How can I calculate the percentage change within a group for multiple columns in R?

Question

I have a data frame with an ID column, a date column (12 months for each ID), and I have 23 numeric variables. I would like to obtain the percentage change by month within each

Answer 1

How about using pct <- function(x) x/lag(x)? (or (x/lag(x)-1)*100, or however you wish to specify pct change exactly) e.g.,

pct(1:3)
[1]  NA 2.0 1.5

Edit: Adding Frank's suggestion

pct <- function(x) {x/lag(x)}

dt %>% group_by(ID) %>% mutate_each(funs(pct), c(V1, V2, V3))

ID Date       V1       V2  V3
1  Jan       NA       NA  NA
1  Feb 1.500000 1.333333 1.2
1  Mar 2.333333 2.000000 1.5
2  Jan       NA       NA  NA
2  Feb 2.000000 3.000000 4.0
2  Mar 3.500000 2.666667 2.0