Change class of multiple columns in data frame without for loop

Question

I want to change the class of multiple columns in an R data frame without either doing it one by one, or using a for loop (and noting this answer). I can do it with either of these methods, but they feel clumsy. Note that I don't necessarily want to change every column.

E.g. I have data frame mydf:

mydf <- data.frame("col1" = c(1, 2, 3),
               "col2" = c("a", "b", "c"),
               "col3" = c("a", "a", "b"), stringsAsFactors = FALSE)

I want to change columns two and three to class factor. (In reality I want to deal with many more than two columns...)

I can either do it column by column in my favourite way, e.g.:

mydf$col2     <- as.factor(mydf$col2)
mydf[, 3]     <- as.factor(mydf[,3])

Or I could use a for loop:

 for (i in 2:3{
   mydf[,i] <- as.factor(mydf[,i])
 }

These work, but feel clunky and suboptimal.

Better ideas?

Scransom · Accepted Answer · 2018-01-10T07:14:52.313

16

OK I worked it out while writing the question, but figured it might as well go up in case it's use to anyone in future:

mydf[,2:3] <- lapply(mydf[,2:3], as.factor)

edited Jan 10 '18 at 07:14

answered Feb 16 '16 at 08:02

Scransom

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2

This solution already appears in the linked question in here http://stackoverflow.com/a/3798348/3001626 – David Arenburg Feb 16 '16 at 08:08
9

For a more general use I think it's better to define a vector with the column names that should be converted, like `factor_cols <- c("col2","col3")`, followed by `mydf[factor_cols] <- lapply(mydf[factor_cols], as.factor)` – RHertel Feb 16 '16 at 08:10
@RHertel thanks that's perfect advice – Scransom Feb 16 '16 at 08:12
1

It's also appears in the linked answer. – David Arenburg Feb 16 '16 at 08:13
@DavidArenburg I didn't check that, but I'm not surprised. – RHertel Feb 16 '16 at 08:15

Change class of multiple columns in data frame without for loop

1 Answers1