[R] Randomly interleaving data frames while preserving order

[Kevin E. Thorpe]

On 03/31/2015 01:44 PM, Duncan Murdoch wrote:
> On 31/03/2015 1:05 PM, Kevin E. Thorpe wrote:
>> Hello.
>>
>> I am trying to simulate recruitment in a randomized trial. Suppose I
>> have three streams (strata) of patients represented by these data frames.
>>
>> df1 <- data.frame(strat=rep(1,10),id=1:10,pid=1001:1010)
>> df2 <- data.frame(strat=rep(2,10),id=1:10,pid=2001:2010)
>> df3 <- data.frame(strat=rep(3,10),id=1:10,pid=3001:3010)
>>
>> What I need to do is construct a data frame with all of these combined
>> where the order of selection from one of the three data frames is
>> randomized but once a stratum is selected patients are selected
>> sequentially from that data frame.
>>
>> To see what I'm looking to achieve, suppose the first five subjects were
>> to come, in order, from strata (data frames) 1, 2, 1, 3 and 2. The
>> expected result should look like this:
>>
>> rbind(df1[1,],df2[1,],df1[2,],df3[1,],df2[2,])
>>      strat id  pid
>> 1      1  1 1001
>> 2      2  1 2001
>> 21     1  2 1002
>> 4      3  1 3001
>> 22     2  2 2002
>>
>> I hope what I'm trying to accomplish makes sense. Maybe I'm missing
>> something obvious, but I really have no idea at the moment how to
>> achieve this elegantly. Since I need to simulate many trial recruitments
>> it needs to be general and compact.
>>
>> I appreciate any advice.
>
> How about something like this:
>
> # Permute an ordered vector of selections:
> sel <- sample(c(rep(1, nrow(df1)), rep(2, nrow(df2)), rep(3, nrow(df3))))
>
> # Create an empty dataframe to hold the results
> df <- data.frame(strat=NA, id=NA, pid=NA)[rep(1, length(sel)),]
>
> # Put the original dataframes into the appropriate slots:
> df[sel == 1,] <- df1
> df[sel == 2,] <- df2
> df[sel == 3,] <- df3
>
> # Clean up the rownames
> rownames(df) <- NULL
>
> Duncan Murdoch
>

Thanks Duncan.

Once you see the solution it is indeed obvious.

Kevin

-- 
Kevin E. Thorpe
Head of Biostatistics,  Applied Health Research Centre (AHRC)
Li Ka Shing Knowledge Institute of St. Michael's
Assistant Professor, Dalla Lana School of Public Health
University of Toronto
email: kevin.thorpe at utoronto.ca  Tel: 416.864.5776  Fax: 416.864.3016