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For instance "X1 + X2 * 3" (without the quotes). In the expression input variables or coefficients can be referenced using their id, e.g. "X1 + a" where "X1" is the variableId of a variable defined elsewhere and "a" is the id of a coefficient defined in a coefficientSet. A variableId or coefficientId should not start with a numerical character and should not contain operators.
Operators
The following operators can be used in the expression: +, -, /, *, ^
Functions
Furthermore the following functions can be used in the expression: sin, cos, tan, asin, acos, atan, sinh, cosh, tanh, asinh, acosh, atanh, log, ln, exp, sqrt, abs, pow, max, min.
isMissing Function
Furthermore the function isMissing can be used to check if an input value is missing (NaN), e.g. for the expression "isMissing(X)" the result will be 1 when X is missing and the result will be 0 when X is not missing.
Furthermore "pi" in lowercase letters is recognised as the standard mathematical constant π. This means that the user cannot use variables or coefficients with id "pi".
Since stable build 2013.02 it is also possible to use the calculated output value of the previous time step, by using "PREVIOUS_OUTPUT_VALUE" in uppercase letters in the expression (can be used multiple times). For each calculation time step "PREVIOUS_OUTPUT_VALUE" is replaced with the previous output value from the output time series. For the first calculation time step in the run period, the previous output value from before the start of the run period is read from the existing output time series in the database (e.g. from a previous run), if present. If for a given calculation time step the previous output value is not present or is unreliable, then "PREVIOUS_OUTPUT_VALUE" is replaced with NaN (missing value).
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