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docs-mini/README.conditions
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docs-mini/README.conditions
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CONDITIONS IN FLIGHTGEAR PROPERTY FILES
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Written by David Megginson, david@megginson.com
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Last modified: $Date$
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This document is in the Public Domain and comes with NO WARRANTY!
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1. Introduction
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---------------
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Some FlightGear property files contain conditions, affecting whether
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bindings or animations are applied. For example, the following
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binding will apply only when the /sim/input/selected/engine[0]
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property is true:
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<binding>
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<condition>
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<property>/sim/input/selected/engine[0]</property>
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</condition>
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<command>property-assign</command>
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<property>/controls/starter[0]</property>
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<value type="bool">true</value>
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</binding>
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Conditions always occur within a property subtree named "condition",
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which is equivalent to an "and" condition.
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2. Comparison Operators
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-----------------------
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The simplest condition is "property". It resolves as true when the
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specified property has a boolean value of true (i.e. non-zero, etc.)
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and false otherwise. Here is an example:
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<condition>
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<property>/sim/input/selected/engine[0]</property>
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</condition>
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For more sophisticated tests, you can use the "less-than",
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"less-than-equals", "greater-than", "greater-than-equals", "equals",
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and "not-equals" comparison operators. These all take two operands,
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either two "property" operands or one "property" and one "value"
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operand, and return true or false depending on the result of the
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comparison. The value of the second operand is always forced to the
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type of the first; for example, if you compare a string and a double,
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the double will be forced to a string and lexically compared. If one
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of the operands is a property, it is always assumed to be first. Here
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is an example of a comparison that is true only if the RPM of the
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engine is less than 1500:
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<condition>
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<less-than>
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<property>/engines/engine[0]/rpm</property>
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<value>1500</value>
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</less-than>
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</condition>
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3. Boolean Operators
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--------------------
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Finally, there are the regular boolean operators "and", "or", and
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"not". Each one surrounds a group of other conditions, and these can
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be nested to arbitrary depths. Here is an example:
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<condition>
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<and>
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<or>
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<less-than>
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<property>/engines/engine[0]/rpm</property>
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<value>1500</value>
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</less-than>
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<greater-than>
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<property>/engines/engine[0]/rpm</property>
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<value>2500</value>
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</greater-than>
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<or>
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<property>/engines/engine[0]/running</property>
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</and>
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</condition>
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The top-level "condition" is an implicit "and".
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4. Approximating if...else
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--------------------------
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There is no equivalent to the regular programming 'else' statement in
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FlightGear conditions; instead, each condition separately must take
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the others into account. For example, the equivalent of
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if (x == 3) ... else if (y == 5) ... else ...
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in FlightGear conditions is
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<condition>
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<equals>
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<property>/x</property>
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<value>3</value>
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</equals>
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<not>
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<equals>
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<property>/y</property>
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<value>5</value>
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</not>
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</condition>
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and then
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<condition>
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<equals>
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<property>/y</property>
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<value>5</value>
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</equals>
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<not>
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<equals>
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<property>/x</property>
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<value>3</value>
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</not>
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</condition>
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and then
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<condition>
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<not>
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<equals>
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<property>/x</property>
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<value>3</value>
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</equals>
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</not>
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<not>
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<equals>
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<property>/y</property>
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<value>5</value>
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</not>
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</condition>
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It's verbose, but it works nicely within existing property-based
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formats and provides a lot of flexiblity.
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5. Syntax Summary
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-----------------
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Here's a quick syntax summary:
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* <and>...</and>
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Contains one or more subconditions, all of which must be true.
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* <condition>...</condition>
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The top-level container for conditions, equivalent to an "and" group
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* <equals>...</equals>
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Contains two properties or a property and value, and is true if the
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properties have equivalent values.
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* <greater-than>...</greater-than>
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Contains two properties or a property and a value, and is true if
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the second property or the value has a value greater than the first
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property.
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* <greater-than-equals>...</greater-than-equals>
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Contains two properties or a property and a value, and is true if
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the second property or the value has a value greater than or equal
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to the first property.
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* <less-than>...</less-than>
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Contains two properties or a property and a value, and is true if
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the second property or the value has a value less than the first
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property.
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* <less-than-equals>...</less-than-equals>
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Contains two properties or a property and a value, and is true if
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the second property or the value has a value less than or equal
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to the first property.
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* <not>...</not>
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Contains one subcondition, which must not be true.
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* <not-equals>...</not-equals>
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Contains two properties or a property and value, and is true if the
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properties do not have equivalent values.
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* <or>...</or>
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Contains one or more subconditions, at least one of which must be
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true.
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* <property>...</property>
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The name of a property to test.
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* <value>...</value>
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A literal value in a comparison.
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