Hi Richard,
Your example will balk over the sequence:
c:= Counter new.
c decrementIfPositive.

Better write 
decrementIfPositive
self value > 0 ifTrue:[value := value - 1].

Regards,
Torge
"Richard A. O'Keefe" wrote:
> 
> "Gary McGovern" <garywork at lineone.net> wrote:
>     [Alan Key wrote:
>         A way around this is to make a class to hold numbers that can be
>         incremented (we used to call this a "gauge").  If you stick one
>         of these in a variable slot, then all will work pretty well.
>         Can you see where you should subclass gauge?
>     ]
>         My guess would be make an IntegerArray with the index changing on
>         incrementing. Close ?
> 
> Perhaps a simpler example is a Counter.
> 
> Create a new subclass Counter of Object with one instance variable 'value'.
> Give it the methods
> 
>     value                         "access method"
>         value ifNil: [value := 0] "lazy initialisation to 0".
>         ^value
> 
>     increment                     "bump value, return self"
>         value := self value + 1
> 
>     decrementIfPositive
>         value > 0 ifTrue: [value := self value - 1]
> 
>     isZero
>         ^self value = 0
> 
> Now you can do
>     |c|
>     c := Counter new.
>     c value                             "==> 0"
>     c isZero                            "==> true"
>     c increment.
>     c isZero                            "==> false"
>     c decrementIfPositive.
>     c decrementIfPositive.
>     c value                             "==> 0"
> 
> You don't want a counter that clamps at 0?  Then roll your own.
> (Note that ++ in C++ and C99 will clamp for one type but not others.)
> You want a method to set a counter to any number?  Add one.