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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. The following operators can be used in the expression: +, -, /, *, ^, sin, cos, tan, asin, acos, atan, sinh, cosh, tanh, asinh, acosh, atanh, log, ln, exp, sqrt, abs, pow. "pi" in lowercase letters is recognised as a standard constant. This means that the user cannot use variables or coefficients with id "pi".
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 database, 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).
Furthermore it is possible to use "if statements" in the expression. This can e.g. be used to get one output value if X is greater than 3 and get another output value if X is equal to or less than 3. For instance in the expression
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