[R] how to randomly eliminate half the entries in a vector?
Esmail Bonakdarian
esmail.js at gmail.com
Wed Feb 18 02:17:51 CET 2009
Hello all,
I need some help with a nice R-idiomatic and efficient solution to a
small problem.
Essentially, I am trying to eliminate randomly half of the entries in
a vector that contains index values into some other vectors.
More details:
I am working with two strings/vectors of 0s and 1s. These will contain
about 200 elements (always the same number for both)
I want to:
1. determines the locations of where the two strings differ
--> easy using xor(s1, s2)
2. *randomly* selects *half* of those positions
--> not sure how to do this. I suppose the result would be
a list of index positions of size sum(xor(s1, s2))/2
3. exchange (flip) the bits in those random positions for both strings
--> I have something that seems to do that, but it doesn't look
slick and I wonder how efficient it is.
Mostly I need help for #2, but will happily accept suggestions for #3,
or for that matter anything that looks odd.
Below my partial solution .. the HUX function is what I am trying
to finish if someone can point me in the right direction.
Thanks
Esmail
------
rm(list=ls())
####################################################
# create a binary vector of size "len"
#
create_bin_Chromosome <- function(len)
{
sample(0:1, len, replace=T)
}
####################################################
# HUX - half uniform crossover
#
# 1. determines the locations of where the two strings
# differ (easy xor)
#
# 2. randomly selects half of those positions
#
# 3. exchanges (flips) the bits in those positions for
# both
#
HUX <- function(b1, b2)
{
# 1. find differing bits
r=xor(b1, b2)
# positions where bits differ
different = which(r==TRUE)
cat("\nhrp: ", different, "\n")
# 2. ??? how to do this best so that each time
# a different half subset is selected? I.e.,
# sum(r)/2 positions.
# 3. this flips *all* positions, should really only flip
# half of them (randomly selected half)
new_b1 = b1
new_b2 = b2
for(i in different) # should contain half the entries (randomly)
{
new_b1[i] = b2[i]
new_b2[i] = b1[i]
}
result <- matrix(c(new_b1, new_b2), 2, LEN, byrow=T)
result
}
LEN = 5
b1=create_bin_Chromosome(LEN)
b2=create_bin_Chromosome(LEN)
cat(b1, "\n")
cat(b2, "\n")
idx=HUX(b1, b2)
cat("\n\n")
cat(idx[1,], "\n")
cat(idx[2,], "\n")
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