Create count per item by year/decade

Question

I have data in a data.table that is as follows:

> x<-df[sample(nrow(df), 10),]
> x      

>                   Importer                 Exporter               


        

Answer 1

We can do this using data.table methods, Create the 'Decade' column by assignment :=, then melt the data from 'wide' to 'long' format by specifying the measure columns, reshape it back to 'wide' using dcast and we use the fun.aggregate as length.

x[, Decade:= year(Date) - year(Date) %%10]
dcast(melt(x, measure = c("Importer", "Exporter"), value.name = "Country"), 
                       Decade + Country~variable, length)
#     Decade        Country Importer Exporter
# 1:   2000      Australia        1        0
# 2:   2000        Ecuador        1        0
# 3:   2000          India        1        0
# 4:   2000         Israel        1        1
# 5:   2000           Peru        1        1
# 6:   2000 United Kingdom        0        1
# 7:   2000  United States        1        3
# 8:   2010         France        0        1
# 9:   2010      Guatemala        1        1
#10:   2010          India        1        0
#11:   2010         Mexico        1        0
#12:   2010         Poland        1        0
#13:   2010  United States        0        2