r - How to subset a row from list based on condition -

i have data.table outcome, has column called hospital. , column called state. outcome has been sorted already. want subset nth hospital each state(if there's not nth returns na state). try solve below way.(since homework, showed third branch causes mistake).

rankall <- function(out, num = "best"){     outcome <- readdata(outcome = out)  //returns data.table sorted rate    ...     outcome <- lapply(outcome, function(x) ifelse(num <= nrow(x), x[num,], c(na,na)))     outcome <- rbindlist(outcome) } 

the original outcome like

> data                                                hospital state    1:                              nyu hospitals center    ny    2:                               doylestown hospital    pa    3:          avera heart hospital of south dakota llc    sd    4:                 glendale adventist medical center    ca    5:                                waterbury hospital    ct   ---                                                         2716:                           desert springs hospital    nv 2717:                   3 rivers community hospital    or 2718: robert wood johnson university hospital @ rahway    nj 2719:                             laredo medical center    tx 2720:                     medical center south arkansas    ar 

and first , second branch produce right result, like

 > head (data)                                hospital state 1:                                   na    ak 2:             crestwood medical center    al 3:              arkansas heart hospital    ar 4:                 mayo clinic hospital    az 5:    glendale adventist medical center    ca 6: st marys hospital , medical center    co      > nrow(data) [1] 54 

however, third condition couldn't work. produce error

error in rbindlist(outcome) :    item 1 of list input not data.frame, data.table or list 

and after debugging found out outcome after condition like(which caused error in last step)

$ak [1] na  $al $al[[1]] [1] "highlands medical center" 

differs first 2 like...

> head(data,2) $ak                            hospital state 1: providence alaska medical center    ak  $al                    hospital state 1: crestwood medical center    al 

so wonder what's wrong third branch. me out, thank much!!!

by way, wonder if refer variable same name of another's. such when called readdata, need pass argument called outcome, prevents me use name argument of rankall function(i use out instead). know in java this.outcome help, how in r.

thank vivek's help, have figured out now. first mis-performance of third branch. it'll work if first convert numto number using as.numeric(num). think it's because num regarded character(since has possible value of "best" , "worst") caused mistake. second regarding naming space strange. though not having performed in own test, work after vivek answer question. so, means can use following code, , r right result.

rankall <- function(outcome, num = "best"){     outcome <- readdata(outcome = outcome)