R- how to dynamically name data frames? [duplicate]

OP is really struggling so instead of a long comment, I'll show him here. Don't care if this gets closed.

The technical (don't do that answer) would be to use assign:

i <- 1
j <- 1
for (f in 1:length(fileList)){
    fileName <- fileList[f]
    X <-read.xls(fileName)

    if(grepl("Drug_Rep", fileName)) {
      print("DRUG")
      print(fileName)
      assign(paste("X_Drug_Rep", i, sep = '_'), X)
      i <- i+1
    } else {
      print("CONTROL")
      print(fileName)
      assign(paste("X_CONTROL", i, sep = '_'), X)
      j <- j+1
    }
  }

But as we recommended, you should use lists instead. Using a for loop, it would look like this:

X_Drug_Rep <- list()
X_CONTROL  <- list()
i <- 1
j <- 1
for (f in 1:length(fileList)){
    fileName <- fileList[f]
    X <-read.xls(fileName)

    if(grepl("Drug_Rep", fileName)) {
      print("DRUG")
      print(fileName)
      X_Drug_Rep[[i]] <- X
      i <- i+1
    } else {
      print("CONTROL")
      print(fileName)
      X_CONTROL[[j]] <- X
      j <- j+1
    }
  }

Finally, how your code would look like without a for loop:

drug.rep.files <- grep("Drug_Rep", fileList, value = TRUE)
control.files  <- grep("Drug_Rep", fileList, value = TRUE, invert = TRUE)

X_Drug_Rep <- lapply(drug.rep.files, read.xls)
X_CONTROL  <- lapply(control.files, read.xls)

Much shorter, no?! Again, this creates two lists. For example, instead of X_Drug_Rep_1, you would access the first Drug_Rep item by doing X_Drug_Rep[[1]].