REVERSE POLISH NOTATION

The Microsoft Flight Simulator SDK has full support for the PostFix notation also known as Reverse Polish Notation (RPN). This is used in various places, such as Gauges and Model Behaviors.

In Reverse Polish Notation, the operators follow their operands - for instance, to add 3 and 4, one would write:

3 4 +

rather than:

3 + 4

If there are multiple operations, operators are given immediately after their second operands, so the conventional expression:

3 - 4 + 5

would be written like this in reverse Polish notation:

3 4 - 5 +

Essentially this means that 4 is first subtracted from 3, then 5 is added to it. An advantage of reverse Polish notation is that it removes the need for the parentheses () that are required by InFix notation (InFix notation is simply the name given to conventional notation for expressions such as these). Using the example above it could possibly have the following two interpretations:

3 - (4 * 5)
(3 - 4) * 5

Both those expressions will give quite different results, yet use the same values and operands. In Reverse Polish Notation this ambiguity is removed. The former example 3 - (4 * 5) of could be written:

3 4 5 * -

which unambiguously means 3 (4 5 *) - which in turn reduces to 3 20 - , which finally gives -17. On the other hand the second "version" could be written

3 4 - 5 *

which unambiguously means (3 4 -) 5 * which gives -5. This can be thought of as a series of stack operations, ie:

• When the RPN script parser comes across a value, it pushes it onto the top of the stack.
• When the script parser comes across an operator, it pops from the stack the number of operands that the operator works on (usually one or two values).
• Whatever value is left on top of the stack at the end of the execution is the result of the calculated expression.

RPN In Microsoft Flight Simulator

Reverse Polish Notation is not a programming language, it is simply a compiler-friendly way of creating complex logical expressions. However, it is used along with some built-in sim functions and variables to create short scripts that are then used to do different things within the sim. Typically these a scripts will use one or more expressions to define what a gauge or model does, and - in their simplest form - the expressions are the names of simulation parameters along with the units in which the element should be expressed - both enclosed in parentheses - followed by operands.

In the following example the expression returns the value of the NAV1 OBS parameter:

(A:NAV1 OBS, degrees)

The "A" before the colon indicates that this parameter is an Aircraft parameter. NAV1 OBS is a simulation variable (see the Simulation Variable document for a full list of variables) and degrees are the units of measure in which the aircraft parameter will display. Let's look at how this would be used along with Reverse Polish Notation to write a small script:

(A:NAV1 OBS, degrees) d (A:PARTIAL PANEL HEADING, bool) (A:PARTIAL PANEL ELECTRICAL, bool) or 0 == if{ (A:PLANE HEADING DEGREES GYRO, degrees) 90 - - } dgrd

This would translate as:

"If the PARTIAL PANEL HEADING is false and the PARTIAL PANEL ELECTRICAL is false, then this expression returns the NAV1 OBS reading minus the ( PLANE HEADING DEGREES GYRO reading minus 90), converted to radians."

Here is a full example of how Reverse Polish Notation is used along with the available parameters and operands within the XML for model behaviors:

<UseTemplate Name="Push_Button_With_Indicator_Template">
<ANIM_NAME>MID_Push_Isolate_Copilot#SUFFIX_ID#</ANIM_NAME>
<NODE_ID>MID_Push_Isolate_Copilot#SUFFIX_ID#</NODE_ID>
<INDICATOR_NODE_ID>MID_Push_Isolate_Copilot_Active#SUFFIX_ID#</INDICATOR_NODE_ID>
<SIMVAR_TO_WATCH_0>INTERCOM MODE</SIMVAR_TO_WATCH_0>
<SIMVAR_TO_WATCH_1>INTERCOM SYSTEM ACTIVE</SIMVAR_TO_WATCH_1>
<GET_STATE_EXTERNAL>(A:INTERCOM MODE, Enum) 0 &gt; (A:INTERCOM SYSTEM ACTIVE, Bool) and sp0</GET_STATE_EXTERNAL>
<SIM_STATE_IS_ON_EXTERNAL>l0</SIM_STATE_IS_ON_EXTERNAL>
<CHECK_STATE_HAS_CHANGED>True</CHECK_STATE_HAS_CHANGED>
<SET_STATE_EXTERNAL>
(&gt;H:#KEY_PREFIX#_Isolate_Copilot_Push)
(A:INTERCOM MODE, Enum) 2 == (A:INTERCOM SYSTEM ACTIVE, Bool) and if{ (&gt;K:TOGGLE_ICS) } els{
(A:INTERCOM SYSTEM ACTIVE, Bool) ! if{
(&gt;K:TOGGLE_ICS)
2 (&gt;K:INTERCOM_MODE_SET)
} els{
2 0 1 3 (A:INTERCOM MODE, Enum) case (&gt;K:INTERCOM_MODE_SET)
}
}
</SET_STATE_EXTERNAL>
<COND_INDICATOR_ACTIVE>(B:AS1000_MID_Isolate_Copilot, Bool)</COND_INDICATOR_ACTIVE>
<TT_DESCRIPTION_ID>@TT_Package.AS1000_MID_PUSH_ISOLATECO_ACTION</TT_DESCRIPTION_ID>
<TOOLTIP_TITLE>@TT_Package.AS1000_MID_PUSH_ISOLATECO_TITLE</TOOLTIP_TITLE>
<BTN_ID>MID_Isolate_Copilot</BTN_ID>
</UseTemplate>

