foreach question - Boost-users - lists.preview.boost.org

newer
[thread] documentation complaints

foreach question

older
[Variant] Tree like structure with...

Neal Becker

15 Oct 2010 15 Oct '10

6:27 p.m.

The following loop iterates over strings in result (which comes from boost::split). Is there a way to include the integer enumerator 'col' into the FOREACH loop? In python that would be spelled for col,s in enumerate (result): ... int col = 0; BOOST_FOREACH (std::string s, result) { H(row, col) = boost::lexical_cast<int> (s); ++col; }

Reply

Sign in to reply online Use email software

Show replies by date

Pete Bartlett

15 Oct 15 Oct

7:35 p.m.

The following loop iterates over strings in result (which comes from boost::split). Is there a way to include the integer enumerator 'col' into the FOREACH loop?

In python that would be spelled for col,s in enumerate (result): ...

int col = 0; BOOST_FOREACH (std::string s, result) { H(row, col) = boost::lexical_cast<int> (s); ++col; }

See this thread http://thread.gmane.org/gmane.comp.lib.boost.user/54162 And the resulting Trac ticket: https://svn.boost.org/trac/boost/ticket/3469 for a essentially negative answer.

Reply

Sign in to reply online Use email software

Jerry Jeremiah

17 Oct 17 Oct

8:49 p.m.

Pete Bartlett writes:

See this thread http://thread.gmane.org/gmane.comp.lib.boost.user/54162 And the resulting Trac ticket: https://svn.boost.org/trac/boost/ticket/3469

for a essentially negative answer.

Personally, I like the answer in that thread that said: #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) The for loop just does a variable declaration in a way that means you won't get bit by using it somewhere that doesn't allow variable declarations. Convoluted but important. Of course, it isn't that generic - it always assumes you want an int index. But then that is what the question said... Jerry

Reply

Sign in to reply online Use email software

Neal Becker

11:59 p.m.

Jerry Jeremiah wrote:

Pete Bartlett writes:

...
See this thread http://thread.gmane.org/gmane.comp.lib.boost.user/54162 And the resulting Trac ticket: https://svn.boost.org/trac/boost/ticket/3469

for a essentially negative answer.

Personally, I like the answer in that thread that said:

#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)

The for loop just does a variable declaration in a way that means you won't get bit by using it somewhere that doesn't allow variable declarations. Convoluted but important.

Of course, it isn't that generic - it always assumes you want an int index. But then that is what the question said...

Jerry

Won't win any beauty contest.

Reply

Sign in to reply online Use email software

Rob Riggs

21 Oct 21 Oct

2:45 p.m.

On 10/15/2010 12:27 PM, Neal Becker wrote:

The following loop iterates over strings in result (which comes from boost::split). Is there a way to include the integer enumerator 'col' into the FOREACH loop?

In python that would be spelled for col,s in enumerate (result): ...

int col = 0; BOOST_FOREACH (std::string s, result) { H(row, col) = boost::lexical_cast<int> (s); ++col; }

Using c++0x: auto myrange = make_iterator_range( boost::make_zip_iterator( boost::make_tuple( boost::counting_iterator(0), result.begin())), boost::make_zip_iterator( boost::make_tuple( boost::counting_iterator(result.size()), result.end()))); BOOST_FOREACH(auto t, myrange) { using boost::tuples::get; H(row, get<0>(t)) = boost::lexical_cast<int>(get<1>(t)); } Which, unfortunately, is quite a bit more verbose and less clear than what you have now. zip iterators and counting iterators are really nice concepts, but they don't make for clear, compact code. And just imagine what that would look like without the type inference provided by c++0x's "auto" keyword. However, the making of the zip iterators for this purpose is such a common idiom in other languages (including Python), and this is easily abstracted into a helper function, that this would be an ideal enhancement to Boost. Something like: template <typename Range> auto make_counting_range(Range& range, size_t start = 0) -> decltype(make_iterator_range( boost::make_zip_iterator( boost::make_tuple( boost::counting_iterator(start), boost::begin(range))), boost::make_zip_iterator( boost::make_tuple( boost::counting_iterator( boost::distance(range) + start), boost::end(range))))) { return make_iterator_range( boost::make_zip_iterator( boost::make_tuple( boost::counting_iterator(start), boost::begin(range))), boost::make_zip_iterator( boost::make_tuple( boost::counting_iterator( boost::distance(range) + start), boost::end(range)))); } Which would then give you: BOOST_FOREACH(auto t, make_counting_range(result)) { using boost::tuples::get; H(row, get<0>(t)) = boost::lexical_cast<int>(get<1>(t)); } (Compiled with GCC 4.4.) Rob

Reply

Sign in to reply online Use email software

5138

Age (days ago)

5144

Last active (days ago)

Download

4 comments

4 participants

tags

participants (4)

Jerry Jeremiah
Neal Becker
Pete Bartlett
Rob Riggs