Scripting Guide - SAS

Chapter 8 Programming Methods 187
Lists and Expressions
• Arg(name,i) obtains the expression held in name (it is not evaluated) and finds the ith argument
• Arg(expression,i) evaluates expression and finds the i th argument
• Arg(Expr(expression),i) finds the i th argument of expression
As another example, if aExpr = Expr(12+13*sqrt(14-15));
• Arg(aExpr,1) yields 12
• Arg(aExpr,2) yields 13*sqrt(14-15)
• Arg(Expr(12+13*sqrt(14-15)),2) yields 13*sqrt(14-15)
To extract an argument of an argument inside an expression, you can nest Arg commands:
• Arg(Arg(aExpr,2),1) yields the first argument within the second argument of aExpr, or 13.
• Arg(Arg(aExpr,2),2) yields Sqrt( 14 - 15 )
• Arg(Arg(Arg(aExpr,2),2),1) yields 14 - 15
• Arg(Arg(Arg(aExpr,2),2),3) yields Empty()
Here is a description of how the last example line unwraps itself:
1. The inner Arg statement is evaluated.
Arg(aExpr,2)
13 * Sqrt( 14 - 15 )
2. Then the next one is evaluated.
Arg(Arg(aExpr,2),2)
// this is equivalent to Arg(Expr (13 * Sqrt( 14 - 15 ) ), 2)
Sqrt( 14 - 15 )
3. Finally, the outer Arg is evaluated.
Arg(Arg(Arg(aExpr,2),2),3)
// this is equivalent to Arg (Expr (Sqrt( 14 - 15 ) ), 3)
Empty()
There is only one element to the Sqrt expression, so a request for the third argument yields Empty(). To
access the two arguments inside the Sqrt expression, try this:
Arg(Arg(Arg(Arg(aExpr,2),2),1),2);
15
HeadName(expressionArg) returns the name of the head of the expression as a string. If the expression
is an infix, prefix, postfix, or other 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 = HeadName (aExpr) results in "Add".
• r = HeadName (Expr(sqrt(r))) results in "Sqrt".
• r = HeadName ({1,2,3}) results in "List".