So, I discovered this:
> TRUE == "TRUE"
[1] TRUE
> TRUE == "BOO"
[1] FALSE
> TRUE == "true"
[1] FALSE
> TRUE == "T"
[1] FALSE
> FALSE == "FALSE"
[1] TRUE
> FALSE == "F"
[1] FALSE
According to the R documentation for logical {base}:
as.logical attempts to coerce its argument to be of logical type. Character strings c("T", "TRUE", "True", "true") are regarded as true, c("F", "FALSE", "False", "false") as false, and all others as NA.
This is actually the case:
> as.logical("T")
[1] TRUE
So apparently, what's going on with TRUE == "T" is not an as.logical sort of conversion. Is there any reasonable explanation for the behavior of these == comparisons?
Here's what I made of it:
From the documentation of logical comparisons ?"==":
At least one of x and y must be an atomic vector, but if the other is a list R attempts to coerce it to the type of the atomic vector: this will succeed if the list is made up of elements of length one that can be coerced to the correct type. If the two arguments are atomic vectors of different types, one is coerced to the type of the other, the (decreasing) order of precedence being character, complex, numeric, integer, logical and raw.
To me it seems like the latter part of this is at work here. TRUE is being coerced to "TRUE" and the actual comparison becomes "TRUE"=="TRUE" instead of TRUE==TRUE.
T always gets converted to TRUE so T=="TRUE" holds. However ``"T"` has no such luck when conversion is to happen to.character and not to.logical. .