Resolving Ambiguities
Note that an ambiguity can arise with wildcard characters since both “*” and “?” have multiple uses. The “*” character may be interpreted as either a multiplication operator or a wildcard character. The “?” character serves as both the single character wildcard and the pool cross section identifier.
Wildcard versus Multiplication
There is a potential for ambiguity in the use of the wildcard character “*”.
Suppose you have a workfile with the series X, X2, Y, XYA, XY2. There are then two interpretations of the wildcard expression “X*2”. The expression may be interpreted as an auto-series representing X multiplied by 2. Alternatively, the expression may be used as a wildcard expression, referring to the series X2 and XY2.
Note that there is only an ambiguity when the character is used in the middle of an expression, not when the wildcard character “*” is used at the beginning or end of an expression. EViews uses the following rules to determine the interpretation of ambiguous expressions:
• EViews first tests to see whether the expression represents a valid series expression. If so, the expression is treated as an auto-series. If it is not a valid series expression, then EViews will treat the “*” as a wildcard character. For example,
y*x
2*x
are interpreted as auto-series, while,
*x
x*a
are interpreted as wildcard expressions.
• You can force EViews to treat “*” as a wildcard by preceding the character with another “*”. Thus, expressions containing “**” will always be treated as wildcard expressions. For example, the expression:
x**2
unambiguously refers to all objects with names beginning with “X” and ending with “2”. Note that the use of “**” does not conflict with the EViews exponentiation operator “^”.
• You can instruct EViews to treat “*” as a series expression operator by enclosing the expression (or any subexpression) in parentheses. For example:
(y*x)
always refers to X times Y.
We strongly encourage you to resolve the ambiguity by using parentheses to denote series expressions, and double asterisks to denote wildcards (in the middle of expressions), whenever you create a group. This is especially true when group creation occurs in a program; otherwise the behavior of the program will be difficult to predict since it will change as the names of other objects in the workfile change.