r - unsplit list, merge factors -

i have following data frame in r:

  c1 c2   1 10    2 20    3 30  b   4 40  b 

i split follows: z = lapply(split(test$c1, test$c2), function(x) {cut(x,2)}) . z then:

$a   [1] (9.99,15] (15,20]   levels: (9.99,15] (15,20]  $b   [1] (30,35] (35,40] levels: (30,35] (35,40]   

i merge factors unsplitting list unsplit(z, test$c2). generates warning:

[1] (9.99,15] (15,20]   <na>      <na>      levels: (9.99,15] (15,20] warning message: in `[<-.factor`(`*tmp*`, i, value = 1:2) :   invalid factor level, nas generated 

i take union of factor levels , unsplit error not happen:

z$a = factor(z$a, levels=c(levels(z$a), levels(z$b))) unsplit(z, test$c2) [1] (9.99,15] (15,20]   (30,35]   (35,40]   levels: (9.99,15] (15,20] (30,35] (35,40]     

in real data frame have big list need iterate on list elements (not two). best way this?

if understood question properly, think making bit more complicated needed. here's 1 solution using plyr. group c2 variable:

require(plyr) ddply(test, "c2", transform, newvar = cut(c1, 2)) 

which returns:

  c1 c2    newvar 1 10  (9.99,15] 2 20    (15,20] 3 30  b   (30,35] 4 40  b   (35,40] 

and has structure of:

'data.frame':   4 obs. of  3 variables:  $ c1    : num  10 20 30 40  $ c2    : factor w/ 2 levels "a","b": 1 1 2 2  $ newvar: factor w/ 4 levels "(9.99,15]","(15,20]",..: 1 2 3 4