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Randomize two categories testing a specific condition R version 3.0.2 windows 32 bit
2 messages · Manos Spanakis, Daniel Nordlund
-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Manos Spanakis
Sent: Saturday, November 23, 2013 1:48 AM
To: R-help at r-project.org
Subject: [R] Randomize two categories testing a specific condition R
version 3.0.2 windows 32 bit
Hello everyone I am new to R. At this time I am trying to create a vector
so i can randomize two categories a,b under the following conditions :
Take a random x from random uniform(0,1) if x<0.5 then the vector takes
value a else takes b . This continues until the number of a's-the number
of
b's > 2 at this point i want to change the possibility that a's and b's
are
assigned to the vector to 0.2 (if x<0.2 then a). This probability i want
to
change again if the number of b's - the number of a's > 2 to 0.8(if x<0.8
then a). However because i have random numbers i may have the above
situation vice versa. (first b's-a's>2 then a's-b's>2) .finally sth like
vector<- a,b,a,a,a,b,b,a . I thought to use a for loop and this is my code
;
set.seed(10)
x<- rep(NA, 20)
tmp<- rep(FALSE, 20)for(i in 1:20) {
x[i]<- runif(1)
tmp<- x[i]<0.5if(2 * sum(tmp) - i + 1 > 2) {
tmp[i]<- x[i]<0.2} else {
tmp[i]<- x[i]<0.8}}
Although the code runs without errors it doesnt return what i want . How
am I wrong? Should i have to use next or break statement here or sth else?
please help me.Thanks in advance
It is not clear to me what your exact boundary conditions are for changing probabilities, or even what your real goal is, but maybe something like the following will get you started. Stage=1 is before there are greater than 2 'a's and 2 'b's.
set.seed(3724)
x <- character(0)
p <- .5
stage <- 1
for(i in 1:20){
x <- c(x, sample(c('a','b'),1,prob=c(p,1-p)))
suma <- sum(x=='a')
sumb <- sum(x=='b')
#check if first condition is met
if(suma > 2 & sumb > 2) {
#select a starting value for changed p value and do this only once
if(stage == 1) {stage <- 2; if(suma >= sumb) p <-.8 else p <- .2}
#check for excess of 'a's or 'b's if(suma - sumb > 2) p <- .2
if(sumb - suma > 2) p <- .8
}
}
If this doesn't do what you want, then write back to R-help describing what your ultimate goal is (i.e., what this method of randomization is supposed to accomplish), and someone may be able to give you better advice.
Hope this is helpful,
Dan
Daniel Nordlund
Bothell, WA USA