IMPORTANT! When writing expressions in an XML file you need to use the markup &gt; and &lt; for the symbols > and <, otherwise you'll get an XML parsing error.

Variable Types

When using RPN to create expressions, you can use different variables, and these will require a prefix so the simulation can correctly identify their type and where they come from. The table below shows the different variable prefixes available to you.

Variable Prefix System Name Description Units
A
Simulation Variable Gets a specified SimVar from a simulation object. Yes
B
Input Events Gets the value of the specified input event (see Input Event Definitions for more information). No
C
Callback Variables This variable prefix is only used when dealing with GPS Variables. Yes
E
Environment Variable This is an environment variable. See the section on Environment Variables, below. Yes
F
Function Library This denotes a built in function from the function library. See the Function Library section below for more details. No
G
Gauge Variables Gets a variable that can be used to transfer unitless data between gauges. No
H
HTML Event An HTML event sent to the JavaScript. These are defined in cockpit panel Model Behaviors, and only go in one direction: from the panel to the JavaScript / HTML code. H: events are not required to be defined ahead of time (similar to L: vars) and can be named with any contiguous string of alphanumeric characters. Each cockpit panel in Microsoft Flight Simulator sends a number of cockpit specific H: events that have no analogous key event, such as pressing individual buttons on an FMS computer. These individual panel specific events can be received by JavaScript instruments. No
I
Instrument Variable Used for variables within components, where the variable scope is the component and its children. No
K
Key Event ID This is a specific variable for a key Event ID for user input.

(>K:TOGGLE_ICS)

Note that some key events require one or more values to be sent, so please see the section SimVars And Keys for more information on this.

No
L
Local Variable

Retrieves and/or creates a user defined local variable. If the local variable has not been defined in any of the associated files then it will be created and set to 0 the very first time it is referenced (and will not persist between runs). You can, however, define a default value for local variables using the following files:

• using the [LocalVars.N] section of the FLT file you can define a non-persistent local variable
• using the [LocalVars] section of the Systems Config file you can create a persistent local variable

This variable can be read and set within the scope of the user aircraft, and can be read by AI Aircraft.

IMPORTANT! "L:" vars can only hold numeric data and nothing else, eg: no strings, no binary values, no structs, etc...

No
M
Mouse Variable Gets the state of the mouse for use in mouse click handlers. Please see the Mouse Variables section below. No
O
Component Variable Used for variables within components, where the variable scope is the component itself. No
P
Program Variable Same as the Environment Variable E: Yes
R
Resource Variable This is used to retrieve a value from an external resource, which can either be a legacy Help ID or Tooltip ID, or something from a custom localization file. See Resource Variables for more details.
X
Calculator Variable This variable is used exclusively when creating Mission Definitions and is for referencing parameters created in the <CalculatorParameterList> element within an RPN calculation (inside a <CalculatorFormula>). No
W
Wwise Event This is a Wwise Event ID and allows you to trigger a Wwise event based on logic driven by the XML. This makes it more flexible than the sounds defined in sound.cfg and the AnimSoundEvents, although more complex to use. No
Z
Custom SimVar These are user-defined variables which are stored in an object's sim. The variable name is not one that has been predefined in the Microsoft Flight Simulator engine code, so anyone can create one with the name they want, as long as it doesn't conflict with an existing SimVar. No

Function Library

The RPN variable prefix F: can be used to access the following built-in functions:

• F:VarO - Set a component variable with the top stack string as the identifier
• F:VarI - Set an instrument variable with the top stack string as the identifier
• F:VarL - Set a local variable with the top stack string as the identifier
• F:VarA - Set a SimVar variable with the top stack string as the identifier
• F:KeyEvent - Call a key Event ID with the top stack string as the identifier and the rest of the stack as the parameters
• F:InputEvent - Call an InputEvent Preset with the top stack string as the identifier and the rest of the stack as the parameters
• F:Format - Format the top + 1 value using the described top stack format rules

