On 10/12/2019 10:32 a.m., Sarah Goslee wrote:
Back to the table part of the question, but using Duncan's example.
x <- c(3.4, 3.4 + 1e-15)
unique(x)
[1] 3.4 3.4
table(x)
x
3.4
2
The question was, why are these different.
table() only works on factors, so it converts the numeric vector to a
factor before tabulation.
factor() tries to do something sensible, and implicitly rounds the numeric data.
factor(x)
[1] 3.4 3.4
Levels: 3.4
Whether you think that is actually sensible or not is up to you, but
if it isn't then you shouldn't use table.
That table uses factors is documented in ?table. A quick read of
?factor didn't find any explicit discussion, other than the
acknowledgement that factor() is lossy in:
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))’.
You can't even get table() to do what you want by being explicit:
table(factor(x, levels = unique(x)))
Error in `levels<-`(`*tmp*`, value = as.character(levels)) :
factor level [2] is duplicated
You could get it to agree with unique if you do the string conversion
yourself. The first result is ugly:
x <- c(3.4, 3.4 + 1e-15)
tab <- table(sprintf("%a", x))
tab
#>
#> 0x1.b333333333333p+1 0x1.b333333333335p+1
#> 1 1
But if you really want you can make it readable:
names(tab) <- as.numeric(names(tab))
tab
#> 3.4 3.4
#> 1 1
Duncan Murdoch
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