# Re: [R] a strange behavior with ifelse

From: Joshua Wiley <jwiley.psych_at_gmail.com>
Date: Fri, 08 Apr 2011 12:34:17 -0700

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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> and provide commented, minimal, self-contained, reproducible code.
>

```--
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/

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