[R] Sum of columns of a data frame equal to NA when all the elements are NA

[Boris Steipe]

Surely the result of summation of non-existent values is not defined, is it not? And since the NA values have been _removed_, there's nothing left to sum over. In fact, pretending the the result in that case is zero would appear audacious, no?

Cheers,
Boris 





> On Mar 21, 2018, at 12:58 PM, Jeff Newmiller <jdnewmil at dcn.davis.ca.us> wrote:
> 
> What do you mean by "should not"?
> 
> NULL means "missing object" in R. The result of the sum function is always expected to be numeric... so NA_real or NA_integer could make sense as possible return values. But you cannot compute on NULL so no, that doesn't work. 
> 
> See the note under the "Value" section of ?sum as to why zero is returned when all inputs are removed.
> -- 
> Sent from my phone. Please excuse my brevity.
> 
> On March 21, 2018 9:03:29 AM PDT, Boris Steipe <boris.steipe at utoronto.ca> wrote:
>> Should not the result be NULL if you have removed the NA with
>> na.rm=TRUE ?
>> 
>> B.
>> 
>> 
>> 
>>> On Mar 21, 2018, at 11:44 AM, Stefano Sofia
>> <stefano.sofia at regione.marche.it> wrote:
>>> 
>>> Dear list users,
>>> let me ask you this trivial question. I worked on that for a long
>> time, by now.
>>> Suppose to have a data frame with NAs and to sum some columns with
>> rowSums:
>>> 
>>> df <- data.frame(A = runif(10), B = runif(10), C = rnorm(10))
>>> df[1, ] <- NA
>>> rowSums(df[ , which(names(df) %in% c("A","B"))], na.rm=T)
>>> 
>>> If all the elements of the selected columns are NA, rowSums returns 0
>> while I need NA.
>>> Is there an easy and efficient way to use rowSums within a function
>> like
>>> 
>>> function(x) ifelse(all(is.na(x)), as.numeric(NA), rowSums...)?
>>> 
>>> or an equivalent function?
>>> 
>>> Thank you for your help
>>> Stefano
>>> 
>>> 
>>> 
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