Overview

Conditional G-code is a powerful feature of Slic3r that allows for arbitrary math to be done in Custom G-code fields as well as setting conditions for specific statements to appear in output from Custom G-code.

Conditional expressions

Conditional expressions has the following syntax:

{if _expression_}

expression may take one of the following forms:

VARIABLE OPERATOR CONSTANT
CONSTANT OPERATOR VARIABLE
{_subexpression_} OPERATOR {_subexpression_}
OPERATOR _subexpression_

subexpression has the same forms as expression.

If _subexpression_s exist, they are evaluated before any other operators.

Available operators:

&& or and: logical AND
|| or or: logical OR
! or not: logical NOT (inversion)
^ or xor: XOR (exclusive-OR)
== or equals: equality comparison
!=: inequality comparison. To use words, use a subexpression.
<: less than (valid for numbers only)
<=: less than or equal to (valid for numbers only)
>: greater than (valid for numbers only)
>=: greater than or equal to (valid for numbers only)
Anything supported by exprtk (except for generic variables, file I/O and expressions that involve {})

The true and false keywords are available.

To apply a single condition to multiple lines, repeat it once for each line:

{if [layer_num] == 10}M104 S210
{if [layer_num] == 10}M600

If a conditional expression evaluates to false, all the characters until the end of the line are removed. If a conditional expression fails to parse, it's silently left untouched.

Expressions may be chained for an implicit AND:

{if [layer_num] == 10}{if [temperature_1] != 210}M104 S210

Conventions

Numerical value of 0 is equivalent to logical false. Any nonzero value is considered to be logical true.
- {if { 1 - 1 } } would result in 0 and thus false.

Value expressions (G-code Math)

Any expression enclosed in two curly brackets, but not starting with {if is evaluated as an arithmetic expression: {foo - bar}. If evaluation fails, the expression is silently left untouched. If any float variables are used, return value will have decimals. If string variables are mixed with numeric variables, they are parsed as floats if they have a dot, or integers otherwise.

Value expressions can be used in conditional expressions by nesting them: {if {foo - bar} > 10}.

All operators supported for conditional expressions are supported for value expressions.

Restrictions

Every expression in {} must resolve to a number.
Nested sub-expressions are evaluated before the current expression is evaluated.
Value Expressions and Conditional G-code are evaluated before any post-processing scripts.

Troubleshooting

If the parser can't understand your statements, it will output nothing for that block.
Remember that Custom G-code placeholders (items in []) are resolved before Conditional G-code.

Slic3r Manual

Conditional G-code