Unevaluated Expressions, ‘expr’ Description Examples Description Enclosing an expression with right single quotes ‘ ‘ will delay the evaluation of the expression. As another example, help(Digits) returns a message stating that the argument to the help…
Evaluate an ( Unevaluated) Expression Description. Evaluate an R expression in a specified environment. Usage eval(expr, envir = parent.frame(), enclos = if(is.list(envir) || is.pairlist(envir)) parent.frame() else baseenv()) evalq(expr, envir, enclos) eval.parent(expr, n = 1) local(expr, envir = new.env()) Arguments, Unevaluated[expr] represents the unevaluated form of expr when it appears as the argument to a function.
expr : object of mode expression or call or an “ unevaluated expression”.: envir: the environment in which expr is to be evaluated. May also be, NULL, a list, a data frame, or an integer as in sys.call. enclos: Relevant when envir is a list or a data frame. Specifies the enclosure, i.e.
where R looks for objects not found in envir.: n: parent generations to go back, The standard term is an unevaluated operand and you can find it in [expr] In some contexts, unevaluated operands appear (5.2.8, 5.3.3, 5.3.7, 7.1.6.2). An unevaluated operand is not evaluated. An unevaluated operand is considered a full-expression.
I do not understand why Unevaluated sometimes does not work. Consider this expression: expr = Hold[a – a] Now let’s say I want to get the length of a-a which is 2. Then I would expect Length[ Unevaluated @@ expr ] to give 2. But instead I get. Length[ Unevaluated @@ expr ] (*1*) which suggests Length is failing to strip away the Unevaluated . Why?, Expr (x) Returns the argument unevaluated (expression-quoting). NameExpr. NameExpr(x) Returns the unevaluated expression of x rather than the evaluation of x. NameExpr is like Expr except that if x is a name, NameExpr returns the unevaluated expression stored in the name rather than the unevaluated name x. Num.
But if I make the Operator forms include Function[…
HoldFirst] and pass-on Unevaluated[expr ] to downstream functions, then Replace/ReplacePart will just receive a Symbol, not the expression it contains. Still thinking this part through…
Details. These functions turn unevaluated expressions (where ‘expression’is taken in a wider sense than the strict concept of a vector ofmodeand type (typeof)expressionused in expression) into characterstrings (a kind of inverse to parse). A typical use of this is to create informative labels for data setsand plots.
