In many cases, a variable may also be undefined. In this case, the variable is set to #0, which identifies a null variable (empty / not initialized). Indeed #0 is a read-only variable used mainly for two purposes:

• check if a variable has been initialized
• reset a variable

At the beginning of the compilation every non system variable is set to #0.

A null variable has no value, it should not be confused with a variable that has a zero value.

#101 = 0    ; Variable #101 has a zero value
#102 = #0   ; Variable #102 is vacant (empty), has no value and cannot be used for some operations

The following piece of G-code provide an example of the operations that can be used with an empty variable

( © 2018 by RosettaCNC Motion                               )
( NULL paramater handling example:                          )
( Every parameter is set to NULL when not initialized       )
( Parameter #0 stores the value NULL and cannot be written  )
 
; Comparing a null to a null variable:
; Define #1 as null (that means that is is empty/not initialized) 
#1 = #0
IF[#1 EQ #0] THEN M109 P"IF[#1 EQ #0] Should return TRUE"
IF[#1 NE #0] THEN M109 P"IF[#1 NE #0] Should return FALSE"
IF[#1 GT #0] THEN M109 P"IF[#1 GT #0] Should return FALSE"
IF[#1 GE #0] THEN M109 P"IF[#1 GE #0] Should return TRUE"
IF[#1 LT #0] THEN M109 P"IF[#1 LT #0] Should return FALSE"
IF[#1 LE #0] THEN M109 P"IF[#1 LE #0] Should return TRUE"
 
; Comparing a zero to a null variable:
; Define #1 as zero that means #1 is equal to 0
#1 = 0
IF[#1 EQ #0] THEN M109 P"IF[#1 EQ #0] Should return FALSE"
IF[#1 NE #0] THEN M109 P"IF[#1 NE #0] Should return TRUE"
IF[#1 GT #0] THEN M109 P"IF[#1 GT #0] Should return FALSE"
IF[#1 GE #0] THEN M109 P"IF[#1 GE #0] Should return TRUE"
IF[#1 LT #0] THEN M109 P"IF[#1 LT #0] Should return FALSE"
IF[#1 LE #0] THEN M109 P"IF[#1 LE #0] Should return TRUE"
 
; Comparing a null variable to a zero:
; #1 is defined as null (that means #1 is vacant)
#1 = #0 
IF[#1 EQ 0] THEN M109 P"IF[#1 EQ 0] Should return FALSE"
IF[#1 NE 0] THEN M109 P"IF[#1 NE 0] Should return TRUE"
IF[#1 GT 0] THEN M109 P"IF[#1 GT 0] Should return FALSE"
IF[#1 GE 0] THEN M109 P"IF[#1 GE 0] Should return TRUE"
IF[#1 LT 0] THEN M109 P"IF[#1 LT 0] Should return FALSE"
IF[#1 LE 0] THEN M109 P"IF[#1 LE 0] Should return TRUE"
 
; Comparing a zero to a zero:
; #1 is defined as zero (that means #1 is equal to 0)
#1 = 0 
IF[#1 EQ 0] THEN M109 P"IF[#1 EQ 0] Should return TRUE"
IF[#1 NE 0] THEN M109 P"IF[#1 NE 0] Should return FALSE"
IF[#1 GT 0] THEN M109 P"IF[#1 GT 0] Should return FALSE"
IF[#1 GE 0] THEN M109 P"IF[#1 GE 0] Should return TRUE"
IF[#1 LT 0] THEN M109 P"IF[#1 LT 0] Should return FALSE"
IF[#1 LE 0] THEN M109 P"IF[#1 LE 0] Should return TRUE"
 
; Comparing a positive number to a null variable:
#1 = 0.1
IF[#1 EQ #0] THEN M109 P"IF[#1 EQ #0] Should return FALSE"
IF[#1 NE #0] THEN M109 P"IF[#1 NE #0] Should return TRUE"
IF[#1 GT #0] THEN M109 P"IF[#1 GT #0] Should return TRUE"
IF[#1 GE #0] THEN M109 P"IF[#1 GE #0] Should return TRUE"
IF[#1 LT #0] THEN M109 P"IF[#1 LT #0] Should return FALSE"
IF[#1 LE #0] THEN M109 P"IF[#1 LE #0] Should return FALSE"
 
; Comparing a negative number to a null variable:
#1 = -0.1
IF[#1 EQ #0] THEN M109 P"IF[#1 EQ #0] Should return FALSE"
IF[#1 NE #0] THEN M109 P"IF[#1 NE #0] Should return TRUE"
IF[#1 GT #0] THEN M109 P"IF[#1 GT #0] Should return FALSE"
IF[#1 GE #0] THEN M109 P"IF[#1 GE #0] Should return FALSE"
IF[#1 LT #0] THEN M109 P"IF[#1 LT #0] Should return TRUE"
IF[#1 LE #0] THEN M109 P"IF[#1 LE #0] Should return TRUE"
 
m2 ( Program End)