Selecting values from a 3-column dataframe in R -

i have 3-dimensional array, variables being x, y , z. x list of places, y list of time, , z list of names. list of names not start @ same initial time across places:

x   y   z x1  1   na x1  2   z2 x1  3   z3 x1  4   z1 x2  1   na x2  2   na x2  3   z5 x2  4   z3 x3  1   z3 x3  2   z1 x3  3   z2 x3  4   z2 

how find first z every x? want output matrix or dataframe be:

x  z x1 z2 x2 z5 x3 z3 

edited, after example data supplied

you can use function ddply() in package plyr

dat <- "x   y   z x1  1   na x1  2   z2 x1  3   z3 x1  4   z1 x2  1   na x2  2   na x2  3   z5 x2  4   z3 x3  1   z3 x3  2   z1 x3  3   z2 x3  4   z2"  df <- read.table(textconnection(dat), header=true, stringsasfactors=false)  library(plyr) ddply(df, .(x), function(x)x[!is.na(x$z), ][1, "z"])     x v1 1 x1 z2 2 x2 z5 3 x3 z3