# Parameter Editor

Model parameters are constants that you can use in the expressions for the species initial amounts and the reaction propensities. The `ID` field is required, but the name is optional (and hidden by default). For the value you can enter a number or any Python expression. You can use the standard mathematical functions and constants as well as other parameters to define the values. You will get an error message if the parameters cannot be evaluated. Below is an example of a valid set of parameters.

    ID        Value        Name
    -----------------------------
    R         sqrt(10)     Radius
    Area      pi * R**2
    Volume    H * Area
    H         5.5          Height

It is permitted to use the mathematical constants `pi` and `e` as parameter identifiers. In this case, the values you assign to them will override their natural values. You may also use the names of Python built-in functions and math functions as parameter identifiers. However, to avoid confusion it best to avoid such names. The following set of parameters are valid, but misleading. Below `lambda` has the value `cos(6)`.

    ID        Value
    ---------------
    pi        3
    e         2
    sin       pi * e
    lambda    cos(sin)