5. Define Flows by Case: activate if Block versus if/then/else Block

Activate if Introduction

Consider the same previous models where the if/then/else blocks are replaced by an activate if block.

Consider the DisplayTempC function introduced in the previous chapter.

Figure 1. DisplayTempC

The activate if block returns the output flows of the branch that is only active, that is say, where the Boolean activation condition is true.

When a branch is active, the equations it contains are evaluated, they are not evaluated otherwise.

Above, when selectC = true, the then branch (first one) is only active, tempOut = ToCelsius(temp) and so tempC is produced. When selectC = false, the else branch is only active, tempOut = temp and tempC is not produced.

For this model to display only either °C or °F temperatures, the activate if block is relevant.

In the same way, when a branch is active, the memories it contains are updated, they are not updated otherwise. Hence, for example the pre operator will return the value produced in the step when the branch it belongs to was previously active.

Consider the DisplayTempAct node modeled with the activate if block:

Figure 2. DisplayTempAct Node

A Swan model must be complete, hence the output flows from an operator instance are connected by wires to a terminator _ when they are not connected to outputs of the root operator as above: for example, tempC is not produced and so connected to _ in the else branch.

For example, in the step 5, selectC = true and maxTempInD = 30.0 and not 100.0, as in the previous cases with the if/then/else construct. This 30.0 value is the last maxTempC value returned in the step 2, the previous time where the first branch is active.

This is because the first branch (like the second branch) is only evaluated when the activation condition of the branch is true. So maxTempC is not evaluated in the first branch in the steps 3 and 4 where selectC = false.

Then maxTempC is compared with the temperature value of the current step to produce the current maximum temperature.

Note that activate if is not relevant to model the display of the °C or °F maximum temperatures, the computation of the maximum temperature must be independent of the active branch and so of the temperature unit.

elsif Branch Introduction

Now, you want to count the following times:

• timeAboveOut, a time during which the difference between the indoor and outdoor temperatures is greater than a threshold. To count this time, you only want to activate a counter when this difference is above the threshold. This time is computed by the TimeCounter operator introduced in the 2. Add Memory chapter.
• timeBelowOut, a time dunring which this previous difference is below the threshold.
• Otherwise, timeAboveOut = timeBelowOut = 0.

The activate if block can optionally contain one or several elsif branches. Only one single branch is active in each step: if several branch conditions are true in the same step, only the first branch from the top of this block whose condition is true is only activated.

Consider the following design of the ThresholdDetection operator. Assume that:

• CONST_THRESHOLD_F = 4.0
• CONST_TIME_CONV = 1

Figure 3. ThresholdDetection Node

The elsif branch becomes active only when tempdF - temp > CONST_THRESHOLD_F.