Error when converting with as.numeric() in R

Question

I have a dataset:

 > x
    Treatment X1 X2
1         T1  6  7
2         T1  5  9
3         T1  8  6
4         T1  4  9
5         T1  7  9
6         T2  3  3
7         T2  1  6
8         T2  2  3
9         T3  2  3
10        T3  5  1
11        T3  3  1
12        T3  2  3

I am trying to find means of the columns X1 and X2. If I run the data as-is, I get an error:

> t1 <- subset(x[2:3], x$Treatment=="T1")
> x_vec <- colMeans(t1, na.rm = TRUE)
Error in colMeans(t1, na.rm = TRUE) : 'x' must be numeric

So, I need to convert X1 and X2 to numeric:

t1$X1 <- as.numeric(as.factor(t1$X1))
t1$X2 <- as.numeric(as.factor(t1$X2))
x_vec <- colMeans(t1, na.rm = TRUE)

But when I do that, I get the wrong result:

> x_vec
 X1  X2 
6.0 4.4

The t1, after conversion to as.numeric(), shows:

Why are the values in X2 changed after converting to numeric?

The problem is that you are running colMeans on every column, and column Treatment is not numeric. — neilfws
– neilfws, Commented Dec 3, 2018 at 1:40
@neilfws - doesn't look like it - t1 only has x[2:3] in it. — thelatemail
– thelatemail, Commented Dec 3, 2018 at 2:03
@thelatemail ah yes, I misread. Then I guess there are non-numeric characters in X1/X2. — neilfws
– neilfws, Commented Dec 3, 2018 at 2:20

mikeck · Accepted Answer · 2018-12-03 14:22:00Z

2

This is a pretty common issue that newer R users hit. The issue is your use of as.factor. running as.numeric on a factor converts the value to the numeric index of the label, rather than converting the label itself to a number. Your can either remove the call to as.factor or run as.character on the factor before calling as.numeric.

Note that some functions like as.data.frame automatically convert characters to factors, which can cause problems. Check out the option stringsAsFactors for more info.

edited Dec 3, 2018 at 14:22

answered Dec 3, 2018 at 1:52

mikeck

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