Re: Using na.omit() on the output of na.omit()

I think this is Shu Fai's point... if lm checked whether an na.omit object were passed instead of just blindly applying na.omit again, then the information would not get lost. Having na.omit work cumulatively could also make this process flow more naturally... it doesn't feel obvious to me that repeatedly calling na.omit needs to lose the attribute.

On March 30, 2026 7:18:43 AM PDT, peter dalgaard <[email protected]> wrote:
>I'm fairly sure that this is deliberate, although hardly something that I bump into every day. na.omit & friends are designed to work in connection with modeling code and there may be code down the line that wants to know about omitted cases. So na.omit cleans the original data by removing some cases and attaching a list of omitted cases; if you do it again, you still have a dataframe and some omitted cases. 
>
>If anything, I would contend that omissions should be cumulative, so if you (say) add a column with some missing values and na.omit again, then the omission list should be augmented rather than replaced.
>
>Also, from a pragmatic viewpoint, there is a chance that na.omit gets called repeatedly by coincidence in the labyrinth of interrelated lm functions.
>
>-pd
>
>> On 27 Mar 2026, at 15.13, Shu Fai Cheung <[email protected]> wrote:
>> 
>> Hi All,
>> 
>> I noticed a behavior of na.omit() that I am not sure is intended.
>> 
>> This is adapted from the example of na.omit()
>> 
>>> DF <- data.frame(x = c(1, 2, 3), y = c(NA, 10, 1))
>>> DF_omitted <- na.omit(DF)
>>> attr(DF_omitted, "na.action")
>> 1
>> 1
>> attr(,"class")
>> [1] "omit"
>>> DF_omitted2 <- na.omit(DF_omitted)
>>> attr(DF_omitted2, "na.action")
>> 1
>> 1
>> attr(,"class")
>> [1] "omit"
>> 
>> In the first call to na.omit(), the output, DF_omitted, correctly has only
>> two rows, with Row 1 removed, and 'na.action' stores the removed row number.
>> 
>> In the second call of na.omit(), no cases are removed because DF_omitted
>> has no missing data. However, the attribute 'na.action' is retained,
>> indicating that Row 1 was removed.
>> 
>> To my understanding of na.omit(), this occurred because, in the second call
>> of na.omit(), no rows were omitted, and so the original object was
>> returned, along with the attribute 'na.action' from the first call.
>> 
>> From the "perspective" of the second call, no rows were omitted.
>> Should 'na.action'
>> be NULL (i.e., not set), as in the following example?
>> 
>>> DF2 <- data.frame(x = c(1, 2, 3), y = c(3, 10, 1))
>>> DF2_omitted <- na.omit(DF2)
>>> attr(DF2_omitted, "na.action")
>> NULL
>> 
>> Or is this behavior of na.omit() intended?
>> 
>> Regards,
>> Shu Fai
>> 
>> [[alternative HTML version deleted]]
>> 
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>
>-- 
>Peter Dalgaard, Professor,
>Center for Statistics, Copenhagen Business School
>Solbjerg Plads 3, 2000 Frederiksberg, Denmark
>Phone: (+45)38153501
>Office: A 4.23
>Email: [email protected]  Priv: [email protected]
>
>______________________________________________
>[email protected] mailing list -- To UNSUBSCRIBE and more, see
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>PLEASE do read the posting guide https://www.R-project.org/posting-guide.html
>and provide commented, minimal, self-contained, reproducible code.

--
Sent from my phone. Please excuse my brevity.
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