6.10.2 Variables

The previous section showed how a sequence of commands could be used to generate a variable. As a final refinement, the whole code sequence can be wrapped up in a defining word (pre-empting the subject of the next section), making it easier to create new variables:

: myvariableX ( "name" -- a-addr ) CREATE 1 cells allot ;
: myvariable0 ( "name" -- a-addr ) CREATE 0 , ;

myvariableX foo \ variable foo starts off with an unknown value
myvariable0 joe \ whilst joe is initialised to 0

45 3 * foo !   \ set foo to 135
1234 joe !     \ set joe to 1234
3 joe +!       \ increment joe by 3.. to 1237

Not surprisingly, there is no need to define myvariable, since Forth already has a definition Variable. Standard Forth does not guarantee that a Variable is initialised when it is created (i.e., it may behave like myvariableX). In contrast, Gforth’s Variable initialises the variable to 0 (i.e., it behaves exactly like myvariable0). Forth also provides 2Variable and fvariable for double and floating-point variables, respectively – they are initialised to 0. and 0e in Gforth. If you use a Variable to store a boolean, you can use on and off to toggle its state.

Variable ( "name" –  ) core “Variable”

Define name and reserve a cell starting at addr. name run-time: ( -- addr ).

AVariable ( "name" –  ) gforth-0.6 “AVariable”

Works like variable, but (when used in cross-compiled code) tells the cross-compiler that the cell stored in the variable is an address.

2Variable ( "name" –  ) double “two-variable”

fvariable ( "name" –  ) floating “f-variable”

Finally, for buffers of arbitrary length there is

buffer: ( u "name" –  ) core-ext “buffer-colon”

Define name and reserve u bytes starting at addr. name run-time: ( -- addr ). Gforth initializes the reserved bytes to 0, but the standard does not guarantee this.