[r] How to convert data.frame column from Factor to numeric

I have a data.frame whose class column is Factor. I'd like to convert it to numeric so that I can use correlation matrix.

> str(breast)
'data.frame':   699 obs. of  10 variables:
 ....
 $ class                   : Factor w/ 2 levels "2","4": 1 1 1 1 1 2 1 1 1 1 ...
> table(breast$class)
  2   4 
458 241
> cor(breast)
Error in cor(breast) : 'x' must be numeric

How can I convert a Factor column to a numeric column?

As an alternative to $dollarsign notation, use a within block:

breast <- within(breast, {
  class <- as.numeric(as.character(class))
})

Note that you want to convert your vector to a character before converting it to a numeric. Simply calling as.numeric(class) will not the ids corresponding to each factor level (1, 2) rather than the levels themselves.

This is FAQ 7.10. Others have shown how to apply this to a single column in a data frame, or to multiple columns in a data frame. But this is really treating the symptom, not curing the cause.

A better approach is to use the colClasses argument to read.table and related functions to tell R that the column should be numeric so that it never creates a factor and creates numeric. This will put in NA for any values that do not convert to numeric.

Another better option is to figure out why R does not recognize the column as numeric (usually a non numeric character somewhere in that column) and fix the original data so that it is read in properly without needing to create NAs.

Best is a combination of the last 2, make sure the data is correct before reading it in and specify colClasses so R does not need to guess (this can speed up reading as well).

From ?factor:

To transform a factor f to approximately its original numeric values, as.numeric(levels(f))[f] is recommended and slightly more efficient than as.numeric(as.character(f)).