CODEWITHSUNDEEP
rclassdataframer-factor
CoolBuffScienceDude
CoolBuffScienceDude

Reputation: 63

Replacing columns of data frame with factor object instead inserts character data?

While trying to plot my data, I found an unexpected behavior which caused my groups to be improperly rearranged and mislabeled.

In short, storing a factor object to several columns of a data frame causes it to be coerced to a character, rather than a factor. This seems related to the previously-answered question here but I still don't understand why it happens.

# x is a factor
(x = factor(c("red", "blue", "green")))
class(x)

# make a data frame
frame = data.frame("y"=1:3, "z"=1:3)

# replacing one column at a time yields a factor
frame[,"y"] = x; class(frame[,"y"])
frame[,"z"] = x; class(frame[,"z"])

# however, replacing >1 column at a time yields a character
frame[,c("y", "z")] = x
class(frame$y); class(frame$z)

Factors in R tend to cause me the most heartburn, somehow! The ordering, the combination of numerical value and character level, the general fiddliness... Anyway, I'm sure it's something I don't understand about the particular properties of data frames. Your help is appreciated!

Upvotes: 3

Views: 98

Answers (1)

MrFlick
MrFlick

Reputation: 206177

So the problem is in the [<-.data.frame function which is what runs when you do an assignment like

 frame[,c("y", "z")] = x

The problem is that when you specify more than one column as you have, if the new value is not a list, it will convert it to a matrix with the correct number of rows and columns and then split it into a list. So the problem with factors is that you cannot store them in a matrix. you can see this if you try

matrix(x, nrow=3, ncol=2)

Again, this casting is happening because you are specifying more than one column, and the new value is not a list. So one way around this is to give a list as the new value instead. 

frame[,c("y", "z")] <- list(x)

So, it's a bit annoying that factors are so scared of matrices, but once you learn to master them, they really are a powerful feature of R. Don't be discouraged!

Upvotes: 3

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