Jpye at 20:05, 14 February 2012

2012-02-14T20:05:13Z

← Older revision		Revision as of 20:05, 14 February 2012
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	Similarly, area <math>{}= \pi*r^2</math> should be written		Similarly, area <math>{}= \pi*r^2</math> should be written
	<source>		<source lang=a4c>
	area = 1{PI} * r^2;		area = 1{PI} * r^2;
	</source>		</source>

Jpye: Created page with "The '''CONSTANT''' statement is used to declare constant-value 'variable' types that may be used as parameters for reusable parametric MODEL types. Real constants which d..."

2012-02-14T20:04:25Z

Created page with "The '''CONSTANT''' statement is used to declare constant-value 'variable' types that may be used as parameters for reusable parametric MODEL types. Real constants which d..."

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The '''CONSTANT''' statement is used to declare constant-value 'variable' types that may be used as parameters for reusable parametric [[MODEL]] types.

Real constants which do not have a default value are usually needed only in libraries of reusable models, such as components.a4l, where the values depend on the end-user’s selection from alternatives in a database. The standard incantation to define a new real constant type is:

<source lang=a4c>
CONSTANT critical_pressure_constant
REFINES real_constant DIMENSION M/L/T^2;
</source>

It is wasteful to define a CONSTANT type and a compiled object to represent a universal constant. For example, the thermodynamic gas constant, R = 8.314... {J/mole/K}, is frequently needed in modeling chemical systems. The SI value of R does not vary with its application. Neither does the value of <math>\pi</math>. Numeric constants of this sort are better represented as a numeric coefficient and an appropriately defined unit conversion, using the [[UNITS]] statement.

Consider the ideal gas law, PV = NRT and the ASCEND unit conversion {GAS_C} which appears in the library ascend4/models/measures.a4l. This equation should be written:

<source lang=a4c>
P * V = n * 1.0{GAS_C} * T;
</source>

Similarly, area <math>{}= \pi*r^2</math> should be written
<source>
area = 1{PI} * r^2;
</source>

The coefficient <tt>1</tt> of <tt>{GAS_C}</tt> and <tt>{PI}</tt> in these equations takes of the dimensionality of and is multiplied
by the conversion factor implied by the [[UNITS]] definition for the part within the braces <tt>{}</tt>. If we check {{src|models/measures.a4l}}, we
find the definition of PI is simply {3.14159...} and the definition of GAS_C is {8.314... J/mole/K} as we ought to expect.

For historical reasons there are a few universal constant definitions in {{src|models/atoms.a4l}}. New modelers should not use them; they are only provided to support outdated models that no one has yet taken the time to update.

== See also ==

* [[UNITS]]
* [[DIMENSION]]
* [[ATOM]]

[[Category:Syntax]]

CONSTANT - Revision history

Jpye at 20:05, 14 February 2012

Jpye: Created page with "The '''CONSTANT''' statement is used to declare constant-value 'variable' types that may be used as parameters for reusable parametric MODEL types. Real constants which d..."