[R] midpoint between two dates

[Pete Brecknock]

eric wrote
> Is there an easy way to get the midpoint between two dates in a data frame
> ? If I have a dataframe that looks like this :
> 
>  head(x)
>       instDay     remDay exp.time mpy
> 1  2006-02-02 2006-04-03       60 0.2
> 2  2006-04-17 2006-08-17      122 0.3
> 4  2006-08-17 2006-10-23       67 0.4
> 6  2006-10-23 2007-04-03      162 0.3
> 8  2007-04-03 2007-05-15       42 0.8
> 11 2007-05-15 2007-08-01       78 0.3
> 
> I would like an additional column that represents the midpoint between
> instDay and newDay. For those days where the time difference is an odd
> number and the midpoint would not be a specific date, it would be OK to
> round up or round down.
> 
> I thought about converting both columns to numeric values and taking the
> difference, dividing by two with modulus operator, then adding to the
> first column and finally converting back to a date. But I'm think there
> must be a more simple way.

How about ...

date1 = as.Date(c("2013-10-10","2013-11-15","2013-12-25"))
date2 = as.Date(c("2013-10-20","2013-11-20","2013-12-30"))

df <- data.frame(id=c(1,2,3),date1,date2)
df$mid <- df$date1 + floor((df$date2-df$date1)/2)

print(df)

  id      date1      date2        mid
1  1 2013-10-10 2013-10-20 2013-10-15
2  2 2013-11-15 2013-11-20 2013-11-17
3  3 2013-12-25 2013-12-30 2013-12-27


HTH

Pete

 



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