

   EEvvaalluuaattee aann ((UUnneevvaalluuaatteedd)) EExxpprreessssiioonn

        eval(expr, envir=sys.frame(sys.parent()),
           enclos=if(is.list(envir) || is.pairlist(envir))
                     sys.frame(sys.parent()))

        evalq(expr, envir, enclos)

   AArrgguummeennttss::

       expr: object of mode `expression' or an ``unevaluated
             expression''.

      envir: the `environment' in which `expr' is to be evalu-
             ated. May also be a list or an integer as in
             `sys.call'.

     enclos: Only relevant if `envir' is a list. Specifies the
             enclosure, i.e. where R looks for objects not
             found in `envir'.

   DDeessccrriippttiioonn::

        This function evaluates the expression `expr' argument
        in the environment specified by `envir' and returns the
        computed value.  If `envir' is not specified, then
        `sys.frame(sys.parent())', the environment where the
        call to `eval' was made is used. The `eval' form evalu-
        ates its first argument before passing it to the evalu-
        ator.  This allows you to assign complicated expres-
        sions to symbols and then evaluate them. The `evalq'
        form is equivalent to `eval(quote(expression), ...)'

   NNoottee::

        Due to the difference in scoping rules, there are some
        differences between R and S in this area. In particu-
        lar, the default enclosure in S is the global environ-
        ment.

        When evaluating expressions in dataframes that has been
        passed as argument to a function, the relevant enclo-
        sure is often the caller's environment, i.e. one needs
        `eval(x, data, sys.frame(sys.parent()))'.

   SSeeee AAllssoo::

        `expression', `quote', `sys.frame', `environment'.

   EExxaammpplleess::

        eval(2 ^ 2 ^ 3)
        mEx <- expression(2^2^3); mEx; 1 + eval(mEx)
        eval({ xx <- pi; xx^2}) ; xx

        a <- 3 ; aa <- 4 ; evalq(evalq(a+b+aa, list(a=1)), list(b=5)) # == 10
        a <- 3 ; aa <- 4 ; evalq(evalq(a+b+aa, 1), list(b=5))         # == 12

        ev <- function() {
             e1 <- sys.frame(sys.parent())
             ## Evaluate a in e1
             aa <- eval(expression(a),e1)
             ## evaluate the expression bound to a in e1
             a <- expression(x+y)
             list(aa = aa, eval = eval(a, e1))
        }
        tst.ev <- function(a = 7) { x <- pi; y <- 1; ev() }
        tst.ev()#-> aa : 7,  eval : 4.14

