r - How to add multiple columns to a data.frame simultaneously? -

i have following dataframe , vector:

ddf = data.frame(a=rep(1,10), b=rep(2,10)) xx = c("c", "d", "e", "f") 

how can new empty columns named items in xx ?

i tried following not work:

ddf = cbind(ddf, data.frame(xx)) error in data.frame(..., check.names = false) :    arguments imply differing number of rows: 10, 4 

following not work:

for(i in 1:length(xx)){     ddf$(xx[i]) = "" }  error: unexpected '(' in: "for(i in 1:length(xx)){ ddf$("  } error: unexpected '}' in "}" 

this there:

ddf[xx] <- na  #   b  c  d  e  f #1  1 2 na na na na #2  1 2 na na na na #3  1 2 na na na na #... 

you can't directly use ddf$xx because try assign column called xx rather interpreting xx. need use [ , [<- functions, using square brackets when dealing character string - ddf["columnname"].

the reason why selects columns because data.frames lists essentially:

is.list(ddf) #[1] true  as.list(ddf) #$a # [1] 1 1 1 1 1 1 1 1 1 1 #  #$b # [1] 2 2 2 2 2 2 2 2 2 2 

...with each column corresponding list entry. if don't use comma specify row, ddf["name",] or column ddf[,"name"], column default.