R apply conversion to multiple columns of data.frame - r

Question

I am wanting to convert several columns in a data.frame from chr to numeric and I would like to do it in a single line. Here is what I am trying to do:
items[,2:4] <- as.numeric(sub("\\$","",items[,2:4]))
But I get an error saying:
Warning message:
NAs introduced by coercion
If I do it column by column though it works:
items[,2:2] <- as.numeric(sub("\\$","",items[,2:2]))
items[,3:3] <- as.numeric(sub("\\$","",items[,3:3]))
items[,4:4] <- as.numeric(sub("\\$","",items[,4:4]))
What am I missing here? Why I specify this command for multiple columns? Is this some odd R idiosyncrasy that I am not aware of?
Example Data:
Name, Cost1, Cost2, Cost3, Cost4
A, $10.00, $15.50, $13.20, $45.45
B, $45.23, $34.23, $34.24, $23.34
C, $23.43, $45.23, $65.23, $34.23
D, $76.34, $98.34, $90.34, $45.09

Answer 1

Your problem is, that gsub converts its x argument to character. If a list (a data.frame is in fact a list) is converted to character something wired happen:
as.character(list(a=c("1", "1"), b="1"))
# "c(\"1\", \"1\")" "1"
# and "c(\"1\", \"1\")" can not convert into a numeric
as.numeric("c(\"1\", \"1\")")
# NA
A one line solution would be to unlist the x argument:
items[, 2:5] <- as.numeric(gsub("\\$", "", unlist(items[, 2:5])))

Answer 2

Yes there is: apply is the command you are looking for:
items<-read.table(text="Name, Cost1, Cost2, Cost3, Cost4
A, $10.00, $15.50, $13.20, $45.45
B, $45.23, $34.23, $34.24, $23.34
C, $23.43, $45.23, $65.23, $34.23
D, $76.34, $98.34, $90.34, $45.09", header=TRUE,sep=",")
items[,2:4]<-apply(items[,2:4],2,function(x){as.numeric(gsub("\\$","",x))})
items
Name Cost1 Cost2 Cost3 Cost4
1 A 10.00 15.50 13.20 $45.45
2 B 45.23 34.23 34.24 $23.34
3 C 23.43 45.23 65.23 $34.23
4 D 76.34 98.34 90.34 $45.09

Answer 3

A more efficient approach would be:
items[-1] <- lapply(items[-1], function(x) as.numeric(gsub("$", "", x, fixed = TRUE)))
items
# Name Cost1 Cost2 Cost3 Cost4
# 1 A 10.00 15.50 13.20 45.45
# 2 B 45.23 34.23 34.24 23.34
# 3 C 23.43 45.23 65.23 34.23
# 4 D 76.34 98.34 90.34 45.09
Some benchmarks of the answers so far
fun1 <- function() {
A[-1] <- lapply(A[-1], function(x) as.numeric(gsub("$", "", x, fixed=TRUE)))
A
}
fun2 <- function() {
A[, 2:ncol(A)] <- as.numeric(gsub("\\$", "", unlist(A[, 2:ncol(A)])))
A
}
fun3 <- function() {
A[, 2:ncol(A)] <- apply(A[,2:ncol(A)], 2, function(x) { as.numeric(gsub("\\$","",x)) })
A
}
Here's some sample data and processing times
set.seed(1)
A <- data.frame(Name = sample(LETTERS, 10000, TRUE),
matrix(paste0("$", sample(99, 10000*100, TRUE)),
ncol = 100))
system.time(fun1())
# user system elapsed
# 0.72 0.00 0.72
system.time(fun2())
# user system elapsed
# 5.84 0.00 5.85
system.time(fun3())
# user system elapsed
# 4.14 0.00 4.14