

   CCoonnddiittiioonnaall EElleemmeenntt SSeelleeccttiioonn

        ifelse(test, yes, no)

   DDeessccrriippttiioonn::

        `ifelse' returns a value with the same shape as `test'
        which is filled with elements selected from either
        `yes' or `no' depending on whether the element of
        `test' is `TRUE' or `FALSE'.  If `yes' or `no' are too
        short, their elements are recycled.

   SSeeee AAllssoo::

        `if'.

   EExxaammpplleess::

        x <- c(6:-4)
        sqrt(x)#- gives warning
        sqrt(ifelse(x >= 0, x, NA))# no warning

        ## Note: the following also gives the warning !
        ifelse(x >= 0, sqrt(x), NA)

