[R] Trying to speed up an if/else statement in simulations

[Rui Barradas]

Hello,

Try creating an index vector. In this case, I've called it 'one'.
Then, 'n.one' is the number of ones in flag1.

(Since data is an R function, I've renamed your example dat).



n <- 10   # number of observations

# First make up some data
set.seed(123)
dat <- data.frame(flag1=rbinom(n, 1, 0.5))
dat$flag2 <- integer(n)  # creates flag2, all zeros.

one <- dat$flag1 == 1  # logical index vector
n.one <- sum(ix)       # how many
dat$flag2[one]  <- rbinom(n.one, 1, 0.95)    # whenever TRUE, set
dat$flag2[!one] <- rbinom(n - n.one, 1, 0.50) # negation of 'one'
dat


Simple, no? And always faster.

Hope this helps,

Rui Barradas

Em 18-06-2012 18:29, nqf escreveu:
> Dear R-help,
>
> I am trying to write a function to simulate datasets of size n which contain
> two time-to-event outcome variables with associated 'Event'/'Censored'
> indicator variables (flag1 and flag2 respectively). One of these indicator
> variables needs to be dependent on the other, so I am creating the first and
> trying to use this to create the second using an if/else statement.
>
> My data structure needs to follow this algorithm (for each row of the data):
> If flag1=1 then flag2 should be 1 with probability 0.95 and zero otherwise
> Else if flag1=0 then flag2 should be 1 with probability 0.5 and zero
> otherwise
>
> I can set up this example quite simply using if else statements, but this is
> incredibly inefficient when running thousands of datasets:
> data<-as.data.frame(rbinom(10,1,0.5))
> colnames(data)<-'flag1'
> for (i in 1:n) {
>    if (data$flag1[i]==1) {data$flag2[i]<-rbinom(1,1,0.95)} else
> {data$flag2[i]<-rbinom(1,1,0.5)}
>   }
>
>
> I think to speed up the simulations I would be better changing to
> vectorisation and using something like:
> ifelse(data$flag1==1,rbinom(1,1,0.95),rbinom(1,1,0.5))
> but the rbinom statements here generate one value and repeat this draw for
> every element of flag2 that matches the 'if' statement on flag1.
>
> Is there a way to assign flag2 to a new bernoulli draw for each subject in
> the data frame with flag1=1?
>
> I hope my question is clear, and thank you in advance for your help.
>
> Thanks,
> Natalie
> PhD student, Reading University
>
> P.S. I am using R 2.12.1 on Windows 7.
>
>
> --
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>
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