To give an example of use, let's use an O: var, defined as:

(O:MyValue)

Now, to alter this value using the F: functions you would use the following to set the variable:

#A_VALUE# MyValue (&gtF:VarO)

And to get the variable:

MyValue (F:VarO)

To use F:Format to create dynamic runtime variables you would flag the value(s) to substitute using % (along with a stack operator, s in the following example):

'replacement' '1st' 'My %s string with a %s' (F:Format)

which would give:

"My 1st string with a replacement" (F:Format)

Mouse Variables

When using the M: identifier, you can check for any one of the following variables:

Variable Description
X

These will return the X/Y position of the mouse in two different ways depending on the InputType value:

• 0 - X and Y will be an absolute position in the screen coordinates.
• 1 - X and Y will represent the value of 2 axis so their values will be normalized between -1 to 1, and 0 will be the point at which the button was held down.
Y
RelativeX

These will return the relative X/Y position of the mouse in two different ways depending on the InputType value:

• 0 - the normalized X/Y position of the mouse relative to the anchor point (where it started the interaction when holding a button down).
• 1 - the relative X and Y will represent the value of 2 axis, where the 0 value is the point being clicked, and the distance from that point on each axis is calculated using the <DragScalar> value.
RelativeY
Event

This variable represents one of the following mouse Events:

• RightSingle
• MiddleSingle
• LeftSingle
• RightDouble
• MiddleDouble
• LeftDouble
• RightDrag
• MiddleDrag
• LeftDrag
• Move
• RightRelease
• MiddleRelease
• LeftRelease
• WheelUp
• WheelDown
• Leave
• Lock
• Unlock
DragPercent THis will be a value between 0 and 1 and is used for drag interactions when using the <DragMode>Trajectory</DragMode> setting in the mouse rect. This would be used to correspond to a position in time along an animation based on the cursor position, and works for most animation paths, curves, and lines but not for loops. It is ideal to use when working with levers, for example.
InputType THis is the input type for the interaction and can be either 0 or 1, where 0 would generally be considered the mouse and 1 would be considered the gamepad. It will affect the X/Y and RelativeX/RelativeY values.

Resource Variables

The resource variable R: can be used to retrieve a value from a resource file, and has two separate "modes":

• R:0 - This is to be used with localization IDs to extract a simple localized string from a file. It also supports legacy projects and systems that use the ToolTip ID (or Help ID) variables that don't require any dynamic elements, although these should not be used in new projects. For example:

(R:0:HELPID_EXTR_LOW_VOLT)

• R:1 - This is the way that all new projects should use the R: variable and is designed to permit you to extract a localized ID from a file, and can also create a dynamic tooltip based on an RPN stack expression. For example:

1 (R:1:@TT_Package.AUDIOPANEL_KNOB_COM_VOLUME_ACTION) (F:Format)

In this example the <Macro> @TT_Package would be the path to the localization file, while AUDIOPANEL_KNOB_COM_VOLUME_ACTION is the value in the file to retrieve, which in this case would be the string "Adjust COM %d volume". The expression then uses the F: variable to perform a substitution which will finally output the string "Adjust COM 1 volume".

Environment Variables

You can access the following environment variables using the E: identifier. Note that these variables are all read-only and cannot be set:

