RepRapFirmware 3.01 and later provide GCode programming constructs to allow conditional execution and iteration, and allow parameter values in GCode commands to be expressions instead of literals.

abort <opt-expression>

This causes all nested macros and the current print file (if any) to be terminated. The expression (if present) is converted to a string, which is included in the message presented to the user and written to the log file.

echo <expression>, <expression>, ...

At least one expression must be provided. The expressions are converted to strings and written to the console, with a space character between each pair. This command is intended for debugging. Example:

echo move.axes[0].homed, move.axes[1].homed, move.axes[2].homed

The general form of the conditional block is:

if <boolean-expression>
  ...
elif <boolean-expression>
  ...
else
  ...

where as usual the “elif” part can be omitted or repeated, and the “else” part can be omitted. The bodies of the if, elif and else parts may contain ordinary GCode commands and/or program elements. Each line in the body must be indented from the corresponding keyword to indicate the extent of the body. The body ends just before the first line that is not indented.

while <boolean-expression>
  ...

The body must be indented from the while keyword. The body ends just before the first line that is not indented.

A loop may contain one or more break statements, which would normally be inside if-constructs:

while <boolean-expression>
  ...
  if <boolean-expression>
    break
  ...

The break statement transfers control to the line following the end of the loop body.

A loop may also contain one or more continue statements, which would normally be inside if-constructs:

while <boolean-expression>
  ...
  if <boolean-expression>
    continue
  ...

The continue statement increments the iteration counter and transfers control back to the start of the loop, ready to evaluate the while-condition again.

Not yet implemented

var <new-variable-name> = <expression>

This creates a new variable and initializes it to expression. The name must not already be in use. Names of the form global.identifier are global names, and names of the form local.identifier or just identifier are local names. The scope of a local name is the remainder of the block in which it is declared.

Not yet implemented

set <existing-variable-name> = <expression>

This re-assigns an existing variable to have value expression. The type of expression (float, int, string, bool) must be the same as when the variable was declared, except that an expression of integer type can be assigned to a variable of floating point type.

This form:

{ <expression> }

may be used in place of any numeric or quoted string within a GCode command. Example:

G1 X{move.axes[0].max-10} Y{move.axes[1].min+10}

Tabs and space characters may be used freely between variable names, keywords, literals and other lexical elements to improve readability.

Sub-expressions may be enclosed in { } or in ( ). The outermost level must always be { } because standard CNC GCode uses ( ) to enclose comments. RepRapFirmware treats ( ) as enclosing subexpressions if they appear inside { }, and as enclosing comments when they do not.

The available types of expressions and variables are: bool, int, float, string, object and array. The only operations available on values of type object are comparison with null and taking a member. The only operations available on values of type array are taking the length (unary prefix operator #) and indexing (operator [ ] ).

The following implicit type conversions will be performed when necessary:

• from type int to type float
• from any type to string

The following names constants are provided:

Decimal integer, floating point, and string literals may be used.

Expressions may use the values of any properties in the RepRapFirmware Object Model (OM). See Object Model of RepRapFirmware to see what is available. An object model property selector must evaluate to a single string, numeric or Boolean value, unless it is the operand of the unary # operator in which case it must evaluate to an array or a string.

[TODO]

Where an expression has multiple binary operators of the same precedence and parentheses are not used to specify the order of evaluation, the operators are evaluated from left to right.

Caution! The multiplication operator * will work when it is used anywhere inside an expression or subexpression enclosed in { } but not otherwise. This is because the * character in a line of GCode normally introduces an end-of-line checksum.

The expression <expr1> ? <expr2> : <expr3> evaluates <expr2> if <expr1> is true, otherwise <expr3>. <expr1> must be Boolean. <expr3> may be another ternary expression. The ternary operator has precedence 1.

The following functions are supported, with their conventional meanings:

Example bed.g file for calibrating a delta printer using conditional GCode. At the start, it homes the printer only if it hasn't already been homed. Then it calibrates the printer by probing a number of points, starting again if probing fails. if calibration yields a standard deviation that is above a limit (set at the end of the loop, in this case >0.03mm), it repeats the calibration process. If calibration fails 5 times for any reason, it quits.

NOTE: If you use this method to iterate the levelling of a bed/gantry mounted on leadscrews (eg Cartesian, CoreXY etc), the maximum deviation corrected is still limited by the S parameter of M671 (default 1mm). If the maximum deviation exceeds this limit, the script will exit with "Error: Some computed corrections exceed configured limit of 1.00mm", as it would if G30 bed levelling was called manually.

; Auto calibration routine for large delta printer
M561                    ; clear any bed transform

; If the printer hasn't been homed, home it
if !move.axes[0].homed || !move.axes[1].homed || !move.axes[2].homed
  G28

; Probe the bed and do auto calibration
G1 X0 Y140 Z10 F10000        ; go to just above the first probe point
while true
  if iterations = 5
    abort "Too many auto calibration attempts"
  G30 P0 X0.00 Y140.00 Z-99999
  if result != 0
    continue
  G30 P1 X70.00 Y121.24 Z-99999
  if result != 0
    continue
  G30 P2 X121.24 Y70.00 Z-99999
  if result != 0
    continue
  G30 P3 X140.00 Y0.00 Z-99999
  if result != 0
    continue
  G30 P4 X121.24 Y-70.00 Z-99999
  if result != 0
    continue
  G30 P5 X70.00 Y-121.24 Z-99999
  if result != 0
    continue
  G30 P6 X0.00 Y-134.85 Z-99999
  if result != 0
    continue
  G30 P7 X-65.57 Y-113.57 Z-99999
  if result != 0
    continue
  G30 P8 X-112.29 Y-64.83 Z-99999
  if result != 0
    continue
  G30 P9 X-130.59 Y-0.00 Z-99999
  if result != 0
    continue
  G30 P10 X-115.90 Y66.91 Z-99999
  if result != 0
    continue
  G30 P11 X-69.45 Y120.29 Z-99999
  if result != 0
    continue
  G30 P12 X0.00 Y70.00 Z-99999
  if result != 0
    continue
  G30 P13 X60.62 Y-35.00 Z-99999
  if result != 0
    continue
  G30 P14 X-52.28 Y-30.19 Z-99999
  if result != 0
    continue
  G30 P15 X0 Y0 Z-99999 S8
  if result != 0
    continue
  if move.calibration.initial.deviation <= 0.03
    break
  echo "Repeating calibration because deviation is too high (" ^ move.calibration.initial.deviation ^ "mm)"
; end loop
echo "Auto calibration successful, deviation", move.calibration.final.deviation ^ "mm"
G1 X0 Y0 Z150 F10000                ; get the head out of the way