sample {base} R Documentation

Random Samples and Permutations

Description

sample takes a sample of the specified size from the elements of x using either with or without replacement.

Usage

sample(x, size, replace = FALSE, prob = NULL)

Arguments

 x Either a (numeric, complex, character or logical) vector of more than one element from which to choose, or a positive integer. size non-negative 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.

Details

If x has length 1 and x >= 1, sampling takes place from 1:x. Note that this convenience feature may lead to undesired behaviour when x is of varying length sample(x). See the resample() example below.

By default size is equal to length(x) so that sample(x) generates a random permutation of the elements of x (or 1:x).

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 that 250 reasonably probable values: this gives results incompatible with those from R < 2.2.0, and there will be a warning the first time this happens in a session.

If replace is false, these probabilities are applied sequentially, that is the probability of choosing the next item is proportional to the probabilities amongst the remaining items. The number of nonzero weights must be at least size in this case.

References

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.

Examples

x <- 1:12
# a random permutation
sample(x)
# bootstrap sampling -- 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!
try(sample(x[x > 10]))# error!

## This is safer, but only for sampling without replacement
resample <- function(x, size, ...)
if(length(x) <= 1) { if(!missing(size) && size == 0) x[FALSE] else x
} else sample(x, size, ...)

resample(x[x >  8])# length 2
resample(x[x >  9])# length 1
resample(x[x > 10])# length 0

[Package base version 2.5.0 Index]