dataframe - R transform column with string as character, not as factor -

i curious behaviour of transform. 2 ways might try creating new column character not factor:

x <- data.frame(letters = letters[1:3], numbers = 1:3) y <- transform(x, alphanumeric = as.character(paste(letters, numbers))) x$alphanumeric = with(x, as.character(paste(letters, numbers))) x y str(x$alphanumeric) str(y$alphanumeric) 

the results "look" same:

> x   letters numbers alphanumeric 1             1          1 2       b       2          b 2 3       c       3          c 3 > y   letters numbers alphanumeric 1             1          1 2       b       2          b 2 3       c       3          c 3 

but inside , 1 has worked:

> str(x$alphanumeric) # did convert character  chr [1:3] "a 1" "b 2" "c 3" > str(y$alphanumeric) # transform didn't  factor w/ 3 levels "a 1","b 2","c 3": 1 2 3 

i didn't find ?transform useful explain behaviour - presumably alphanumeric coerced being factor - or find way stop (something stringsasfactors = false data.frame). safest way this? there similar pitfalls beware of, instance apply or plyr functions?

this not issue transform as data.frames, stringsasfactors set, default, true. add argument should false , you'll on way:

y <- transform(x, alphanumeric = paste(letters, numbers),                stringsasfactors = false) str(y) # 'data.frame': 3 obs. of  3 variables: #  $ letters     : factor w/ 3 levels "a","b","c": 1 2 3 #  $ numbers     : int  1 2 3 #  $ alphanumeric: chr  "a 1" "b 2" "c 3" 

i use within instead of transform, , seems not have problem:

y <- within(x, {   alphanumeric = paste(letters, numbers) }) str(y) # 'data.frame':  3 obs. of  3 variables: #  $ letters     : factor w/ 3 levels "a","b","c": 1 2 3 #  $ numbers     : int  1 2 3 #  $ alphanumeric: chr  "a 1" "b 2" "c 3" 

this because takes approach similar with approach: create character vector , add (via [<-) existing data.frame.

you can view source of each of these typing transform.data.frame , within.data.frame @ prompt.

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as other pitfalls, that's broad of question. 1 thing comes mind right waya apply create matrix data.frame, columns coerced single type.