Question

Total newb R question, but here it is: lets say I want to create a data frame with two columns, one with all years in a range, and the other with every month in each year. When I’m done, I should have this:

year month
1990 1
1990 2
1990 3

Et cetera. This seems like a pretty obvious job for cbind, to turn a range into a column, and repeat, to produce 12 instances of each year. This works great, but only for an even number of years in the range. So, for instance:

df <- data.frame(cbind(year=rep(c(1990:2000), 12)))

Works fine. And so does this:

df <- data.frame(cbind(year=rep(c(1990:2000), 12), month=c(1:12)))

But this produces overt nonsense:

df <- data.frame(cbind(year=rep(c(1990:2001), 12), month=c(1:12)))

The first line of code produces 12 instances of each year in the range, just as you’d expect; the second line produces the desired result. The third line produces 12 instances of each year, where each year only gets one month number. Thus:

year month 
1990  1
1990  1
1990  1

Is there a way around this that doesn’t require always adding a year and trimming it off later?

Answer 1

You are looking for expand.grid

 df <- expand.grid(year = 1990:2001, month = 1:12)