Calculating %changes with the By()

Question

I am an inexperienced R user and have been struggling with the By() function and would appreciate your help. The task is simple, I have a longitudinal dataset (How do I decl

Answer 1

Your suggested output is not "%change" (but rather fractional difference) while this illustrates a method getting "percent of original" using the initial value as the basis for the change:

> dat$pctTemp <-  unlist(
            tapply(dat$Temp, dat$ID, function(x) c(NA, 100*x[-1]/x[1]) )
                        )
> dat
    ID     Date      Temp   pctTemp
1  AAA 1/1/2003 0.7498817        NA
2  AAA 1/2/2003 0.6666616  88.90223
3  AAA 1/3/2003 0.7730799 103.09358
4  AAA 1/4/2003 0.6290236  83.88305
5  AAA 1/5/2003 0.7333124  97.79041
6  BBB 1/1/2003 0.7073398        NA
7  BBB 1/2/2003 0.7649865 108.14980
8  BBB 1/3/2003 0.6622015  93.61858
snipped

If you want interval change, you can divide diff(x) by the prceding values

> dat$pctTemp <-  unlist(tapply(dat$Temp, dat$ID, function(x) c(NA, 100*diff(x)/x[-length(x)]) )  )
> dat
    ID     Date      Temp      pctTemp
1  AAA 1/1/2003 0.7498817           NA
2  AAA 1/2/2003 0.6666616 -11.09776868
3  AAA 1/3/2003 0.7730799  15.96287574
4  AAA 1/4/2003 0.6290236 -18.63407501
5  AAA 1/5/2003 0.7333124  16.57946178
6  BBB 1/1/2003 0.7073398           NA
7  BBB 1/2/2003 0.7649865   8.14979813
snipped