[R] Inserting rows of interpolated data

[Benstead, Jonathan]

Dear help list - I have light data with 5-min time-stamps. I would like to insert four 1-min time-stamps between each row and interpolate the light data on each new row. To do this I have come up with the following code:

lightdata <- read.table("Test_light_data.csv", header = TRUE, sep = ",")  # read data file into object "lightdata"
library(chron)
mins <- data.frame(times(1:1439/1440)) # generate a dataframe of 24 hours of 1-min timestamps
Nth.delete <- function(dataframe, n)dataframe[-(seq(n, to=nrow(dataframe), by=n)),] # function for deleting nth row
empty <- data.frame("1/9/13", Nth.delete(mins, 5), "NA") # delete all 5-min timestamps in a new dataframe
colnames(empty) <- c("date", "time", "light") # add correct column name to empty timestamp dataframe
newdata <- rbind(lightdata, empty)

I get the following error message:

Warning message:
In `[<-.factor`(`*tmp*`, ri, value = c(0.000694444444444444, 0.00138888888888889,  :
  invalid factor level, NAs generated

Digging into this a little, I can see that the two time columns are doing what I need and APPEAR to be similar in format:

> head(lightdata)
    date    time       light
1 1/9/13 0:00:00 -0.00040925
2 1/9/13 0:05:00 -0.00023386
3 1/9/13 0:10:00 -0.00032155
4 1/9/13 0:15:00 -0.00017539
5 1/9/13 0:20:00 -0.00029232
6 1/9/13 0:25:00 -0.00038002

> head(empty)
    date     time light
1 1/9/13 00:01:00    NA
2 1/9/13 00:02:00    NA
3 1/9/13 00:03:00    NA
4 1/9/13 00:04:00    NA
5 1/9/13 00:06:00    NA
6 1/9/13 00:07:00    NA

but they clearly are not as far as R is concerned, as shown by str:

> str(lightdata)
'data.frame':   288 obs. of  3 variables:
 $ date : Factor w/ 1 level "1/9/13": 1 1 1 1 1 1 1 1 1 1 ...
 $ time : Factor w/ 288 levels "0:00:00","0:05:00",..: 1 2 3 4 5 6 7 8 9 10 ...
 $ light: num  -0.000409 -0.000234 -0.000322 -0.000175 -0.000292 ...

> str(empty)
'data.frame':   1152 obs. of  3 variables:
 $ date : Factor w/ 1 level "1/9/13": 1 1 1 1 1 1 1 1 1 1 ...
 $ time :Class 'times'  atomic [1:1152] 0.000694 0.001389 0.002083 0.002778 0.004167 ...
  .. ..- attr(*, "format")= chr "h:m:s"
 $ light: Factor w/ 1 level "NA": 1 1 1 1 1 1 1 1 1 1 ...

In the first (original) dataframe, light is a factor, while in the dataframe of generated timestamps, the timestamps are actually still in fractions of a day.

Presumably this is why rbind is not working? Can anyone help? By the way, I know I can use na.approx in zoo to do the eventual interpolation of the light data. It's getting there that has me stumped for now.

Many thanks, Jon (new R user).