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?

class="post-text" itemprop="text">

class="normal">Answer

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)))