Scripting Guide - SAS

186 Programming Methods Chapter 8
Lists and Expressions
Table 8.1 Compare Eval, Name Expr, and Expr
(Continued)
Command and result
NameExpr(x);
a+b+c
Expr(x);
x
Explanation
Returns the expression that was stored in x, which is a+b+c (unpacks the
outside layer).
Returns the expression x (packs one layer).
JSL also supports functions to access and traverse expressions, all of them either a name or a literal
expression as an argument. In the following, expressionArg is either a single name or a compound
expression to be taken literally.
NArg(expressionArg) finds the number of arguments in expressionArg.
The expressionArg can be a name holding an expression, an expression evaluated to an expression, or a
literal expression quoted by Expr().
NArg (name) obtains the expression held in name (it is not evaluated) and returns the number of
arguments
NArg (expression) evaluates expression and returns the number of arguments
NArg (Expr(expression)) returns the number of arguments to literal expression.
For example, if aExpr = {a+b,c,d,e+f+g};
• NArg(aExpr) results in 4.
• NArg(Arg(aExpr,4)) results in 3.
• NArg(Expr({1,2,3,4})) results in 4.
Head(expressionArg) returns the head of the expression without any arguments. If the expression is an
infix, prefix, or postfix special character operator, then it is returned as the functional equivalent.
The expressionArg can be a name holding an expression, an expression evaluated to an expression, or a
literal expression quoted by Expr().
For example, if aExpr = expr(a+b);
• r = Head(aExpr) results in Add().
• r = Head (Expr(sqrt(r))) results in Sqrt().
• r = Head({1,2,3}) results in {} .
Arg(expressionArg,indexArg) extracts the specified argument of the symbolic expression, resulting in
an expression.
For example,
Arg(expressionArg,i) extracts the i th argument of expressionArg
The expressionArg can be a name holding an expression, an expression evaluated to an expression, or a
literal expression quoted by Expr().