Built-in functions : rand_var function

rand_var function

rand_var(X,Y)

Returns a random number between X and Y, with a new value every time step.

Input: numeric, numeric

Result: numeric

Comment:

This function is used for doing stochastic modelling and Monte-Carlo simulations, i.e. one or more processes in the model (like giving birth or dying) have a random element to them.

rand_var gives a new result for every call, and if it is used in an expression that is replicated to make an array, each element's random value will be different.

rand_var uses the pseudo-random sequence generator built into the c++ compiler which Simile is using to create executable models. The sequence is initialized with a value generated from the process ID and clock time when Simile starts up, so no two runs will produce the same results. However, if it is required that a model has exactly the same behaviour each time it runs, despite including calls to rand_var, this can be achieved by means of a tool that sets the seed to a given value; see Initializing pseudo-random sequence.

Historical note: You may come across some models that use a rand(X,Y) function. This behaves like rand_const if Simile deduces that the model element will only be called at initialisation time, and like rand_var if the equation contains some variable that changes over time. The use of this function is now deprecated because the semantics of the two uses are so very different. Also, there are some situations when you need to be able to over-ride this behind-the-scenes decision about how the function should behave.

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