#5251: [with patch, needs review] preparser bug in parsing the backslash "solve
right" notation
--------------------+-------------------------------------------------------
Reporter: was | Owner: mhansen
Type: defect | Status: assigned
Priority: major | Milestone: sage-3.3
Component: misc | Resolution:
Keywords: |
--------------------+-------------------------------------------------------
Comment (by was):
I read the stopping condition code, and realized it is easy to make up
many other examples that break the \ notation, even after the above patch
is applied:
{{{
sage: A = matrix(QQ,2,2,[1..4])
sage: matrix(QQ,2,1,[1/3,"2/3"]) # valid notation
[1/3]
[2/3]
sage: A \ matrix(QQ,2,1,[1/3,"2/3"])
------------------------------------------------------------
File "<ipython console>", line 1
A ._backslash_(
matrix(QQ,Integer(2),Integer(1),[Integer(1)/Integer(3),)"2/3"])
^
SyntaxError: invalid syntax
sage: A \ matrix(QQ,2,1,[1/3,'2/3'])
------------------------------------------------------------
File "<ipython console>", line 1
A ._backslash_(
matrix(QQ,Integer(2),Integer(1),[Integer(1)/Integer(3),)'2/3'])
^
SyntaxError: invalid syntax
sage: A \ matrix(QQ,2,1,[1/3,2*3])
------------------------------------------------------------
File "<ipython console>", line 1
A ._backslash_(
matrix(QQ,Integer(2),Integer(1),[Integer(1)/Integer(3),Integer(2))*Integer(3)])
^
SyntaxError: invalid syntax
}}}
The point of the stopping condition is that one should be able to do,
e.g.,
{{{
A \ stuff # this does something,
}}}
and get
{{{
A._backslash_(stuff) # this does something
}}}
instead of
{{{
A._backslash_(stuff # this does something)
}}}
Another example:
{{{
A \ B + C
}}}
should become
{{{
A._backslash_(B) + C
}}}
Now that is *not* correctly parsed:
{{{
sage: preparse('A \ B + C')
'A ._backslash_( B + C)'
}}}
It should be parsed to
{{{
A._backslash_(B) + C
}}}
Here is another that isn't correctly parsed:
{{{
sage: preparse( 'A\eval("C+D")' )
'A._backslash_(eval()"C+D")'
}}}
It should be
{{{
A._backslash_(eval("C+D"))
}}}
Finally, the / was there before because of precedence. I.e., the
following
is now wrong after the patch:
{{{
sage: preparse('A \\ x / 5')
'A ._backslash_( x / Integer(5))'
}}}
but we should get
{{{
sage: preparse('A \\ x / 5')
'A ._backslash_( x ) / Integer(5)'
}}}
since precedence of backslash should be left to right (just like that of
/).
----
Comments:
1. I find the backslash notation -- which is from Matlab -- very useful
and do use it all the time. Otherwise, I wouldn't have noticed the bug I
reported in this ticket.
2. Clearly whoever (=me, of course!) implemented this in preparser.py did
a very bad job, and this is an extremely buggy feature with the potential
to lead to serious confusion and wrong answers. It's not the sort of
thing that should be implemented without using a more sophisticated
parsing technique.
3. I almost think it would be better that this feature didn't exist given
the bugs I listed above.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/5251#comment:2>
Sage <http://sagemath.org/>
Sage - Open Source Mathematical Software: Building the Car Instead of
Reinventing the Wheel
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