2013/10/21

character handling: mean() vs sd()

Here’s a weird R error/bug/nuance I came across today:

What would you expect the following lines of code to return?

x = c('1', '2', '3')
mean(x)
sd(x)

Well, apparently it is:

# mean(x)
[1] NA

# sd(x)
[1] 1

So, sd() silently converts its input to numeric, while mean() does not. More evidence of this is in the source:

> mean
function (x, ...) 
UseMethod("mean")
<bytecode: 0x000000001165e790>
<environment: namespace:base>

> sd
function (x, na.rm = FALSE) 
sqrt(var(if (is.vector(x)) x else as.double(x), na.rm = na.rm))
<bytecode: 0x000000001158eb00>
<environment: namespace:stats>

One hour of my work day was spent sorting this out. You've been warned.

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