[R] Question about avoid the for loop

Dear R-helpers, I would like to generate a binary random variable within a stratum's stratum. Here is a simple example. ## x is the first level strata index, here I have 3 strata. x=c(rep(1,5), rep(2,5), rep(3,5)) ## within x, there is a second strata indexed by t=0 and t=1

Re: [R] Question about avoid the for loop

Hi Carrie, Here are two options: # Option 1 d - data.frame(x, t) y - with(d, ifelse(t == 0, rbinom(2, 1, 0.2), rbinom(3, 1, 0.8))) y # Option 2 -- more general case, e.g. you do not know # how many 0's and 1's you have within each strata spd - with(d, split(d, x)) do.call(c, lapply(spd,

Re: [R] Question about avoid the for loop

Thanks! Jorge Just one more question I don't get it even after checking help For option, why just using with(d,...), ifelse works on stratum indexed by x automatically ? Since in with, we didn't specify the stratum is indexed by x, what if you have another categorical variable in the data ? Thanks

Re: [R] Question about avoid the for loop

Hi Jorge, I found a problem. I just want to check if the answer is random, I change the code as follows: d - data.frame(x, t) y - with(d, ifelse(t == 0, rbinom(2, 1, 0.5), rnorm(3))) cbind(x, t,y) x t y [1,] 1 0 0.000 [2,] 1 0 0.000 [3,] 1 1 0.8920037 [4,] 1 1

Re: [R] Question about avoid the for loop

Hi Carrie, It works just fine in this case because you have the same number of 0's and 1's within each strata. If that would not be the case, option 1 would not work. That's why I provided you a second option. Best, Jorge On Thu, Jun 3, 2010 at 7:24 PM, Carrie Li wrote: Thanks! Jorge Just

Re: [R] Question about avoid the for loop

Yes, in my case here, each strata has the same number of 0's and 1's. But I want the y to be randomly generated within each strata, so y should have some difference across the strata. (at least for the rnorm part we would see much clear randomness) (I hope what I am asking here is clear to you. )

Re: [R] Question about avoid the for loop

Hi Carrie, Try running the following: # function to create y using binomials and normals # -- this function is based on option 2 makey - function(d){ spd - with(d, split(d, x)) do.call(c, lapply(spd, function(comp) with(comp, ifelse(t == 0, rbinom(sum(t==0), 1, 0.2),