Question

I’m trying to create a script that will loop through some data frames and change their column names to lowercase.

firmNames <- c("file1","file2","file3","file4","file5")
for(i in 1:length(firmNames)){
    fileName <- paste("data/",firmNames[i],".RData",sep="")
    load(fileName)
    upper <- names(get(firmNames[i]))
    lower <- tolower(upper)  # transforms to lower case
    names(get(firmNames[i])) <- lower  # This is the offending line
    save(get(firmNames[i]), file = fileName)
}

This causes a syntax error:

Error in names(get(firmNames[i])) <- lower : 
could not find function "get<-"

If I use assign, it still fails:

Error in save(get(firmNames[i]), file = fileName) : 
object ‘get(firmNames[i])’ not found
In addition: Warning message:
In assign(names(get(firmNames[i])), lower) :
only the first element is used as variable name

Strangely, printing names(get(firmNames[i])) displays perfectly, as does lower. They are both of mode character. What am I missing?

Answer 1

A straightforward, readable, and sanity-preserving solution is to assign the data.frame you want to modify to a temporary object (here named X). Work on that, and when you’ve got it in shape, overwrite the original object with the correctly modified one:

# SETUP
firmNames <- c("file1","file2","file3","file4","file5")
file1 <- data.frame(ALLIGATOR=1:4, BLUEBIRD=rnorm(4))

file1
#   ALLIGATOR    BLUEBIRD
# 1         1 -0.37122263
# 2         2 -0.13939213
# 3         3 -0.79044157
# 4         4 -0.06837244


# MODIFY COLUMN NAMES
for (i in 1) {
    X <- get(firmNames[i])
    names(X) <- tolower(names(X))
    assign(firmNames[i], X)
}

file1
#   alligator    bluebird
# 1         1 -0.37122263
# 2         2 -0.13939213
# 3         3 -0.79044157
# 4         4 -0.06837244