Built-in functions : rand_const function

rand_const function

rand_const(X,Y)

Returns a random number between X and Y at the start of the simulation or when the submodel instance is created. The random-number generator is not called again, and so the value stays the same until the simulation is reset.

Input: numeric, numeric

Result: numeric

Comment:

The main use of this function is to assign values to a set of instances of a multiple-instance submodel (fixed-membership or population). For example, to randomly assign initial sizes to a set of trees in a multiple-instance tree submodel, we could use the equation:

size = rand_const(12,20)

or to assign random locations to the trees, we could use the equations:

x = rand_const(0,50)

y = rand_const(0,100)

which would randomly place the trees in the left-hand half of a one-hectare plot, assuming that the values are in metres.

The use of rand_const() is deprecated because it cannot be made to behave in the same way as rand_var when implicitly replicating over an array. It is implemented by internal conversion to at_init(rand_var(x,y)) and this form should be used in full to make the replicatio behaviour clear.

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 also 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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