CODEWITHSUNDEEP
r
BurninLeo
BurninLeo

Reputation: 4474

Why can't I test which.max() against 0

I am trying to compare the result of which.max against 0 to check if any maximum value was found in the data. For explanation: There are about 24.000 vectors I must check for their maximum's index.

Here is a litte example:

tmp <- which.max(c(NA, NA, NA, NA))
str(tmp)
tmp == 0
as.integer(tmp) == 0
as.numeric(tmp) == 0

It results in FALSE, FALSE, and FALSE - although str(tmp) returns int(0).

What I did as workaround is this:

tmp <- which.max(c(NA, NA, NA, NA))
isTRUE(as.logical(tmp))

This works in differenetiation if which.max() found a maximum or not. However I do not understand why the above comparisons do not work.

Bonus question: Is there another function than str() that had easily shown me the tmp object's structure to instantly understand the comparison fail?

Thanks!

Upvotes: 1

Views: 878

Answers (3)

Tommy
Tommy

Reputation: 40861

It would probably be better if which.max returned 0 or -1 when no max value was found. Then you could more easily check it. regexpr for instance returns -1 and match allows you to specify what is returned in the case of no match (which this arguably is similar to).

...but you can check the return value's length yourself: 

x <-  which.max(c(NA, NA, NA, NA))
if(length(x)) {
  # Do stuff with x...
}

...or wrap it into a function to simplify things:

which.max2 <- function(x) {
    x <-  which.max(x)
    if(length(x)) x else 0L
}

which.max2(c(NA,-Inf,NA)) # 2
which.max2(c(NA,NA))      # 0

Upvotes: 2

Joshua Ulrich
Joshua Ulrich

Reputation: 176698

int(0) means it's a zero-length vector (i.e. it has no elements, it's empty, etc.). Even if you do something like tmp == integer(0) you get a zero-length logical vector, not TRUE or FALSE.

According to ?which.max:

Value:

 Missing and ‘NaN’ values are discarded.

 an ‘integer’ of length 1 or 0 (iff ‘x’ has no non-‘NA’s), giving
 the index of the _first_ minimum or maximum respectively of ‘x’.

So you want to check to see whether length(tmp) > 0.

Upvotes: 5

David Robinson
David Robinson

Reputation: 78620

You can whether tmp is numeric(0) as:

length(tmp) == 0

Upvotes: 3

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