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I am trying to do something with [R] which should be extremely simple: convert values in a data.frame to numbers, as I need to test for their values and r does not recognize them as number.

When I convert a decimal number to numeric, I get the correct value:

> a <- as.numeric(1.2)
> a
[1] 1.2

However, when I extract a positive value from the data.frame then use as.numeric, the number is rounded up:

> class(slices2drop)
[1] "data.frame"
> slices2drop[2,1]
[1] 1.2
Levels: 1 1.2
> a <- as.numeric(slices2drop[2,1])
> a
[1] 2

Just in case:

> a*100
[1] 200

So this is not a problem with display, the data itself is not properly handled.

Also, when the number is negative, I get NA:

> slices2drop[2,1] <- -1
> a <- as.numeric(slices2drop[2,1])
> a
[1] NA

Any idea as to what may be happening?

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  • 1
    Note that the first time you show us slices2drop[2,1] it returns a factor. That is your issue - you're working with a factor - not numeric data. Commented Nov 18, 2013 at 20:12
  • 1
    See ?factor, Warning section. Commented Nov 18, 2013 at 20:14
  • 1
    Please take a look at this question: stackoverflow.com/questions/3418128/… Commented Nov 18, 2013 at 20:14

2 Answers 2

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11

This problem has to do with factors. To solve your problem, first coerce your factor variable to be character and then apply as.numeric to get what you want.

> x <- factor(c(1, 1.2, 1.3)) # a factor variable
> as.numeric(x)  
[1] 1 2 3

Integers number are returned, one per each level, there are 3 levels: 1, 1.2 and 1.3, therefore 1,2,3 is returned.

> as.numeric(as.character(x)) # this is what you're looking for
[1] 1.0 1.2 1.3

Actually as.numeric is not rounding your numbers, it returns a unique integer per each level in your factor variable.

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1

I faced a similar situation where the conversion of factor into numeric would generate incorrect results.

When you type: ?factor

The Warning mentioned with the factor variables explains this complexity very well and provides the solution to this problem as well. It's a good place to start working with...

Another thing to consider is that, such conversion would transform NULLs into NAs

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