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comp.lang.c++

Parsing - is this a sensible idea?

gw7rib

11/16/2008 9:16:00 PM

I have a program that needs to do a small amount of relatively simple
parsing. The routines I've written work fine, but the code using them
is a bit long-winded.

I therefore had the idea of creating a class to do parsing. It could
be used as follows:

int a, n, x, y;
Parser par;
par << string;
if (par >> "From" >> ' ' >> x >> ' ' >> "to" >> ' ' >> y) a = 1;
else if (par >> "Number" >> ' ' >> n) a = 2;
else a = 3;

Then if string is "From 3 to 5" this will set a=1, x=3, y=5. If the
string is "Number 2" this will set a=2 and n=2. If string is
"Other" then a=3. For convenience, I'll assume that an input of "From
4 other" is allowed to alter the value of x while returning a=3.

I think I could write a class that would do this. It would need to
keep track of whether the current parsing was succeeding and, if so,
how far through the string it had got. It would need overloaded >>
operators, obviously, some of them taking references. And it would
need a conversion operator, which I think would need to be to void *,
which would not only return whether the current parse had succeeded
but would also reset the flag and counter ready for another attempt.

So my questions are, is this a sensible thing to try to do, and are
there any potential snags that I haven't spotted?

Thanks.
Paul.
6 Answers

Erik Wikström

11/16/2008 9:42:00 PM

0

On 2008-11-16 22:16, gw7rib@aol.com wrote:
> I have a program that needs to do a small amount of relatively simple
> parsing. The routines I've written work fine, but the code using them
> is a bit long-winded.
>
> I therefore had the idea of creating a class to do parsing. It could
> be used as follows:
>
> int a, n, x, y;
> Parser par;
> par << string;
> if (par >> "From" >> ' ' >> x >> ' ' >> "to" >> ' ' >> y) a = 1;
> else if (par >> "Number" >> ' ' >> n) a = 2;
> else a = 3;
>
> Then if string is "From 3 to 5" this will set a=1, x=3, y=5. If the
> string is "Number 2" this will set a=2 and n=2. If string is
> "Other" then a=3. For convenience, I'll assume that an input of "From
> 4 other" is allowed to alter the value of x while returning a=3.
>
> I think I could write a class that would do this. It would need to
> keep track of whether the current parsing was succeeding and, if so,
> how far through the string it had got. It would need overloaded >>
> operators, obviously, some of them taking references. And it would
> need a conversion operator, which I think would need to be to void *,
> which would not only return whether the current parse had succeeded
> but would also reset the flag and counter ready for another attempt.
>
> So my questions are, is this a sensible thing to try to do, and are
> there any potential snags that I haven't spotted?

If you need to parse a lot you should probably try a tool like yacc or
some other parser-generator. If you only need to be able to parse a very
small grammar (and want a good exercise) you can try to write the state-
machine by hand.

You example looks like a runtime-construct (though, perhaps you can make
it compile-time with some fancy template meta-programming) which does
not sound like a good idea to me.

--
Erik Wikström

gw7rib

11/16/2008 10:09:00 PM

0

On 16 Nov, 21:42, Erik Wikström <Erik-wikst...@telia.com> wrote:
> On 2008-11-16 22:16, gw7...@aol.com wrote:
> > I have a program that needs to do a small amount of relatively simple
> > parsing. The routines I've written work fine, but the code using them
> > is a bit long-winded.
>
> > I therefore had the idea of creating a class to do parsing. It could
> > be used as follows:
>
> > int a, n, x, y;
> > Parser par;
> > par << string;
> > if (par >> "From" >> ' ' >> x >> ' ' >> "to" >> ' ' >> y) a = 1;
> > else if (par >> "Number" >> ' ' >> n) a = 2;
> > else a = 3;
>
> > Then if string is "From 3 to 5" this will set a=1, x=3, y=5. If the
> > string is "Number     2" this will set a=2 and n=2. If string is
> > "Other" then a=3. For convenience, I'll assume that an input of "From
> > 4 other" is allowed to alter the value of x while returning a=3.
>
> > I think I could write a class that would do this. It would need to
> > keep track of whether the current parsing was succeeding and, if so,
> > how far through the string it had got. It would need overloaded >>
> > operators, obviously, some of them taking references. And it would
> > need a conversion operator, which I think would need to be to void *,
> > which would not only return whether the current parse had succeeded
> > but would also reset the flag and counter ready for another attempt.
>
> > So my questions are, is this a sensible thing to try to do, and are
> > there any potential snags that I haven't spotted?
>
> If you need to parse a lot you should probably try a tool like yacc or
> some other parser-generator. If you only need to be able to parse a very
> small grammar (and want a good exercise) you can try to write the state-
> machine by hand.

I don't think I'm going to be doing that much parsing, though I'll
bear that in mind if i do.

> You example looks like a runtime-construct (though, perhaps you can make
> it compile-time with some fancy template meta-programming) which does
> not sound like a good idea to me.

How my example works - par >> "text" will check to see whether the
next bit of the string to be parsed contains the characters "text".
par >> n will check to see if the next bit of the string is a number,
and if so, set n to that number. par >> ' ' will skip whitespace. The
routine doesn't build up a "template" of what the string is supposed
to look like, it just checks each bit of it in turn, as I would have
thought any parser needs to.

Thanks for any further thoughts.
Paul.

