Change the class from factor to numeric of many columns in a data frame

Question

What is the quickest/best way to change a large number of columns to numeric from factor?

I used the following code but it appears to have re-ordered my data.

> head(stats[,1:2])   rk                 team 1  1 Washington Capitals* 2  2     San Jose Sharks* 3  3  Chicago Blackhawks* 4  4     Phoenix Coyotes* 5  5   New Jersey Devils* 6  6   Vancouver Canucks*  for(i in c(1,3:ncol(stats))) {     stats[,i] <- as.numeric(stats[,i]) }  > head(stats[,1:2])   rk                 team 1  2 Washington Capitals* 2 13     San Jose Sharks* 3 24  Chicago Blackhawks* 4 26     Phoenix Coyotes* 5 27   New Jersey Devils* 6 28   Vancouver Canucks* 

What is the best way, short of naming every column as in:

df$colname <- as.numeric(ds$colname) 

Answer 1

You have to be careful while changing factors to numeric. Here is a line of code that would change a set of columns from factor to numeric. I am assuming here that the columns to be changed to numeric are 1, 3, 4 and 5 respectively. You could change it accordingly

cols = c(1, 3, 4, 5);     df[,cols] = apply(df[,cols], 2, function(x) as.numeric(as.character(x))); 

Answer 2

Further to Ramnath's answer, the behaviour you are experiencing is that due to as.numeric(x) returning the internal, numeric representation of the factor x at the R level. If you want to preserve the numbers that are the levels of the factor (rather than their internal representation), you need to convert to character via as.character() first as per Ramnath's example.

Your for loop is just as reasonable as an apply call and might be slightly more readable as to what the intention of the code is. Just change this line:

stats[,i] <- as.numeric(stats[,i]) 

to read

stats[,i] <- as.numeric(as.character(stats[,i])) 

This is FAQ 7.10 in the R FAQ.

HTH