Variable Units Description
ABSOLUTE TIME Seconds This returns the seconds since 12:00 am 1/1/1AD Zulu Time See the Note On Zulu Time for more information.
ZULU TIME Seconds This returns the seconds since midnight (00:00 Zulu Time) on the current day.
ZULU DAY OF WEEK Number This returns the current Zulu Time day of the week as an integer value between 0 and 6 (inclusive), where 0 is Monday and 6 is Sunday. See the Note On Zulu Time for more information.
ZULU DAY OF MONTH Number This returns the current Zulu Time day of the month as an integer value between 1 and 31 (inclusive). See the Note On Zulu Time for more information.
ZULU MONTH OF YEAR Number This returns the current Zulu Time month of the year as an integer value between 1 and 12 (inclusive), where 1 is January and 12 is December. See the Note On Zulu Time for more information.
ZULU DAY OF YEAR Number This returns the current Zulu Time day of the year as an integer value between 0 and 365 (365 only on a leap year). See the Note On Zulu Time for more information.
ZULU YEAR Number This returns the current Zulu Time year as an integer value. See the Note On Zulu Time for more information.
ZULU SUNRISE TIME Seconds This returns the seconds since midnight until the sunrise based on Zulu Time. See the Note On Zulu Time for more information.
ZULU SUNSET TIME Seconds This returns the seconds since midnight until the sunset based on Zulu Time. See the Note On Zulu Time for more information.
LOCAL TIME Seconds This returns the seconds since midnight (00:00 local time) on the current day.
LOCAL DAY OF WEEK Number This returns the current day of the week as an integer value between 0 and 6 (inclusive), where 0 is Monday and 6 is Sunday, within the local time reference.
LOCAL DAY OF MONTH Number This returns the current day of the month as an integer value between 1 and 31 (inclusive), within the local time reference.
LOCAL MONTH OF YEAR Number This returns the current month of the year as an integer value between 1 and 12 (inclusive), where 1 is January and 12 is December, within the local time reference.
LOCAL DAY OF YEAR Number This returns the current day of the year as an integer value between 0 and 365 (365 only on a leap year), within the local time reference.
LOCAL YEAR Number This returns the current year (local time) as an integer value.
TIME ZONE OFFSET Seconds This returns the offset between the local time and Zulu Time. See the Note On Zulu Time for more information.
TIME OF DAY

Enum

(Number)

This will return an integer value representing the approximate time of day where:

• 0 = dawn
• 1 = day
• 2 = dusk
• 3 = night
TOOLTIP UNITS

Enum

(Number)

This will return an integer value representing the current units used to display tooltip information, where:

• 0 = default
• 1 = metric (SI)
• 2 = US (imperial)
UNITS OF MEASURE

Enum

(Number)

This indicates the units used to express measurements and will return an integer value representing the following:

• 0 - English
• 1 - Metric with altitude in feet
• 2 - Metric with altitude in meters
SIMULATION RATE Number This is used to get/set the internal rate of passing time within the simulation(independently of the visual frame rate). The value will be clamped between 0.25 and 128, and a value of 1 means that 1 second of game time is the same as 1 second of "real" time, and lower values will make the game time slower, and higher values will make the game time faster.
SIMULATION TIME Seconds This returns a value for the seconds since the simulation has been started.
SIMULATION DELTA TIME Number This returns a value for the time passed between the last simulation frame and the current one.
IS IN VR Boolean This returns either 1 (TRUE) ir 0 (FALSE) to indicate whether the simulation is in VR mode or not.

Note On Zulu Time

It should be be noted that Zulu Time is calculated off of the local time using a table of time-zones. For example, if the simulation is in a timezone that is +9 hours, then Zulu Time will be considered as current time - 9h.

Expression Operators

There is a long list of operators that can be used within RPN stacks:

Operator Operation Args Example Result
+ Addition 2 7 2 + 9
-

Subtraction.

For example, if the stack contains A B -, then the calculation is A - B.

2 (L:Val) 125 - The local value Val minus 125.
/

Division.

For example, if the stack contains A B /, then the calculation is A / B.

2 16 4 / 4
* Multiplication. 2 10 2 * 20
%

Taking modulo. Returns the signed remainder of a dividend (argument 1) and a divisor (argument 2).

NOTE: The divisor (argument 2) will always be considered as positive, regardless of the actual sign.

2

7.2 2 %

7.2 -2 %

-7.2 2 %

-7.2 -2 %

1.2

1.2

-1.2

-1.2

pmod

Absolute modulo. Returns the absolute remainder (ie: positive) of a dividend (argument 1) and a divisor (argument 2).

NOTE: The divisor (argument 2) will always be considered as positive, regardless of the actual sign.

2

7.2 2 pmod

7.2 -2 pmod

-7.2 2 pmod

-7.2 -2 pmod

1.2

1.2

0.8

0.8

++ Increment by 1. 1 158 ++ 159
-- Decrement by 1. 1 1005 -- 1004

/-/

neg

Negates a number (essentially multiplying the value by -1). 1 11 /-/ -11

Comparison Operators

== Will be TRUE if values are equal, or FALSE otherwise. 2 (L:Value) 0 == if{ A } If the L value is 0, then operation A will be performed.
!= Will be TRUE if values are not equal, or FALSE otherwise. 2 (L:Value) 0 != if{ A } If the L value is not 0, then operation A will be performed.
>

Will be TRUE if one value is greater than another, or FALSE otherwise.

NOTE: When using RPN in XML files, this must be written as &gt; otherwise the XML will not parse correctly.

2 (L:Val1) (L:Val2) > if{ A } els{ B } If Val1 is greater than Val2, operation A is performed, otherwise operation B is performed.
<

Will be TRUE if one value is less than another, or FALSE otherwise.