Joe Smith

11/17/2008 12:19:00 AM

0


Paul wrote:
>How my example works - par >> "text" will check to see whether the
>next bit of the string to be parsed contains the characters "text".
>par >> n will check to see if the next bit of the string is a number,
>and if so, set n to that number. par >> ' ' will skip whitespace. The
>routine doesn't build up a "template" of what the string is supposed
>to look like, it just checks each bit of it in turn, as I would have
>thought any parser needs to.

It is definately possible.

The only part that sticks out of your design as really weird is the
side effects of the conversion operator. I would prefer to have the
operator>> overloads return copies of the original with the changed
member variables. If you use a reference counting smart pointer for
the string your class would no larger than 4 integers on most
platforms (one for pointer, one for its reference count, one for the
position and less than 1 for the flag). The cost of copying four
integers is not terrible. If all the lines you want to parse are
fairly short like in your examples, you won't be making too many
copies. This is likely a reasonable tradeoff for avoiding the magic
in the operator void*().

In general though the returning copies is not scalable. On the other
hand your design has limited scalablility too, as advanced parsing
requires more sophisiticated techniques. But considerering your
examples, it sounds like you don't need a powerful parser, but
want something to parse simple strings, so all this might be just fine
for you.

James Kanze

11/17/2008 9:39:00 AM

0

On Nov 16, 11:09 pm, gw7...@aol.com wrote:
> On 16 Nov, 21:42, Erik Wikström <Erik-wikst...@telia.com> wrote:
> > On 2008-11-16 22:16, gw7...@aol.com wrote:
> > > I have a program that needs to do a small amount of
> > > relatively simple parsing. The routines I've written work
> > > fine, but the code using them is a bit long-winded.

> > > I therefore had the idea of creating a class to do
> > > parsing. It could be used as follows:

> > > int a, n, x, y;
> > > Parser par;
> > > par << string;
> > > if (par >> "From" >> ' ' >> x >> ' ' >> "to" >> ' ' >> y) a = 1;
> > > else if (par >> "Number" >> ' ' >> n) a = 2;
> > > else a = 3;

> > > Then if string is "From 3 to 5" this will set a=1, x=3,
> > > y=5. If the string is "Number 2" this will set a=2 and
> > > n=2. If string is "Other" then a=3. For convenience, I'll
> > > assume that an input of "From 4 other" is allowed to alter
> > > the value of x while returning a=3.

> > > I think I could write a class that would do this. It would
> > > need to keep track of whether the current parsing was
> > > succeeding and, if so, how far through the string it had
> > > got. It would need overloaded >> operators, obviously,
> > > some of them taking references. And it would need a
> > > conversion operator, which I think would need to be to
> > > void *, which would not only return whether the current
> > > parse had succeeded but would also reset the flag and
> > > counter ready for another attempt.

> > > So my questions are, is this a sensible thing to try to
> > > do, and are there any potential snags that I haven't
> > > spotted?

> > If you need to parse a lot you should probably try a tool
> > like yacc or some other parser-generator. If you only need
> > to be able to parse a very small grammar (and want a good
> > exercise) you can try to write the state- machine by hand.

> I don't think I'm going to be doing that much parsing, though
> I'll bear that in mind if i do.

> > You example looks like a runtime-construct (though, perhaps
> > you can make it compile-time with some fancy template
> > meta-programming) which does not sound like a good idea to
> > me.

> How my example works - par >> "text" will check to see whether
> the next bit of the string to be parsed contains the
> characters "text".

I think that that's what I really don't care for in it. One
expects >> to read, not to check.

What's wrong with just using boost::regex?

--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

Jerry Coffin

11/19/2008 12:11:00 AM

0

In article <52baba84-3b2a-40fc-b95b-
e8d9e3660c5a@a17g2000prm.googlegroups.com>, gw7rib@aol.com says...
> I have a program that needs to do a small amount of relatively simple
> parsing. The routines I've written work fine, but the code using them
> is a bit long-winded.
>
> I therefore had the idea of creating a class to do parsing. It could
> be used as follows:

Depending on what you're doing, I'd consider using a regular expression
library such as boost::regex, or a template-based parser generator such
as boost::Spirit 2.

--
Later,
Jerry.

The universe is a figment of its own imagination.

rwp

11/19/2008 8:16:00 AM

0

I wrote a class like that a few years ago and it turned out to be quite
useful

Example code:

string Part1, Part2, Key;
parse_str(Line) >> Part1 >> "%" >> Key >> "%" >> Part2 >> "";
if (Key.size() != 0) ...


The class was modelled after the Rexx parse command so it uses some
special strings like
"." for word
"10" to go to position 10 in the string
"+10" to go 10 positions forward in the string
"," to go to the next line in the string etc.

The construction of the class is as follows

parse_str(const string& in_s) ...
Constructor that just saves the string variable internally

//method that picks up integer variable to assign value to and returns the
object to enable
//continuing using >> operators
parse_str& operator>>(int& ival)
{
wordstep();
(this->*m_try_assign)();
m_pvar = (void*)&ival;
m_try_assign = &parse_str::try_assign_int;
m_wordmatch = 1;
return *this;
}

// method that recognizes special strings and search items
parse_str& operator>>(const char* in_psz)...

// method that converts a part of the parse string to an integer
int try_assign_int() ...


// variables

void* m_pvar; // pointer to variable to set value to
const string m_str; // string passed in as argument to constructor
int (t_parse_string::* m_try_assign)(void); function pointer to method
that assigns variable





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