) and x >= 1
, sampling via
sample
takes place from 1:x
. Note that this
convenience feature may lead to undesired behaviour when x
is
of varying length in calls such as sample(x)
. See the examples.
Otherwise x
can be any R object for which length
and
subsetting by integers make sense: S3 or S4 methods for these
operations will be dispatched as appropriate.
For sample
the default for size
is the number of items
inferred from the first argument, so that sample(x)
generates a
random permutation of the elements of x
(or 1:x
).
It is allowed to ask for size = 0
samples with n = 0
or
a lengthzero x
, but otherwise n > 0
or positive
length(x)
is required.
Noninteger positive numerical values of n
or x
will be
truncated to the next smallest integer, which has to be no larger than
.Machine$integer.max
.
The optional prob
argument can be used to give a vector of
weights for obtaining the elements of the vector being sampled. They
need not sum to one, but they should be nonnegative and not all zero.
If replace
is true, Walker's alias method (Ripley, 1987) is
used when there are more than 200 reasonably probable values: this
gives results incompatible with those from R < 2.2.0.
If replace
is false, these probabilities are applied
sequentially, that is the probability of choosing the next item is
proportional to the weights amongst the remaining items. The number
of nonzero weights must be at least size
in this case.
sample.int
is a bare interface in which both n
and
size
must be supplied as integers.
Argument n
can be larger than the largest integer of
type integer
, up to the largest representable integer in type
double
. Only uniform sampling is supported. Two
random numbers are used to ensure uniform sampling of large integers.
For sample
a vector of length size
with elements
drawn from either x
or from the integers 1:x
.
For sample.int
, an integer vector of length size
with
elements from 1:n
, or a double vector if
n >= 2^31.
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
Ripley, B. D. (1987) Stochastic Simulation. Wiley.
RNG
about random number generation.
CRAN package sampling for other methods of weighted sampling without replacement.
x < 1:12 # a random permutation sample(x) # bootstrap resampling  only if length(x) > 1 ! sample(x, replace = TRUE) # 100 Bernoulli trials sample(c(0,1), 100, replace = TRUE) ## More careful bootstrapping  Consider this when using sample() ## programmatically (i.e., in your function or simulation)! # sample()'s surprise  example x < 1:10 sample(x[x > 8]) # length 2 sample(x[x > 9]) # oops  length 10! sample(x[x > 10]) # length 0 ## safer version: resample < function(x, ...) x[sample.int(length(x), ...)] resample(x[x > 8]) # length 2 resample(x[x > 9]) # length 1 resample(x[x > 10]) # length 0 ## R 3.x.y only sample.int(1e10, 12, replace = TRUE) sample.int(1e10, 12) # not that there is much chance of duplicates