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the FOREACH macro

 
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Guest
PostPosted: Sun Dec 31, 2006 12:04 am    Post subject: the FOREACH macro Reply with quote


Hi,

I would like to do something like this, which is the WISH case:

Quote:
typedef map<int, double> hash;
hash a;
// ...
FOREACH(a, key, value, hash)
std::cout << key << "->" << value << "\n";

I already can do something similar, which is the ALREADY-HAVE case:

Quote:
FOREACH(hash, i, hash)
std::cout << i->first << "->" << i->second << "\n";

with a macro that I defined this way (inspired by the iteration macros
in the Boost Graph Library):

Quote:
#define FOREACH(MNAME, INAME, MapType) \
for (MapType::iterator \
INAME = MNAME.begin(); INAME != MNAME.end(); ++INAME)

The complete source code for the example above is at the bottom of this
message.

You may rightfully argue that this macro makes little sense, because
you can just have a regular loop there. However, in my real code
(what I gave you here is just a show case) there are two loops and I
want to put then into a macro.

Now, the problem is that I can't declare in the for-loop the variables
key and value as shown in the WISH case. I don't have this problem in
the ALREADY-HAVE case, becasue I only have to declare an iterator.

Thanks for reading. Any ideas how to get closer to my WISH case?


Thanks & best,
Irek

*********************************************

#include <iostream>
#include <map>

#define FOREACH(MNAME, INAME, MapType) \
for (MapType::iterator \
INAME = MNAME.begin(); INAME != MNAME.end(); ++INAME)

using namespace std;

typedef map<int, double> hash;

int main()
{
hash a;

a[0] = 1.71;
a[1] = 3.14;

FOREACH(a, i, hash)
cout << i->first << "->" << i->second << "\n";

return 0;
}


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Guest
PostPosted: Mon Jan 01, 2007 2:21 am    Post subject: Re: the FOREACH macro Reply with quote


irek.szczesniak (AT) gmail (DOT) com wrote:

Quote:
Hi,

I would like to do something like this, which is the WISH case:

typedef map<int, double> hash;
hash a;
// ...
FOREACH(a, key, value, hash)
std::cout << key << "->" << value << "\n";

Hi,

It might be something like this?

#define FOREACH(A, Key, Value, Hash) \
for (Hash::iterator INAME = A.begin(); INAME != A.end(); ++INAME) \
for (bool continue_ = true; continue_;) \
for (Hash::key_type Key = INAME->first; continue_;) \
for (Hash::mapped_type Value = INAME->second;
continue_; continue_ = false) \
/**/

I of course recommand to use BOOST_FOREACH Smile
http://www.boost-consulting.com/vault/index.php?directory=Algorithms

--
Shunsuke Sogame


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Guest
PostPosted: Mon Jan 01, 2007 9:43 pm    Post subject: Re: the FOREACH macro Reply with quote


Hi John,

Thanks for your input. However, using std::for_each complicates things
too much: I want to see beneath the FOREACH loop what's happening in my
code and not to read the function object.


Best,
Irek

John L Fjellstad wrote:

Quote:
irek.szczesniak (AT) gmail (DOT) com writes:

Hi,

I would like to do something like this, which is the WISH case:

typedef map<int, double> hash;
hash a;
// ...
FOREACH(a, key, value, hash)
std::cout << key << "->" << value << "\n";

Would it be easier to use for_each()?

Something like this:
#########
#include <iostream
#include <map
#include <cstdlib

struct PrintOut
{
private:
int key_;

public:
PrintOut( int key ) : key_(key) {}

void operator() ( const std::pair<int, double>& p )
{
if (key_ == p.first)
std::cout << p.first << "->" << p.second << "\n";
}
};

int
main()
{
std::map<int, double> a;

a.insert( std::make_pair<int,double>(0, 1.71) );
a.insert( std::make_pair<int,double>(1, 3.14) );

std::for_each( a.begin(), a.end(), PrintOut(1) );

return EXIT_SUCCESS;
}
#########


--
John L. Fjellstad
web: http://www.fjellstad.org/ Quis custodiet ipsos custodes
Replace YEAR with current four digit year
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Guest
PostPosted: Mon Jan 01, 2007 9:43 pm    Post subject: Re: the FOREACH macro Reply with quote
Hi Shunsuke,

This is awesome! It's interesting how you introduced the local loop
variables Key and Value. It's a nice trick. Naturally, there is the
performance penalty for this trick: the continue_ variable is
initialized, then evaluated twice, and then a value is assigned.
Nonetheless, I'll pay this penalty just to have a more readable code.

And the BOOST_FOREACH looks interesting, and I'll look closer at it.


Best,
Irek

pstade.mb (AT) gmail (DOT) com wrote:

Quote:
irek.szczesniak (AT) gmail (DOT) com wrote:

Hi,

I would like to do something like this, which is the WISH case:

typedef map<int, double> hash;
hash a;
// ...
FOREACH(a, key, value, hash)
std::cout << key << "->" << value << "\n";

Hi,

It might be something like this?

#define FOREACH(A, Key, Value, Hash) \
for (Hash::iterator INAME = A.begin(); INAME != A.end(); ++INAME) \
for (bool continue_ = true; continue_;) \
for (Hash::key_type Key = INAME->first; continue_;) \
for (Hash::mapped_type Value = INAME->second;
continue_; continue_ = false) \
/**/

I of course recommand to use BOOST_FOREACH Smile
http://www.boost-consulting.com/vault/index.php?directory=Algorithms

--
Shunsuke Sogame
--

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Daniel K. O.
Guest
PostPosted: Wed Jan 03, 2007 12:49 am    Post subject: Re: the FOREACH macro Reply with quote
irek.szczesniak (AT) gmail (DOT) com escreveu:
Quote:
This is awesome! It's interesting how you introduced the local loop
variables Key and Value. It's a nice trick. Naturally, there is the
performance penalty for this trick: the continue_ variable is
initialized, then evaluated twice, and then a value is assigned.
Nonetheless, I'll pay this penalty just to have a more readable code.

Even if C++ haters say otherwise, the C++ compilers whe have today are
much more than "high-level assemblers".

I just compiled that code with "gcc -O1" (4.1.1) and checked the
generated assembly. There's nothing there that ressembles any variable
other than the iterator itself (tested with a std::map).

I doubt that any mainstream compiler will fail to do such basic
optimization.


--
Daniel K. O.

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