Sum of values within a week

Question

I want to sum the values in each column of a frame by week. I can do the mean, but the sum doesn't work for some reason:

> zoo.data <- zoo(data.frame(x=11:20,y=1:10),as.Date(1:10,origin="1970-01-01"))
> apply.weekly(zoo.data, mean)
            x y
1970-01-04 12 2
1970-01-11 17 7
> apply.weekly(zoo.data, sum)
1970-01-04 1970-01-11 
        42        168 

What's going on?

Answer 1

This is a result of the fact that the xts authors have decided to add a mean.xts method to mimic the old behavior of base R (and which is essentially colMeans). mean.xts is now dispatched on xts objects instead of mean.default, and apply.weekly temporarily converts your zoo object to an xts internally.

R> apply.weekly(zoo.data, mean)
            x y
1970-01-04 12 2
1970-01-11 17 7
R> apply.weekly(zoo.data, mean.default)
1970-01-04 1970-01-11 
         7         12 

---

But, I think this is what you want to do:

R> apply.weekly(zoo.data, colMeans)
            x y
1970-01-04 12 2
1970-01-11 17 7
R> apply.weekly(zoo.data, colSums)
             x  y
1970-01-04  36  6
1970-01-11 119 49