Dear Peter,
Thank you for the explanations. Some further questions are inlined below.
Best regards,
Craig
Peter Dalgaard wrote:
Craig P. Pyrame wrote:
Stavros Macrakis wrote:
It gives me a headache, too! I think you'll have to wait for a more
expert user than me to supply explanations of these behaviors and
their rationales.
Thanks, Stavros. I hope someone with expertise will shed more light
on this, and in the meantime I'll try to learn more from the manuals.
Well first, you (and others) should shake any bees relating to "raw"
objects out of your bonnet. The raw type should be viewed as an add-on
enabling access to low-level data structures, not as a fundamental
data type in R. The original data modes are logical, numeric,
character, with "upwards" coercion rules (and some further twiddles
for storage modes of numerics).
I think I've seen examples in the list's archives where downwards
coercion rules were used, but I am not sure.
One particular aspect is that in the original system, logical works as
a "wild card" type that can be coerced to all the others (hence NA has
mode "logical", which causes some confusion at first notice).
That's right, I find it confusing at first.
....
Why does this fail:
> r = as.raw(TRUE) > ifelse(TRUE, r, r) => error
This gives an error which I take for saying that raw cannot be coerced
to logical, but yes it can:
> as.logical(r) => TRUE
Wrong. Or rather, irrelevant. The error is
> ifelse(TRUE,r,r)
Error in ans[test & !nas] <- rep(yes, length.out = length(ans))[test &
: incompatible types (from raw to logical) in subassignment type fix
Notice "subassignment type fix". This corresponds to this sort of
operation
> r <- as.raw(TRUE)
> x <- TRUE
> x[1] <- r
Error in x[1] <- r : incompatible types (from raw to logical) in
subassignment type fix
Now, notice that when the coercion is "bigger to smaller" the fix is
to coerce on the left side
What do you mean by "coercion from bigger to smaller"? What I thought
should happen in the example above is to have x coerced upwards to logical.
> y <- rep(TRUE,5)
> y[1] <- 5
> y
[1] 5 1 1 1 1
So the thing that might be expected here would be that x be coerced to
raw before the assignment,
No, I would expect upwards coercion, just as in the example immediately
above. I would expect r to be coerced to logical.
but this is not implemented. I.e. you could have had the effect of.
> x <- as.raw(x)
> x[1] <- r
> x
[1] 01
No, I would rather expect to have the effect of:
> x[1] <- as.logical(r)
This is designed not to happen automatically. I didn't write the code
but I suppose one possible reason is that the raw mode does not allow
missing values (or negative numbers for that matter, everything is
0-255).
It also cuts the other way - no subassignment fixes are defined for
"raw":
> x <- as.raw(1:10)
> x[3] <- NA
Error in x[3] <- NA : incompatible types (from logical to raw) in
subassignment type fix
> x[3] <- TRUE
Error in x[3] <- TRUE : incompatible types (from logical to raw) in
subassignment type fix
> x[3] <- 4
Error in x[3] <- 4 : incompatible types (from double to raw) in
subassignment type fix
> x[3] <- 4L
Error in x[3] <- 4L : incompatible types (from integer to raw) in
subassignment type fix
I don't understand any of these. If you can do the following:
> as.raw(TRUE) => 1
> as.raw(4L) => 04
> as.raw(-1) => 00, with a warning
> as.raw(NA) => 00, with a warning
Why can't this be done automatically. What would be wrong in having this
done automatically, with a warning if needed?
and raw can even be used as the condition vector in ifelse:
> ifelse(r, 1, 2) => 1
Yes, but that's a different issue. Ifelse has the C language
convention that anything nonzero is TRUE.
> ifelse(pi,2,1)
[1] 2
That's a news for me. The man page does not explain that! It says
"test: an object which can be coerced to logical mode" and also "Missing
values in test give missing values in the result". If ifelse were
working the C way, I would expect 1 as the result here:
> ifelse(as.raw(1), 1, 2) => error
> as.raw(1) == 0 => FALSE
The raw 1 is nonzero.
Best regards,
Craig
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