[R] a strange behavior with ifelse

[Joshua Wiley]

Hi,

The problem is that (a == 1 | b == 1) returns some NAs.  NAs are not
treated as strictly TRUE or FALSE.  From the documentation (see
?ifelse) "Missing values in 'test' give missing values in the result".

You need to use a construct designed to force a TRUE/FALSE value OR
something that handles NAs (like is.na()).

HTH,

Josh

On Fri, Apr 8, 2011 at 10:56 AM, wgu <wgu at uab.edu> wrote:
> I have used R for years but run into a seemingly simple problem involving 'ifelse'.  condensed code like this
>
>  a=c(2,NA,NA,NA,2,2,NA,2,NA,2)
>  b=c(NA,1,1,NA,2,2,2,2,2,2)
>
> #I want to combined a and b into c so that c would be a valid number either a or b is not missing
>
> c=ifelse(a==1|b==1,1,ifelse(a==2|b==2,2,NA))
> cbind(a,b,c)
>
>          a  b  c
>  [1,]  2 NA NA
>  [2,] NA  1  1
>  [3,] NA  1  1
>  [4,] NA NA NA
>  [5,]  2  2  2
>  [6,]  2  2  2
>  [7,] NA  2 NA
>  [8,]  2  2  2
>  [9,] NA  2 NA
> [10,]  2  2  2
>
> look at rows 1, 7 and 9, c supposes to be 2 but got NA instead. seemingly, the second 'ifelse' doesn't perform as it supposes.
>
> my R version is 2.12.2
>
> any suggestion
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-- 
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/