# Built-in functions : dt function

dt function

dt(I)

Returns the duration of the level I time step.

Input: numeric. This is the time step level.

Result: numeric

Examples:

dt(1) --> 0.1 (for a model whose top-level time step was set to 0.1 in the Run Control panel)

dt(2) --> 0.001 (for a model whose 2nd-level time step was set to 0.001 in the Run Control panel)

If you are starting off with Simile, it is unlikely you will need to understand the concept of "time step index". You will probably just be making models with a single time step, hence one level, so don't worry about anything except the use of dt(1).

The main use of the dt function is to engineer the addition or removal of a specified amount of a substance into or out of a compartment. The only handle we have for causing changes to the amount in a compartment are flows, and flows are expressed as a rate per unit of time (whatever time unit is used for the model, e.g. year). This creates a problem if we want to add or remove a specified amount of substance at some instant in time. For example, consider a model with a time unit of year, a time step of 0.1, and with a compartment X from which we want to remove 5 units at the instant that some condition, which only lasts for 1 time step (0.1 years), is met. If we simply had a flow out that was zero when the condition was not met, and was 5 when the condition was met, then for one time step the flow would be 5 (units per year): hence, only 0.5 units would be removed in the 1/10th of a year, not the 5 we intended. What we need to do is to artificially inflate the flow rate by a factor of 10 (in this case, with a time step of 0.1) for this one time step. We do this by dividing the flow rate (in our flow equation) by 0.1 (in this case), or by dt(1) in general. The flow rate then appears to be 50 units per year for that one time step, giving a loss of 5 units in the one time step. Bingo!

The actual flow expression for the case considered above would be:

flowout = 5/dt(1)

Special case

Using the argument zero in the function, i.e. dt(0), is equivalent to saying dt(n), where n is the time step index of the submodel in which the function is used. This is useful, because when a submodel time step index is changed, it is then not necessary to edit the dt() functions within it to preserve the meaning.