NOTE: When using RPN in XML files, this should be written as &lt;. It may not be strictly necessary, but it is good practice.

2 (L:Val1) (L:Val2) < if{ A } els{ B } If Val1 is less than Val2, operation A is performed, otherwise operation B is performed.
>= Will be TRUE if one value is greater than or equal to another, or FALSE otherwise. 2 (L:Val1) (L:Val2) >= if{ A } els{ B } If Val1 is greater than or equal to Val2, operation A is performed, otherwise operation B is performed.
<= Will be TRUE if one value is less than or equal to another, or FALSE otherwise. 2 (L:Val1) (L:Val2) <= if{ A } els{ B } If Val1 is less than or equal to Val2, operation A is performed, otherwise operation B is performed.
?

Ternary operator.

With this, the third operand determines whether the first value (the operation is TRUE) or second value (the operation is FALSE) is selected.

3 X Y A:INTERCOM SYSTEM ACTIVE ? If the SimVar INTERCOM SYSTEM ACTIVE evalutaes as TRUE then X is selected otherwise Y is selected.

Bit Operators

& Bitwise AND. 2 3 2 & 2
| Bitwise OR. 2 8 5 | 13
^ Bitwise XOR. 2 17 4 ^ 21
~ Bitwise NOT. 1 8 ~ -9
>>

Shift the right operand N number of bits.

NOTE: When using RPN in XML files, this must be written as &gt;&gt; otherwise the XML will not parse correctly.

2 40 1 >> 20
<<

Shift the left operand N number of bits.

NOTE: When using RPN in XML files, this should be written as &lt;&lt;. It may not be strictly necessary, but it is good practice.

2 5 3 << 40

Logical Operators

!, NOT Logical NOT. 1 (L:Val) ! (>L:Val) Toggles the variable Val.
&&, AND Logical AND. 2 (L:Val) 0xFF00 && (>L:Val) The variable Val is ANDed with the hexadecimal value 0xFF00
||, OR Logical OR. 2 (L:Val) 135046 OR (>L:Val) The variable Val is ORed with 135046.

Numerical Operators

abs Absolute value (essentially just forces any value to be positive). 1 -15 abs 15

int

flr

Calculates the nearest integer value which is less than the source value. 1 88.69 flr 88
rng Returns TRUE if the third operand lies between the range created by the first and second values. 3 1 10 3 rng True
cos

Cosine.

1 pi cos -1
lg Logarithm to base 10. 1 20 lg 1.30102999566
min Returns the minimum of two values. 2 11 3 min 3
sin

Sine.

1 pi sin 0
acos

Arc cosine.

NOTE: return value is in radians.

1 pi acos -1
ctg

Cotangent.

1 1 ctg 0.642093
ln Natural logarithm. 1 10 ln 2.302585
sqr Square. 1 4 sqr 16
asin Arc sine. 1 -1 asin -1.570796
eps Floating-point relative accuracy. 1 1 eps  2^(-52)
log Logarithm of the first operand, to the base of the second operand. 2 16 2 log 4
pi

Pi - Puts $\pi$ on the stack.

0 pi 3.14159
sqrt Square root. 1 16 sqrt 4
atg2

Arc tangent with two inputs.

2 2 1 atg2 0.463647
exp Exponent - e to the power of the operand. 1 1 exp 2.718282
max Returns the maximum of two values. 2 127 256 max 256
pow Power of - the first value to the power of the second. 2 3 8 pow 6561
tg

Tangent.

1 pi tg 0
atg Arc tangent with one input. 1 1 atg 0.785398
sign Returns the sign of the top number on the stack (-1 or 1 and note that 0 is considered positive). 1

-9 sign

0 sign or 160 sign

-1

1

dec Returns the decimal part of a floating point number. 1 3.14 dec 0.14

Special Operators

div Integer Division. The result of this operator is always an integer 2 9 4 div 2
ceil Rounds the value up to the nearest integer (always larger than - or equal to - the source value). 1 11.4 ceil 12
near

Rounds the value to the nearest integer, where a value of 0.5 or greater is rounded up, and all other values are rounded down.

1 8.4 near 8

dnor

d360

rdeg

Normalizes an angle expressed in degrees, such that the result is always between 0 and 360. 1 -45 dnor 315
rddg Converts radians to degrees. 1 pi rddg 180
dgrd Converts degrees to radians. 1 180 dgrd pi
rnor Normalizes an angle expressed in radians, such that the result is always between 0 and 2 pi 1 -2.18166 rnor 4.10152
if{ .... }

If statement.

