zero weights in weighted.mean

I wonder about the last sentence in the Details section of the
documentation of 'weighted.mean':

"However, zero weights _are_ handled specially and the corresponding ?x?
values are omitted from the sum."

The return value of weighted.mean.default is

sum((x * w)[w != 0])/sum(w)

and indeed, it looks as if zero weights are getting special treatment,
but what is wrong with the alternative (equivalent?) expression

sum(x * w) / sum(w)?

Is it a good idea to remove zeros from a vector before applying 'sum' to
it? I don't think so. Anyway, the sentence in the documentation seems to
be uncalled for.

On 14/07/2021 6:00 a.m., G?ran Brostr?m wrote:

I wonder about the last sentence in the Details section of the
documentation of 'weighted.mean':

"However, zero weights _are_ handled specially and the corresponding ?x?
values are omitted from the sum."

The return value of weighted.mean.default is

sum((x * w)[w != 0])/sum(w)

and indeed, it looks as if zero weights are getting special treatment,
but what is wrong with the alternative (equivalent?) expression

sum(x * w) / sum(w)?

Is it a good idea to remove zeros from a vector before applying 'sum' to
it? I don't think so. Anyway, the sentence in the documentation seems to
be uncalled for.

Inf*0 is not zero.? Setting weights to zero on infinite observations (or 
NA, or NaN) will give different results in your two expressions.

Gentlemen,

At the risk of beating a dead horse, but in he spirit of learning
more about R, aren't the two expressions functionally the same? One
drops values where weight is zero. The other (in the case where we
and infinity * 0, something one would not expect to see in data) also
drops data as in R infinity * 0 = Nan. In either case the observation
would be dropped. I am certain I am missing something, but I don't
know what I am missing.

John

John David Sorkin M.D., Ph.D. Professor of Medicine University of
Maryland School of Medicine Associate Director for Biostatistics and
Informatics Baltimore VA Medical Center Geriatrics, Research,
Education, and Clinical Center Chief, Biostatistics and Informatics 
University of Maryland School of Medicine Division of Gerontology,
Geriatrics and Palliative Care Senior Statistician University of
Maryland Center for Vascular Research 10 North Greene Street GRECC
(BT/18/GR) Baltimore, MD 21201-1524 (Phone) 410-605-7119 (Fax)
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