Folks, i thought it should be straightforward but after a few hours poking
around, I decided it's best to post my question on this list.
I have a data frame consisting of a (large) number of date columns, which are
read in from a csv file as character string. I want to convert them to Date
type. Following is an example, where the first column is of integer type, while
the rest are type character.
> head(df)
TRANSNO TRANS.START_DATE TRANS.END_DATE DIVISION FASB
1 250897 7/1/2010 7/31/2010 PSTRUCT Z
2 250617 8/1/2010 8/31/2010 PSTRUCT Z
3 250364 4/1/2011 6/30/2011 PLR Z
4 250176 4/1/2011 6/30/2011 PLR Z
5 250176 4/1/2011 6/30/2011 PLR Z
6 250364 4/1/2011 6/30/2011 PLR Z
> sapply(df, class)
TRANSNO TRANS.START_DATE TRANS.END_DATE DIVISION
FASB
"integer" "character" "character" "character"
"character"
I thought it's just a matter of applying with a as.Date,
df[,c(2,3)] = apply(df[,c(2,3)], 2, function(x)as.Date(x,"%m/%d/%Y"))
Well, the Date conversion fails and I got,
TRANSNO TRANS.START_DATE TRANS.END_DATE DIVISION FASB
1 250897 14791 14821 PSTRUCT Z
2 250617 14822 14852 PSTRUCT Z
3 250364 15065 15155 PLR Z
4 250176 15065 15155 PLR Z
5 250176 15065 15155 PLR Z
6 250364 15065 15155 PLR Z
The character columns are indeed converted, but they became integer, not Date
type. OK, that's strange and so I started reading the help pages.
It turns out in apply, the result is coerced to some basic vector types. And
apparently Date is not one of the basic vector types.
"In all cases the result is coerced by
as.vector<http://127.0.0.1:19182/library/base/help/as.vector> to one of the
basic vector types before the dimensions are set, so that (for example) factor
results will be coerced to a character array. "
The question then is how type conversion can be carried out on some columns of
a data frame without using a loop.
Thanks.
Horace Tso
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