how to pivot/unpivot (cast/melt) data frame?

Question

How can I 'unpivot' a table? What is the proper technical term for this?

UPDATE: The term is called melt

I have a data frame for countries and data for each year

Country     2001    2002    2003
Nigeria     1       2       3
UK          2       NA       1

And I want to have something like

Country    Year    Value
Nigeria    2001    1
Nigeria    2002    2
Nigeria    2003    3
UK         2001    2
UK         2002    NA
UK         2003    1

Answer 1

I still can't believe I beat Andrie with an answer. :)

> library(reshape)
> my.df <- read.table(text = "Country     2001    2002    2003
   + Nigeria     1       2       3
   + UK          2       NA       1", header = TRUE)
> my.result <- melt(my.df, id = c("Country"))
> my.result[order(my.result$Country),]
     Country variable value
   1 Nigeria    X2001     1
   3 Nigeria    X2002     2
   5 Nigeria    X2003     3
   2      UK    X2001     2
   4      UK    X2002    NA
   6      UK    X2003     1