How to rewrite this Stata code in R?

Question

One of the things Stata does well is the way it constructs new variables (see example below). How to do this in R?

foreach i in A B C D {  
    forval n=1990         


        

Answer 1

Both Spacedman and Joshua have very valid points. As Stata has only one dataset in memory at any given time, I'd suggest to add the variables to a dataframe (which is also a kind of list) instead of to the global environment (see below).

But honestly, the more R-ish way to do so, is to keep your factors factors instead of variable names.

I make some data as I believe it is in your R version now (at least, I hope so...)

Data <- data.frame(
    popA1989 = 1:10,
    popB1989 = 10:1,
    popC1989 = 11:20,
    popD1989 = 20:11
)

Trend <- replicate(11,runif(10,-0.1,0.1))

You can then use the stack() function to obtain a dataframe where you have a factor pop and a numeric variable year

newData <- stack(Data)
newData$pop <- substr(newData$ind,4,4)
newData$year <- as.numeric(substr(newData$ind,5,8))
newData$ind <- NULL

Filling up the dataframe is then quite easy :

for(i in 1:11){

  tmp <- newData[newData$year==(1988+i),]
  newData <- rbind(newData,
      data.frame( values = tmp$values*Trend[,i],
                  pop = tmp$pop,
                  year = tmp$year+1
      )
  )
}

In this format, you'll find most R commands (selections of some years, of a single population, modelling effects of either or both, ...) a whole lot easier to perform later on.

And if you insist, you can still create a wide format with unstack()

unstack(newData,values~paste("pop",pop,year,sep=""))

---

Adaptation of Joshua's answer to add the columns to the dataframe :

for(L in LETTERS[1:4]) {
  for(i in 1990:2000) {
    new <- paste("pop",L,i,sep="")  # create name for new variable
    old <- get(paste("pop",L,i-1,sep=""),Data)  # get old variable
    trend <- Trend[,i-1989]  # get trend variable
    Data <- within(Data,assign(new, old*(1+trend)))
  }
}