continuation question: add row in r dataframe unique factor in column showing percent change month    testing <- data.frame(   month = c("mtd: 12", "mtd: 12", "mtd: 11", "mtd: 12", "mtd: 12", "mtd: 12"),    year = c(2012, 2013, 2014, 2015, 2013, 2014),    client = c("a.", "a.", "a.", "b.", "b.", "b."),    revenue = c(320, 205, 166l, 152, 150, 138),   col1 = c(651, 485, 533, 3932, 171, 436),    col2 = c(478, 335, 305, 238, 115, 251),    col3 = c(73, 69, 57, 6, 67, 57),    col4 = c(6.7, 6.1, 5.5, 6.4, 13.1, 5.5) )  # subset month=12 rows test12 <- testing[testing$month=="mtd: 12", ] test12 <- test12[order(test12$client, test12$year), ]  # define function calculate percent change pctchange <- function(x) {   l <- length(x)   c(na, 100 * (x[-1] - x[-l]) / x[-l]) }  # calculate percent change columns, client change <- apply(test12[, c("...