Note there is no space between the if and the opening {.

1 (L:Val) 0 == if{ A } Operation A is carried out if Val is 0.
els{ .... }

Else statement.

Note there is no space between the els and the opening {. Also note that yuou cannot have an els without a previous if.

1 (L:Val1) (L:Val2) <= if{ A } els{ B } If Val1 is less than or equal to Val2, operation A is carried out, otherwise operation B is carried out.
quit

The quit statement allows expression evaluation to stop completely.

Can be used - for example - to avoid the use of nesting if{ statements.

0 pi quit (L:Val1) (L:Val2) <= if{ A } els{ B } pi. The rest of the script is ignored.
g0...gn

Goto.

Execution will jump to the specified label, which was set using a colon followed by the label number.

0 g2 Execution jump to :2
case Case statement.   50 40 30 20 10 5 (L:value) case

The "5" indicates there are five case values, which are selected depending on the evaluation of (L:value).

If the evaluation is equal to or greater than 0, but less than 1, the result is 10. If the evaluation is equal to or greater than 1, but less than 2, the result is 20, and so on.

seed Sets the seed for the random function. 1 23488 seed N/A
rand Retrieves a random number between 0 and 1. 0 rand rand rand 0.987551
0.058777
0.478326

String Operators

lc Converts a string to lowercase. 1 'AbCd20' lc 'abcd20'

uc

cap

Converts a string to uppercase. 1 'abCD50' uc 'ABCD50'
chr Converts an integer to an ASCii symbol. 1 88 chr 'X'
ord Converts an ASCii symbol to an integer. 1 'B' ord 66
scat Concatenates two input strings. 2 'abc' 'xyz' scat 'abcxyz'
schr Finds the position of a specific symbol in a string. Positions start at 0. 2 'abcd' 'd' schr 3
scmp

Compares two strings.

NOTE: This operation is case sensitive.

2 (M:Event) 'LeftSingle' scmp 0 == if{ A }els{ B } Performs A if the left mouse button has been pressed, otherwise performs B
scmi

Compares two strings.

NOTE: this operation is not case sensitive.

2 'left' 'Left' scmi 0 == if{ 'yes' } 'yes'
sstr

Finds the position of substring B in string A. Will return -1 if the substring cannot be found. Positions start at 0.

NOTE: This operation is case sensitive.

2 'abcxyz' 'cx' sstr  2
ssub Extracts a substring from string A based on the (integer) positions given as the "from" (B) and "to" (C) values, inclusive. Positions start at 0, and note that you may supply a negative value to start N number of characters from the end of the given string. 3

'abcxyz' 1 2 ssub

'abcxyz' -3 2 ssub

'bc'

'xy'

symb Extracts a single character from the string at the given (integer) position. Positions start at 0. 2 'abc' 1 symb 'b'

Stack Operators

b

Backup the stack.

Essentially this lets you retrieve a value from before performing an operation. Note that when an operation takes multiple parameters it will only save the first value in the backup. For example 1 2 + b would contain 2 and 1 would be lost.

0 (L:MyValue) neg sp0 b sp1 This would have -(L:MyValue) in sp0 and (L:MyValue) in sp1, since b is the value saved before doing the neg operation.
c Clears the stack. 0 stack: 1 2 3 c stack:
d Duplicates the value that is on the top of the stack. 1 stack: 5 d stack: 5 5
p Pops and discards the top value on the stack. 1 stack: 1 2 3 p stack: 1 2
r Reverses the top and second values on the stack. 2 stack: 1 2 3 r stack: 1 3 2
s0, s1, … s49 Stores the top value in an internal register, but does not pop it from the stack. 1 stack: 1 2 3 s0 stack: 1 2 3
s0: 3
l0, l1, … l49 Loads a value from a register to the top of the stack. 1 stack: 1 2 3 s0 l0 stack: 1 2 3 3
sp0, sp1, … sp49 Stores the top value and pops it from the stack. 1 stack: 1 2 3 sp0 stack: 1 2
sp0: 3

Notes:

• Formatted strings use a similar but slightly different syntax.
• All strings should be written using single quotes, for example: 'abcxyz'.
• Hexadecimal numbers can be entered using the 0x convention (the hex value can use upper or lower case, for example: 0xff or 0xFF00AA00)
• Octal numbers can be entered by using a leading zero. For example, 022 is octal 18. This means you must be careful not to have leading zeros on decimal numbers.
• Scientific notation can be used to represent values, for example: 5E2 represents 5 x (10 to the power of 2) and 5E-2 represents 5 x (10 to the power of -2), giving 500 and 0.005 respectively.

