rearrange set of items randomly
On 08-Nov-11 08:59:38, Ted Harding wrote:
On 08-Nov-11 07:46:15, flokke wrote:
Sorry, but I dont think that I get what you mean (I am a quite new user though) I hae a data file of a sample, so I cannot use random numbers, the only thing I want to do is to randomly reassign the order of the items.
The simplest method of randomly re-ordering is to use
sample() to re-arrange (1:N) randomly, where N is the
number of items in the data. Then use the result to
access the items. Example:
D <- data.frame(X1=c(1.1,2.1,3.1,4.1),
X2=c(1.2,2.2,3.2,4.2))
N <- nrow(D)
ix <- sample((1:N))
D
# X1 X2
# 1 1.1 1.2
# 2 2.1 2.2
# 3 3.1 3.2
# 4 4.1 4.2
N
# [1] 4
ix
# [1] 3 2 4 1
D[ix,]
# X1 X2
# 3 3.1 3.2
# 2 2.1 2.2
# 4 4.1 4.2
# 1 1.1 1.2
Note that the defaults for sample() are (see ?sample):
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').
and the default for the 'replace' option is "FALSE",
so sample((1:N)) samples N from (1:N) without replacement,
i.e. a random permutation.
Ted.
While I am at it, an alternative to this use of sample() is to use order() to find the permutation which re-arranges a set of random numbers into increasing order. This in effect returns a random permutation of (1:N). Hence, instead of "ix <- sample(1:N))" in the above, you could use: ix <- order(runif(N)) Ted. -------------------------------------------------------------------- E-Mail: (Ted Harding) <ted.harding at wlandres.net> Fax-to-email: +44 (0)870 094 0861 Date: 08-Nov-11 Time: 09:30:33 ------------------------------ XFMail ------------------------------