> toString(ddf$year) [1] "1983, 1983, 1983, 1983" > > toString(ddf$period) [1] "M01, M02, M03, M04" > > paste(toString(ddf$year), toString(ddf$period)) [1] "1983, 1983, 1983, 1983 M01, M02, M03, M04"