[R] NA preserved in logical call - I don't understand this behavior because NA is not equal to 0

[David Winsemius]

On Jul 18, 2010, at 12:02 PM, stephen sefick wrote:

> I am confused by the behavior of the below piece of code.  The NAs are
> making it past the logical call ==0.  I am sure that I am missing
> something.  I just don't understand this behavior.  Thanks for your
> help in advance.
>
>
> ########code#######################################################
> left <- (structure(list(measurment_num = c(2, 2.2, 2.4, 2.6, 2.8,  
> 2.82,
> 3, NA, NA, NA), bankfull_depths_m = c(1.29, 1.28, 1.23, 0.18,
> -0.05, 0, -0.09, NA, NA, NA)), .Names = c("measurment_num",  
> "bankfull_depths_m"
> ), row.names = c(10L, 11L, 12L, 13L, 14L, 20L, 15L, 16L, 17L,
> 18L), class = "data.frame"))
>
> if(sum(left[,"bankfull_depths_m"]==0, na.rm=TRUE) == 1){
> left_min <- left[left[,"bankfull_depths_m"]==0, "measurment_num"]
> }
>
> left

Why aren't you looking at left_min? That is what the code is supposed  
to be changing. I do not get an error and left_min[1] (now) =='s  2.82.

Regarding why there are the 3 NA's, ... you need to look at the  
documentation for Extract in the section very appropriately entitled:
"NAs in indexing"

I have been bitten by that behavior more times than I care to admit  
and I am not the only person that thinks that aspect of the R language  
is badly designed. See Aniko Szabo's comments:

http://radfordneal.wordpress.com/2008/09/21/design-flaws-in-r-3-%E2%80%94-zero-subscripts/

The problem is in data.frame[ and any NA in a logical vector will  
return a row of NA's. This can be avoid by wrapping which() around the  
logical vector which seems entirely wasteful or using subset().


David Winsemius, MD
West Hartford, CT