Strings

The following sections contain information specific to the output of strings and how the RPN should be formatted for this.

Formatting Numbers

If you wish any numbers to be formatted in a specific way then you need to use the exclamation mark symbol along with a designated letter, eg: !x!. This letter must be lowercase, and can only be one of the following:

• s: the number should be formatted as a string.
• d: the number should be formatted as an integer. Note that if the number is not already an integer, it will be rounded (not truncated) to the nearest integer.
• f: the number should be formatted as a float.

When setting up number formatting in this way, you can (optionally) choose to preceed the formatting letter by a number. This number specifies the minimum number of digits to display.

When working with decimal numbers, the following rules will be applied:

• If d is preceded by the digit "0", then leading zeros are added if necessary.
• If d is preceded by "-", text is left-aligned.
• If d is preceded by "+", a "+" symbol is indicated in front of the number when the number is greater than 0 (a "-" is always used to indicate numbers less than 0).
• If d is preceded by " " (space), leading spaces are added if necessary.

For floating point numbers, the following rule applies:

• If a decimal point is used in the formatting number, the digit after the decimal point specifies the number of digits to display after the decimal point.

Below you can find some examples of number formatting using various letters and values:

Example Result Description
%( 12.34 )%!4.3f! 12.340 The 4 in 4.3 is ignored.
%( 12.34 )%!04.3f! 12.340 Leading "0"s are not added to floating point numbers.
%( 12345.6789 )%!4.3f! 12345.679 The number before decimal point does not limit the number of digits displayed before decimal point.
%( 34.56 )%!+d! +35 Rounding, not truncation, has occurred.
%( 234 )%!5d! 234 Two leading spaces have been prefixed to the number.
%( "foo" )%!5s! foo Two leading spaces have been prefixed to the string
%( 234 )%!3s! 234 The number is output as a string, with a minimum of three digits.

Conditional Gauge Strings

The format of conditions (if, then, else, and case statements) in gauge strings is different from that in other scripts. In gauge strings use the %{if}, %{else}, and %{end} constructs to choose which text to display. Note that these keywords are case-sensitive and must be typed in lowercase. Also, there must not be a space between the "%" and the "{". An if statement can be used without a corresponding else, in which case nothing is displayed if the result of the condition is false. The syntax for usage is one of the following:

%(CONDITIONAL)%{if}TEXT TO DISPLAY IF TRUE%{else}TEXT TO DISPLAY IF FALSE%{end}
%(CONDITIONAL)%{if}TEXT TO DISPLAY IF TRUE%{end}  

For example:

%( 1 )%{if}ON%{else}OFF%{end}

would give the output ON, whereas:

%( 0 )%{if}The value is true%{else}The value is false%{end}

would give the output: The value is false.

Escape Codes

It is also possible to insert escape code sequences into gauge strings.

Escape Code Example Description
\{tabs=50R,60C, 244L} Set 3 tab stops; the first is right-aligned, the second is centered, and last is left-aligned.
\{fnt1} Switch to the first alternate font specified as a child of the gauge text element
\{fnt} Return to the default font
\{up} Superscript
\{dn} Subscript
\{md} Normal (neither superscript nor subscript)
\{bo} Bold
\{ul} Underline
\{itl} Italic
\{strk} Strikeout
\{blnk} Blink
\{rev} Reverse background/foreground color for text
\{nr} Normal -- clear all properties previously set.
\{lcl} Line color
\{blc} Background line color
\{clr} Color
\{bck} Background color
\{dplo=X} Put a degrees symbol above the next character after the ?=?
\{dpl=XY} Make X superscript and Y subscript
\{lsp=23} Set line spacing to 23
\{lsp} Set line spacing to default
\{ladj=L} Set horizontal text alignment to left. (use ?C? for center or ?R? for right)
\{line=240} Draw a horizontal line with width 240
\{lmrg=20} Set the left margin to 20
\{rmrg=30} Set the right margin to 30
\{img1} Insert image #1 (a text element can have image children)

Examples

The following table shows a few examples of formatted strings using RPN:

