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Re: If this isn't a bug . . .

  • To: mathgroup at
  • Subject: [mg102137] Re: If this isn't a bug . . .
  • From: pfalloon <pfalloon at>
  • Date: Thu, 30 Jul 2009 05:33:54 -0400 (EDT)
  • References: <h4p3ne$j8n$>

On Jul 29, 7:11 pm, AES <sieg... at> wrote:
> If this isn't a bug, sure seems like it should be!
>    In[247]:= Do[y=2; y=3, {1}]
>    In[248]:= %
>    Out[248]= 3
>    In[249]:= Do[y=4, {1}]
>    In[250]:= %
>    < No Out[250] >

That does seem bizarre. After experimenting a little, it seems like
the cause of this is in the behaviour of the CompoundExpression (";")
function (or operator).

(Note that the expression "y=2; y=3" has FullForm "CompoundExpression
[y=2, y=3]".)

>From the documentation for CompoundExpression: "expr1;expr2; returns
value Null. If it is given as input, the resulting output will not be
printed. Out[n] will nevertheless be assigned to be the value of
Subscript[expr, 2]. "

So, each time a CompoundExpression appears in an input In[n], it goes
and sets Out[n] to the output of its last non-Null argument. My guess
is that this happens regardless of whether you are inside a Do loop or

This would explain why the following holds:

In[52]:= {Do[y=2, {1}]}
Out[52]= {Null}
In[51]:= {Out[-1]}
Out[51]= {Null}

Whether this should be considered a bug or an acceptable behaviour is
another question...

Note that a more desirable consequence of this CompoundExpression
behaviour is the following:

In[47]:= y = 3;
In[48]:= %
Out[48]= 3


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