In certain contexts it is possible to specify an expression returning more than a single value. This is done by simply specifying several subexpressions, separated by commas. The maximum allowed number of expressions is given by the particular context. E.g. the expression volau-volad, volad
provides two values, the auto volumes minus the additional volumes as first value and the additional volumes as second value.
A special case is the empty expression, i.e. an expression with no operands at all. Empty expression will always return a zero value as result.
While the expressions in Enif are much more powerful than those in EMME/2, they remain essentially compatible with EMME/2 expressions. All attributes, operators and intrinsic functions which are available in EMME/2 are also available in Enif.
Expressions are always stored as normal strings, which also implies that they can easily be saved to files and read back when needed. However, when expressions are actually used to provide values, their string representation is automatically compiled into an efficient internal RPN token list. Special cases, such as empty expressions, constants or expressions consisting of a single attribute are recognized as such, so that they can be handLED with less overhead. These features allow for a very efficient evaluation of the same expression for large numbers of network elements.
Since the user can enter and modify expressions at any time, it may happen that he or she enters an invalid expression. For this reason, expressions entered or modified interactively by the user are compiled immediately after the return key is pressed. If the expression is found to be invalid, the background of the expression field becomes red (or to be more precise: changes to a special color which is configurable in the user preferences) and the cu
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