The data.table::melt function is used to convert a wide table into a long table, that is, the columns are collapsed into a key and value relationship.
If, say, we wanted to convert the dstate table into a long table consisting of a column with State, Region, Income, and Area as the values, we could use:
dmelted <- data.table::melt(dstate, id.vars=c("State","Region"), measure.vars=c("Income","Area"))
dmelted
An easy way to remember this is that id.vars are the columns that will remain constant, in this case, State, Region, and measure.vars indicate the columns that will be collapsed as shown as follows:
# State Region variable value # 1: Connecticut Northeast Income 5348 # 2: Maine Northeast Income 3694 # 3: Massachusetts ...
