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

David Winsemius dwinsemius at comcast.net
Wed Mar 21 18:57:27 CET 2018

```> On Mar 21, 2018, at 9:58 AM, 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.

But your perspective above that sentence is at odds with the note in the Details section of the same document:

> sum(NULL)
[1] 0
> ?sum
starting httpd help server ... done
> sum( integer(0) )
[1] 0

Best;
David

> --
> 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
>>>
>>>
>>>
>>>        (oo)
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>
> ______________________________________________
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> and provide commented, minimal, self-contained, reproducible code.

David Winsemius
Alameda, CA, USA

'Any technology distinguishable from magic is insufficiently advanced.'   -Gehm's Corollary to Clarke's Third Law

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