Thanks for your help Petr

I think I understand better now.


> > > Masechaba$unique[which(is.na(unique(Masechaba$PROPDESC))==FALSE)]=TRUE
>                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> This seems to be strange. At first sight I am puzzlet what result I shall
> expect from such construction.
>
> About your comment above: I looked at the construction again. This was
absolutely unsuited to what I actually had in mind and based on a wrong
understanding of how unique() functions. When one uses is.na(unique(x)) the
answer is FALSE as long a there is something unique - which there will
always be as long as there are any values. It therefore produces a list of
FALSE the same length as the result of unique. which() gives the indices
namely 1:length(unique(x))

The desired result (a true or false value indicating whether a variable is a
duplicate or not is obtained by

Masechaba$unique=duplicated(match(Masechaba$PROPDESC,unique(Masechaba$PROPDESC)))

Thanks again for your help

regards
Christiaan

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