Question

I have a data frame that looks like this, with two key columns and then a count of things that come in three different types.

  Year Month Urban Suburban Rural
1    1     1    11       12    13
2    1     2    21       22    23

I want to expand each row so that it lists the type as a factor and then the number in that type, so something like this:

  Year Month     Type Number
1    1     1    Urban     11
2    1     1 Suburban     12
3    1     1    Rural     13
4    1     2    Urban     21
5    1     2 Suburban     22
6    1     2    Rural     23

is there a function that does this painlessly?

Answer 1

This is precisely what the reshape and reshape2 packages are designed to do:

require(reshape2)
x <- read.table(text = "Year Month Urban Suburban Rural
1    1     1    11       12    13
2    1     2    21       22    23")

#Specify the variables that are your ID variables. The others will form your "long" data
x.m <- melt(x, id.vars = c("Year", "Month"))
#-----  
Year Month variable value
1    1     1    Urban    11
2    1     2    Urban    21
3    1     1 Suburban    12
...

There is a paper in the journal of statistical software that’s a great place to get started.