How to calculate percentage change from different rows over different spans

Question

I am trying to calculate the percentage change in price for quarterly data of companies recognized by a gvkey(1001, 1384, etc...). and it\'s corresponding quart

Answer 1

df <- read.table(text="gvkey  PRCCQ
1  1004  5.500
2  1004  4.450
3  1004  4.500
4  1004  8.010
5  1005  4.450
6  1005  4.500",header=TRUE)

library(plyr)
library(quantmod)
ddply(df, "gvkey", transform, DeltaCol = Delt(PRCCQ,k=3))
#error

Delt2 <- function(x,k) {
  if(length(x)>k) as.vector(Delt(x1=x,k=k)) else rep(NA,length(x))
}

ddply(df, "gvkey", transform, DeltaCol = Delt2(PRCCQ,k=3))
#  gvkey PRCCQ  DeltaCol
#1  1004  5.50        NA
#2  1004  4.45        NA
#3  1004  4.50        NA
#4  1004  8.01 0.4563636
#5  1005  4.45        NA
#6  1005  4.50        NA