Why does Ifelse fail to replace NAs?

Question

I have a dataset where one column contains entries of yes, no, and NA. I want to replace any NA with 1, and replace any non-NA entry with 0. Ifelse replaces the non-NA entries with 0, but does NOT replace the NA entries with 1. I need to use the is.na() command for that. Why does is.na() work where ifelse does not?

I define a reproducible example below that starts with the column defined as a factor since that's how I got the data.

    q <-as.factor(c(NA, "yes",  "no",   "yes", NA))

    ## Does not work
    q <- ifelse(q == "NA", 1, 0)
q    
### Returns: [1] NA  0  0  0 NA

    ## Does not work
    q[q == "NA"] <- 1
q    
### Returns: [1] NA  0  0  0 NA    

    ## This works
    q[is.na(q)] <- 1
q
### Returns: [1] 1 0 0 0 1

Some other entries exist, but they do not seem to have this precise problem. https://stackoverflow.com/a/8166616/1364839 -- This answer shows that is.na() works but not why ifelse fails.

There's nothing wrong with `ifelse`. You are doing two completely different logical comparisons. The string "NA" has nothing to do with `NA`. — joran, Jun 18 '13 at 15:00
`> q <- ifelse(q == NA, 1, 0)` ...Outputs, `[1] NA NA NA NA NA` — Dr. Beeblebrox, Jun 18 '13 at 15:06
You cannot do any comparisons with NA value, any comparison with NA will result in NA. Try this: `c(NA=="yes",NA==NA,NA=="NA",NA==1,is.na(NA))` — zx8754, Jun 18 '13 at 15:06
@DTRM I updated the comment - I am in need of coffee, look at `?is.na`, in fact, instead of `ifelse` just do `as.integer( is.na( q ) )` - UPDATE - oh, that is what you are doing. So what is this question?!! — Simon O'Hanlon, Jun 18 '13 at 15:09
Stackoverflow etiquette question: now that you two cleared this up for me, should I click "Answer Your Question," refer to you two, and then write up the main points? — Dr. Beeblebrox, Jun 18 '13 at 15:14

score 4 · Accepted Answer · answered Jun 18 '13 at 15:19

You really don't need ifelse() here, not least because if you don't know the value of something (which is what NA indicates!) how can you compare its value with something else?

> NA == NA ## yes, even NA can't be compared with itself
[1] NA

Instead, use is.na() to identify whether something is NA or not. is.na() returns TRUE if an element is NA and FALSE otherwise. Then we can use the fact that FALSE == 0 and TRUE == 1 when we coerce to numeric:

q <-as.factor(c(NA, "yes",  "no",   "yes", NA))
q

as.numeric(is.na(q))

> as.numeric(is.na(q))
[1] 1 0 0 0 1

If that is too much typing then

> is.na(q) + 0
[1] 1 0 0 0 1

works via the same trick except + is doing the coercion for you.

Great thanks! Also, @SimonO101 above pointed out `ifelse( is.na(q) , 1, 0)` as yet another approach. — Dr. Beeblebrox, Jun 18 '13 at 15:37
@DTRM Not really, if you look at the guts of `ifelse` you'll see it is overkill for something like this. More generally useful perhaps yes, but not in regard to your question. — Gavin Simpson, Jun 18 '13 at 15:44

Why does Ifelse fail to replace NAs?

1 Answers1