On Sun, Dec 13, 2009 at 11:51 PM, Eric Niebler wrote:

The Dude wrote:

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Hello,

Boost::foreach is very useful for iterating over a sequence and doing something that depends only on the iterated element, e.g., <code> BOOST_FOREACH(const Foo &f, foos) cout << f.bar() << endl; </code>

However, I often need to iterate over a sequence and do some operation that depends on both the iterated element and the iteration index. E.g., I would like something like <code> BOOST_FOREACH(size_t i, const Foo &f, foos) cout << "The bar of element " << i << " is " << f.bar() << endl; </code>

Is there an easy way to do so?

Why not:

int index = 0;

BOOST_FOREACH(const Foo &f, foos) { // ... stuff ... ++index; }

?

-- Eric Niebler BoostPro Computing http://www.boostpro.com

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Hello, Thanks for you answer. I'm not sure how to answer the "why not"? The code you write certainly will work, but so would the predecessor to BOOST_FOREACH in the first place, no? So here's my attempt: 1. For shorter loops, this changes 2 LOCs to 5. 2. For longer loops, the iteration code changes its meaning if it appears before the ++index or after. 3. The variable index has scope outside the loop. 4. Other people think so, e.g., the author's of D language http://en.wikipedia.org/wiki/D_(programming_language)#Example_1 It's true that none of these points is really a proof. Still, I'd be really happy to hack my own INDEX_FOREACH, but the 500+ LOCs of BOOST_FOREACH left me daunted. Thanks & Bye, TD

Eric Niebler wrote:

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Why not:

int index = 0;

BOOST_FOREACH(const Foo &f, foos) { // ... stuff ... ++index; }

What value BOOST_FOREACH has comes primarily from its ability to simplify something that's already pretty darn simple -- a plain for(;;) loop. I strongly resist any effort to make BOOST_FOREACH more complicated, unless the wins are truly significant. In this case, I don't think they are.

Maybe this shouldn't be a BOOST_FOREACH feature at all. As with Python's builtin enumerate() function, maybe what we want here is an iterator adapter that dereferences to a std::pair?

On Sun, Dec 13, 2009 at 7:11 PM, OvermindDL1 wrote:

On Sun, Dec 13, 2009 at 6:41 PM, Nat Goodspeed wrote:

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Eric Niebler wrote:

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...

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   Why not:

    int index = 0;

    BOOST_FOREACH(const Foo &f, foos)     {       // ... stuff ...       ++index;     }

...

What value BOOST_FOREACH has comes primarily from its ability to simplify something that's already pretty darn simple -- a plain for(;;) loop. I strongly resist any effort to make BOOST_FOREACH more complicated, unless the wins are truly significant. In this case, I don't think they are.

Maybe this shouldn't be a BOOST_FOREACH feature at all. As with Python's builtin enumerate() function, maybe what we want here is an iterator adapter that dereferences to a std::pair?

Ooo, that makes sense, as long as it can still be done quickly (ala the in index = 0; example above): BOOST_FOREACH(size_t i, const Foo &f, ENUMERATE(foos))

Or maybe: BOOST_FOREACH_ENUMERATE(size_t i, const Foo &f, foos)

Er, meant my first example to be: BOOST_FOREACH(tuple t, ENUMERATE(foos))

Nat Goodspeed wrote:

Maybe this shouldn't be a BOOST_FOREACH feature at all. As with Python's builtin enumerate() function, maybe what we want here is an iterator adapter that dereferences to a std::pair?

Couldn't you just do a BOOST_FOREACH over combine(count_range(), foos) for that? (uses zip_iterator)

2009/12/13 The Dude :

  It's true that none of these points is really a proof. Still, I'd be really happy to hack my own INDEX_FOREACH, but the 500+ LOCs of BOOST_FOREACH left me daunted.

It's not that hard: #define INDEX_FOREACH(index,a,b) \ for(unsigned int index = static_cast<unsigned int>(-1); \ index == static_cast<unsigned int>(-1);) \ BOOST_FOREACH(a,b) if(++index,true) INDEX_FOREACH(i, const Foo &f, foos) cout << "The bar of element " << i << " is " << f.bar() << endl;