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Re: Can This be Possible? A bug in Set (=) ?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg110828] Re: Can This be Possible? A bug in Set (=) ?
  • From: David Bailey <dave at removedbailey.co.uk>
  • Date: Thu, 8 Jul 2010 03:13:24 -0400 (EDT)
  • References: <i11p5t$p2a$1@smc.vnet.net>

On 07/07/10 12:41, James Stein wrote:
> The definition of Set (=) is straight forward:  "*lhs=rhs*
>
> evaluates  *rhs* and assigns the result to be the value of  *lhs*. From then
> on,  *lhs* is replaced by  *rhs* whenever it appears."
>
>
> How then to explain the output of cell 4 below, when cells 1 through 5 are
> evaluated in sequence (but not all at once, e.g. do NOT select all five
> cells and then press shift-return) ?
>
>
>   (* cell 1 *)
> foo = 1;
> Print [ foo ]
>
>
> (* cell 2 *)
> foo = 2;
> Print [ foo ]
>
>
> (* cell 3 *)
> ClearAll [ evaluate, foo ];
> evaluate[n_Integer] := Module [ {nb, id, sel},
>     Print [ "entering evaluate[", n, "], foo=", foo ];
>     nb = EvaluationNotebook [ ];
>     id = "cell "<>  ToString [ n ];
>     (* find desired string and select it: *)
>     sel = NotebookFind [ nb, id ];
>     If [ sel == $Failed, Return [ sel ] ];
>     (* select entire cell containing string: *)
>     SelectionMove [ nb, All, Cell ];
>     (* set foo by evaluating selected cell: *)
>     SelectionEvaluate [ nb ];
>     (* verify[??] that foo has a new value *)
>     Print [ "exiting  evaluate[", n, "], foo=", foo ];
>     Return [ foo ];
>     ];
>
>
> (* cell 4 *)
> foo = 0;
> evaluate [1];
> foo
> evaluate [2];
> foo  (* returns 0 *)
>
>
> (* cell 5 *)
> foo  (* returns 2 *)
>
>
> If you enter the above, not all in a single cell but in five separate
> cells, then evaluate each cell in turn, you will discover that consecutive
> evaluations of 'foo' (the last two) return different values.
>
>
> How can this be?  I was under the impression that Mathematica maintains a
> single symbol table.  In this table will be an entry for "foo", with various
> associated definitions, values, syntax information, attributes, etc.  So how
> is the following sequence be explained?
>
>    Assign foo the value 2.  (penultimate line in cell 4)
>
>    Display foo (value is 0)  (last line in cell 4)
>
>    Display foo (value is 2)  (last line in cell 5)
>
>
The crucial point here is that SelectionEvaluate *queues* the evaluation 
to happen when the previous one is complete - it is not analogous to 
calling a function. Although I have not worked through your example in 
detail, I am pretty sure this is what is involved here.

SelectionEvaluate is a fairly specialised thing to use - do you have a 
good reason to do so - for example a call to Switch to select among 
various strings might solve your problem more elegantly (and much more 
efficiently!).

David Bailey

http://www.dbaileyconsultancy.co.uk


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