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

Question

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?

Answer 1

breast$class <- as.numeric(as.character(breast$class)) 

If you have many columns to convert to numeric

indx <- sapply(breast, is.factor) breast[indx] <- lapply(breast[indx], function(x) as.numeric(as.character(x))) 

Another option is to use stringsAsFactors=FALSE while reading the file using read.table or read.csv

Just in case, other options to create/change columns

 breast[,'class'] <- as.numeric(as.character(breast[,'class'])) 

or

 breast <- transform(breast, class=as.numeric(as.character(breast))) 

Answer 2

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)).