Nitu, Laurentiu L@urent|u@N|tu @end|ng |rom |r@@erhe@|th@c@
Thu Apr 25 19:20:47 CEST 2019

```Thank you very much for your answer. Yes, meanwhile I found out …

Plus, that index that which() is returning  applied on the matrix is calculated as:

matrix[rowIndices + nrow(matrix) * (colIndices - 1)]

From: William Dunlap <wdunlap using tibco.com>
Sent: Thursday, April 25, 2019 10:03 AM
To: Nitu, Laurentiu <Laurentiu.Nitu using fraserhealth.ca>
Cc: r-help using r-project.org

is.na<http://is.na>(DF) is a matrix for a data.frame DF.  The semantics of '[" are different for matrices
and data.frame and that can cause confusion

> DF <- data.frame(X=c(101,NA,NA), Y=c("one","two",NA), row.names=c("i","ii","iii"))
> is.na<http://is.na>(DF) # returns a matrix when given a data.frame
X     Y
i   FALSE FALSE
ii   TRUE FALSE
iii  TRUE  TRUE
> which(is.na<http://is.na>(DF)) # returns a vector when given a data.frame
 2 3 6
> which(is.na<http://is.na>(DF), arr.ind=TRUE) # returns a length(dim(matrix))-column matrix when given an array
row col
ii    2   1
iii   3   1
iii   3   2
> DF[!is.na<http://is.na>(DF)] # as.matrix(DF)[ !is.na<http://is.na>(as.matrix(DF)) ]
 "101" "one" "two"

Bill Dunlap
TIBCO Software
wdunlap tibco.com<http://tibco.com>

On Thu, Apr 25, 2019 at 9:27 AM Nitu, Laurentiu <Laurentiu.Nitu using fraserhealth.ca<mailto:Laurentiu.Nitu using fraserhealth.ca>> wrote:
Hello,

I have this data frame [algae] in the package DMwR. I thought I understand how to refer an element but I cannot explain...

is.na<http://is.na>(algae) is giving us the a logical vector with TRUE being the na's.
which(is.na<http://is.na>(algae)) gives the positions on the elements in the data frame where is.na<http://is.na> returns TRUE.

However which(is.na<http://is.na>(algae)) returns

  648  838  862 1055 1056 1057 1058 1059 1060 1061 1062 1161 1199 1262 1399 1462 1599 1662 1799 1828 1999 2055 2056 2057 2058 2059 2060 2061 2062 2063
 2116 2184 2199

Weirs since:

> dim(algae)

 200  18

If I refer back algae[which(is.na<http://is.na>(algae))) I get a vector of NA's...

What are the values returned by which(is.na<http://is.na>(algae))?

Thanks a lot for your help

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