Looping

for

One of the main uses of arrays is to allow code to be simplified through the use of loops. So we will conclude this chapter on arrays by introducing some basic looping constructs and showing how they are used in conjunction with array features.

In general, the for keyword is used to represent looping. But there are many different contexts in which for can be used. Several of the examples in this chapter used for to generate a collection of equations. When for is used within an equation section, any equations contained within the for loop are generated for each value of the loop index variables. In this way, we can easily generate many equations that have the same overall structure and only vary by the value of the loop index variable. The general syntax for a for loop in an equation section is:

equation
  for i in 1:n loop
    // equations
  end for;

Note that the loop index variable (e.g., i in this case) does not have to be declared. It is also worth noting that these variables only exists within the scope of the for loop (not before or after the loop).

For loops can, of course, be nested. For example:

equation
  for i in 1:n loop
    for j in 1:n loop
      // equation
    end for;
  end for;

They can also appear in other contexts. For example, they can appear in initial equation sections or in algorithm Sections.

Another case where the for keyword can be seen is in our discussion of Array Comprehensions. In that case, the for construct is not used to generate equations or statements, but to populate the various elements in an array.

while

There is another type of loop in Modelica and that is the while loop. The while loop is not used very often in Modelica. The reason is that Modelica, unlike a general purpose language, is an equation oriented language. Furthermore, it imposes a requirement that a model should include an equal number of equations and unknowns. Such a model is considered a “balanced model”.

The reason that the while construct is not widely used is because a balanced model requires that the number of equations is predictable (by the compiler). Because a for loop is bounded and the number of values of the index variable is always known (because it is always derived from a vector of possible values), the number of equations it will generate is always known. The same cannot be said of a while loop. As such, while loops are only practical in the context of algorithm Sections (typically in the definition of Functions).