String Example Description
Fuel Pressure The text will appear exactly as entered: Fuel Pressure
Fuel Capacity: %(A:FUEL TOTAL CAPACITY)%!1.2f! The fuel capacity of the aircraft will be given as a floating point number accurate to two decimal places, following the initial string, such as:
Fuel Capacity: 80.55
%(A:ENG ON FIRE:1 A:ENG ON FIRE:2 ! if{ 'Warning: Engine Fire' } ) The text string "Warning: Engine Fire" will appear if either or both of the engines are on fire.
%( 1 )%{if}ON%{else}OFF%{end} The text ON would be rendered. If there is no {else} statement, then no text will be displayed if the condition evaluates to false.
%( 3 )%{case}%{ :0 }AIRPORT%{ :1 }INTERSECTION%{ :2 }NDB%{ :3 }VOR%{ :4 }MARKER%{end} A case statement can be used to select a text string from a group of strings. The case numbers do not have to be sequential. The example would produce the result:
VOR
%((C:Mission:OnScreenTimerValue) 60 / 60 / flr )%!02d!: %((C:Mission:OnScreenTimerValue) 60 / flr 60 %)%!02d!: %((C:Mission:OnScreenTimerValue) flr 60 %)%!02d!. %((C:Mission:OnScreenTimerValue) 10 * flr 10 % )%!01d! Takes the custom on-screen timer value and displays the time in hours, minutes, seconds and tenths of a second.
The !02d! indicates that the text output should be displayed with two digits (for example, 06). The % signs inside the string refer to the modulus operator, and the colons and period between the statements will appear on the screen as text, for example:
01 : 14: 08 . 2
85 %% To output the percent character, use two percent signs in the script:
85 %
%(10 s2 1 s1)%{loop}%( l1 )%!s! %( l1 ++ s1 l2 <)%{next} This statement sets up two registers s1 and s2, with the numbers 1 and 10, then loops to print out:
1 2 3 4 5 6 7 8 9
Whatever value is on top of the stack when the %{next} statement is reached is evaluated as a boolean to determine if execution of the loop should continue.

SimVars And Keys

When creating add-ons for aircraft - primarily Instruments - you will need to be able to access Simulation Variables and change them based on user input in the form of Key Events. Both using SimVar values and using input key events is done using operations written in RPN, as explained below. Note that you can also find many use-cases and examples in the different ModelBehavior templates

SimVars

SimVars permit you to get the state of a component, or the state of something in the simulation, and they are generally accesed using the following RPN format:

(A:[simvar_name]:[simvar_index], [units])

Note that the "index" value is only required by a few SimVars. Here are some simple examples:

(A:SIM ON GROUND, Bool)
(A:AIRSPEED TRUE, Knots)
(A:TURB ENG IGNITION SWITCH EX1:1, Enum)

Sometimes a SimVar will require two additional index values, for example the SimVar CIRCUIT CONNECTION ON requires a circuit index and a bus index. Normally this would be written like this:

2 (>A:BUS LOOKUP INDEX, Number) (A:CIRCUIT CONNECTION ON:45, Bool)

However, some SimVars will permit an alterative "shorthand" method of using the two indices, like this:

2 (A:1:CIRCUIT CONNECTION ON:45, Bool)

Here, the 1: after the A: is used to tell the sim to send the top value on the stack to the SimVar. This shorthand method is ONLY applicable to the following SimVars:

• BUS CONNECTION ON
• BATTERY CONNECTION ON
• ALTERNATOR CONNECTION ON
• CIRCUIT CONNECTION ON
• EXTERNAL POWER CONNECTION ON
• BUS BREAKER PULLED
• BATTERY BREAKER PULLED
• ALTERNATOR BREAKER PULLED
• CIRCUIT BREAKER PULLED
• EXTERNAL POWER BREAKER PULLED

Finally it should be noted that sometimes you will see simulation variables written using snake case, and prefixed with SIMVAR_, as that is how they appear in the simulation code itself. However when using them in RPN, you should omit the SIMVAR part as well as the _. For example:

SIMVAR_CIRCUIT_ON -> (A:CIRCUIT ON:1, Bool)

Keys

Key events permit you to change the state of a component or other aspect of the simulation based on some form of input. When checking for a key event, you would use the following RPN format:

[0] (>K:[key_event_name])
[1] [0] (>K:2:[key_event_name])

There are two formats showing for accessing key events because many of them require not just one but two parameters, and so the formatting above shows how this is achieved. Below are two "real world" examples, one with a single parameter and the other with 2 parameters:

1 (>K:TOGGLE_EXTERNAL_POWER)
50 1 (>K:2:PANEL_LIGHTS_POWER_SETTING_SET)


Converting InFix To PostFix

Unfortunately there is no easy way to automatically convert InFix expressions into RPN (PostFix) expressions, especially with the use of SimVars and things specific to the Microsoft Flight Simulator SDK. However, there are third-party tools that may be of some use to you and that can work quite well, although they are not perfect. One in particular may be worth looking at which you can find from the link below: