Constants

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Constants

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Constants are symbolic representations of numerical quantities that do not change during or in between simulation runs. Constants must be defined in 20-sim, using the constants keyword. After the keyword, lines with constant declarations can be entered. Every line must be finished by a semicolon (;). An example is given below:

 

constants

 real V = 1 {volume, m3};                // the volume of barrel 1

 real hidden D = 0.5 {length, m};        // the length of part 1

 ...

 

Types

20-sim currently supports four types of constants: boolean, integer, real and string. These types must be specified explicitly. The use of the type real is shown in the example above.

Hidden

To prevent users from inspecting constants, the keyword hidden can be applied. This keyword should follow the data type.

Constant names

Constant names can consist of characters and underscores. A constant name must always start with a character. Constant names are case-sensitive. No reserved words may be used for constants. If a reserved word is used, an error message will pop-up while checking the model. In the example above the names V and D are used.

Constant Values

It is advised to assign every constant a value. Otherwise 20-sim will assign a default value (0, false or an empty string), which can easily lead to misinterpretations. Unlike parameter values, constant values cannot be overridden in the Simulator.

Quantity and Unit

If a constant is known to represent a physical value, you can define the corresponding quantity and unit. 20-sim will use this information to do a unit check on equations. The use of quantities and units is optional. In the example above the quantities volume and length are used with the units m3 and m.

Example

constants

 real pi = 3.14159265359;

 real two pi = 6.28318530718;

 integer i = 10 {length,m};                // vehicle length

 integer j = 1 {time,s};

 

constants

 string filename = 'data.txt';

 integer i = 1;

 real state = 0.0;

 boolean t = true, f = false;

Tip

You can easily enter constants by using the Add button Declarations in the Equation Editor Taskbar.
You can easily select quantities and units by using the Units button Units of the Equation Editor Taskbar.