[R] cumsum on chron objects

[Gabor Grothendieck]

On 5/17/05, Sebastian Luque <sluque at mun.ca> wrote:
> Hi,
> 
> Is there some alternative to cumsum for chron objects? I have data frames
> that contain some chron objects that look like this:
> 
> DateTime
> 13/10/03 12:30:35
> NA
> NA
> NA
> 15/10/03 16:30:05
> NA
> NA
> ...
> 
> and I've been trying to replace the NA's so that a date/time sequence is
> created starting with the preceding available value. Because the number of
> rows with NA's following each available date/time is unknown, I've split
> the data frame using:
> 
> splitdf <- split(df, as.factor(df$DateTime))
> 
> so that I can later use lapply to work on each "block" of data. I thought
> I could use cumsum and set the NA's to the desired interval to create the
> date/time sequence starting with the first row. However, this function is
> not defined for chron objects. Does anybody know of alternatives to create
> such a sequence?
> 


The 'zoo' package has na.locf which stands for Last Occurrence Carried
Forward, which is what I believe you want.   

First let us create some test data, x:

> library(chron); library(zoo)
> x <- chron(c(1.5, 2, NA, NA, 4, NA))  
> x
[1] (01/02/70 12:00:00) (01/03/70 00:00:00) (NA NA)            
[4] (NA NA)             (01/05/70 00:00:00) (NA NA)            


> # na.locf is intended for zoo objects but we can convert
> # the chron object to zoo, apply na.locf and convert back:

> chron(as.vector(na.locf(zoo(as.vector(x)))))
[1] (01/02/70 12:00:00) (01/03/70 00:00:00) (01/03/70 00:00:00)
[4] (01/03/70 00:00:00) (01/05/70 00:00:00) (01/05/70 00:00:00)