[R] Using transform to add a date column to a dataframe

[Gavin Simpson]

On Tue, 2008-12-23 at 05:24 -0800, Tom La Bone wrote:
> I would like to add a column to the airquality dataset that contains the date
> 1950-01-01 in each row. This method does not appear to work:
> 
> > attach(airquality)
> > data1 <- transform(airquality,Date=as.Date("1950-01-01"))
> 
> Error in data.frame(list(Ozone = c(41L, 36L, 12L, 18L, NA, 28L, 23L, 19L,  : 
>   arguments imply differing number of rows: 153, 1
>  
> I can't decipher what the error message is trying to tell me. Any
> suggestions on how to do this?

It says that the two arguments have different numbers of observations.
The reason for which should now be pretty obvious as you provided a
single Date whereas airquality has 153 observations.

You did read ?transform , which points out this "problem"? ;-)

Anyway, don't assume R recycles everything if it is not of sufficient
length to match other arguments. In this case, repeat the date as many
times as there are rows in airquality:

> data(airquality)
> data1 <- transform(airquality, Date = rep(as.Date("1950-01-01"),
nrow(airquality)))
> head(data1)
  Ozone Solar.R Wind Temp Month Day       Date
1    41     190  7.4   67     5   1 1950-01-01
2    36     118  8.0   72     5   2 1950-01-01
3    12     149 12.6   74     5   3 1950-01-01
4    18     313 11.5   62     5   4 1950-01-01
5    NA      NA 14.3   56     5   5 1950-01-01
6    28      NA 14.9   66     5   6 1950-01-01

Also, the attach(airquality) call in your example doesn't do anything
that affects your example so is redundant.

HTH

G

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