r - operations on a variable from a data.frame with time points -

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i stuck in problem , send call (i went through similar questions not find need, although seems simple) : have original dataframe 50 patients 3 time points (reproducible code below) , variable of interest called "ht". goal study variations of ht between these 3 time points, example according variable (ex here numerical variable "a"). 

    id  <-  rep(c(seq(1,50,1)),3)     time  <- factor(rep(c("day1", "day2", "day3"), c(50,50,50)), levels=c("day1", "day2", "day3"), labels=c("day1", "day2", "day3"), ordered=true)     ht  <- rnorm(150, mean=30, sd=3)      <- rnorm(150, mean=7, sd=10)     dfrm  <-  cbind (id,time,ht,a)      > head(dfrm)       id time       ht           [1,]  1    1 28.64048 11.1595852  [2,]  2    1 28.30068  4.2925773  [3,]  3    1 32.51943 21.2013316  [4,]  4    1 30.66561  0.6980816  [5,]  5    1 28.92749 22.2756818  [6,]  6    1 33.82217 14.2877789

i study differences between ht values between day 1 , day2, between day 2 , day 3, between day 1 , day 3. insert calcultations form indidual data id values, , other variables. not know if better build new data.frame, guess yes. suggest simple way proceed? thank in advance. denis

 library(plyr)  dfrm <- data.frame(dfrm)  out <- ddply(dfrm, .(id), function(x) {         x <- x[order(x$time),]         x$diff <- c(na, diff(x$ht))         x$diff2 <- c(na, na, diff(x$ht, 2))         x     })

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