Modelica.Utilities.Examples

Information

This package contains quite involved examples that demonstrate how to use the functions of package Modelica.Utilities. In particular the following examples are present.

• Function calculator is an interpreter to evaluate expressions consisting of +,-,*,/,(),sin(), cos(), tan(), sqrt(), pi. For example: calculator("1.5*sin(pi/6)");
   
• Function expression is the basic function used in "calculator" to evaluate an expression. It is useful if the expression interpreter is used in a larger scan operation (such as readRealParameter below).
   
• Function readRealParameter reads the value of a parameter from file, given the file and the parameter name. The value on file is interpreted with the Examples.expression function and can therefore be an expression.
   
• Model readRealParameterModel is a test model to demonstrate the usage of "readRealParameter". The model contains 3 parameters that are read from file "Modelica.Utilities/data/Examples_readRealParameters.txt".

Package Content

Name	Description
calculator	Interpreter to evaluate simple expressions consisting of +,-,*,/,(),sin(), cos(), tan(), sqrt(), pi
expression	Expression interpreter that returns with the position after the expression (expression may consist of +,-,*,/,(),sin(), cos(), tan(), sqrt(), pi
readRealParameter	Read the value of a Real parameter from file
readRealParameterModel	Demonstrate usage of Examples.readRealParameter/.expression

Modelica.Utilities.Examples.calculator

Information

Syntax

result = calculator(expression);

Description

This function demonstrates how a simple expression calculator can be implemented in form of a recursive decent parser using basically the Strings.scanToken(..) and Strings.scanDelimiter(..) function.

The following operations are supported (pi=3.14.. is a predefined constant):

   +, -
   *, /
   (expression)
   sin(expression)
   cos(expression)
   tan(expression)
   sqrt(expression)
   pi

Example

  calculator("2+3*(4-1)");  // returns 11
  calculator("sin(pi/6)");  // returns 0.5

Inputs

Name	Description
string	Expression that is evaluated

Outputs

Name	Description
result	Value of expression

Modelica.Utilities.Examples.expression

Information

Syntax

             result = expression(string);
(result, nextIndex) = expression(string, startIndex=1, message="");

Description

This function is nearly the same as Examples.calculator. The essential difference is that function "expression" might be used in other parsing operations: After the expression is parsed and evaluated, the function returns with the value of the expression as well as the position of the character directly after the expression.

This function demonstrates how a simple expression calculator can be implemented in form of a recursive decent parser using basically the Strings.scanToken(..) and scanDelimiters(..) function. There are 2 local functions (term, primary) that implement the corresponding part of the grammar.

The following operations are supported (pi=3.14.. is a predefined constant):

   +, -
   *, /
   (expression)
   sin(expression)
   cos(expression)
   tan(expression)
   sqrt(expression)
   pi

The optional argument "startIndex" defines at which position scanning of the expression starts.

In case of error, the optional argument "message" is appended to the error message, in order to give additional information where the error occurred.

This function parses the following grammar

  expression: [ add_op ] term { add_op term }
  add_op    : "+" | "-"
  term      : primary { mul_op primary }
  mul_op    : "*" | "/"
  primary   : UNSIGNED_NUMBER
              | pi
              | ( expression )
              | functionName( expression )
  function  :   sin
              | cos
              | tan
              | sqrt

Note, in Examples.readRealParameter it is shown, how the expression function can be used as part of another scan operation.

Example

  expression("2+3*(4-1)");  // returns 11
  expression("sin(pi/6)");  // returns 0.5

Inputs

Name	Description
string	Expression that is evaluated
startIndex	Start scanning of expression at character startIndex
message	Message used in error message if scan is not successful

Outputs

Name	Description
result	Value of expression
nextIndex	Index after the scanned expression

Modelica.Utilities.Examples.readRealParameter

Information

Syntax

result = readRealParameter(fileName, name);

Description

This function demonstrates how a function can be implemented that reads the value of a parameter from file. The function performs the following actions:

1. It opens file "fileName" and reads the lines of the file.
2. In every line, Modelica line comments ("// ... end-of-line") are skipped
3. If a line consists of "name = expression" and the "name" in this line is identical to the second argument "name" of the function call, the expression calculator Examples.expression is used to evaluate the expression after the "=" character. The expression can optionally be terminated with a ";".
4. The result of the expression evaluation is returned as the value of the parameter "name".

Example

On file "test.txt" the following lines might be present:

// Motor data
J        = 2.3     // inertia
w_rel0   = 1.5*2;  // relative angular velocity
phi_rel0 = pi/3

The function returns the value "3.0" when called as:

readRealParameter("test.txt", "w_rel0")

Inputs

Name	Description
fileName	Name of file
name	Name of parameter

Outputs

Name	Description
result	Actual value of parameter on file

Modelica.Utilities.Examples.readRealParameterModel

Information

Model that shows the usage of Examples.readRealParameter and Examples.expression. The model has 3 parameters and the values of these parameters are read from a file.

Parameters

Name	Description
file	File on which data is present
J	Inertia [kg.m2]
phi_rel0	Relative angle [rad]
w_rel0	Relative angular velocity [rad/s]