From: boostusr@pbjzone.com

On 2/9/2012 11:26 PM, John M. Dlugosz wrote:

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On 2/9/2012 3:08 PM, Bill Buklis wrote:

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What am I missing? How can count_if be properly expressed as count. To me it seems like it can't.

The value to compare against would be part of the 'pred', either hard-coded into it, or bound as an argument to a function that takes two arguments.

That makes sense, but how would that work? For example, let's say I wanted to count the number of non-zero values in an array. With count_if I can easily do this:

namespace bll = boost::lambda; int data[] = { 1, 0, 2, 0, 3, 0, 4, 0, 5, 0 }; size_t k = boost::count_if(data, bll::_1 != 0);

The documentation implies that this is possible with boost::count using a filtered adaptor. Is it?

This line isn't valid: size_t n = boost::count(data | boost::adaptors::filtered(bll::_1 != bll::_2), 0);

and this line will return 0 matches: size_t n = boost::count(data | boost::adaptors::filtered(bll::_1 != 0), 0);

I'm sure I must be missing the obvious somewhere. Thanks in advance for enlightening me.

I don't think you're missing anything, I think the example is wrong. A working alternative to: boost::count_if(data, bll::_1 != 0) would be: boost::distance(data | filtered(bll::_1 != 0)) since boost::distance counts the total number of elements in the range. Regards, Nate

On 2/12/2012 7:03 AM, John M. Dlugosz wrote:

On 2/10/2012 11:39 AM, Bill Buklis wrote:

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What am I missing? How can count_if be properly expressed as count. To me it seems like it can't.

I agree. "count returns the number of elements x in rng where x == val is true." If you filter the original range using the predicate instead, you don't want to then compare them all against some constant. If you did so, you would need a special value such that every one of them compared true.

You just want to count the number of items in the range you get after filtering. The obvious boost::size requires a bidirectional range, and is probably less efficient then we would want.

Nathan mentioned boost::distance, which is hard to find in the documentation. But I see it listed on the synopsis under Forward Range functions. I guess it's missing from the page doc/range/concepts/forward_range.html

In general, it is true that having two algorithms "do something with or to each element" and "same, but selecting elements via a predicate" can be generalized to using the first (only) with a range filter.

But count and count_if are not such a pair. count also has a built-in predicate, but of the form el==value, rather than acting implicitly on all the elements.

—John

Indeed, the documentation must be wrong. Hopefully it can be appropriately fixed soon. That is an excellent suggestion about using distance. I had not thought of that, nor, come to think of it, did I even know that there was a boost::distance function. But, it makes sense. On the other hand, I do have to wonder if using "distance" for "count" would obfuscate the code. In general, I think I would rather use count_if just for clarity. But, at least the statement that the "_if functions aren't required", still holds true even if it may not be rather obvious which one to use. -- Bill

Hi Bill, On Tue, Feb 14, 2012 at 11:26 AM, Bill Buklis wrote:

struct data {    int a;    int b; };

// function object that takes two data parameters - contents unimportant for the example struct match_data;

std::vector<data>    data_range; data                            search_value;

boost::find_if( data_range, boost::bind(match_data(), _1, search_value) );

Interesting question. How about: boost::begin(data_range | filtered(boost::bind(match_data(), _1, search_value))); ? HTH, Nate P.S. Had to compile it to prove it to myself: (cygwin g++ 4.5.3, old boost trunk) $ cat simple.cpp #include #include #include #define BOOST_TYPEOF_SILENT #include #include <iostream> #include <vector> using boost::adaptors::filtered; struct data { int a; int b; }; struct match_data { typedef bool result_type; bool operator()(const data& lhs, const data& rhs) { return lhs.a == rhs.a; } }; int main() { std::vector<data> data_range; data search_value; #if 0 data data1; data1.a = 3; data data2; data1.a = 5; data_range.push_back(data1); data_range.push_back(data2); search_value.a = 5; #endif BOOST_AUTO(it, boost::begin(data_range | filtered( boost::bind(match_data(), _1, search_value))).base()); if(it != data_range.end()) std::cout << "Found: " << it->a; else std::cout << "Not found."; }

Den 14-02-2012 21:16, Nathan Ridge skrev:

The broader point, I think, is that the statement in the documentation for range adaptors that they obsolete the "_if" variants of algorithms, is misleading at best.

Thanks to all for the discussion and issues raised. We'll moderate the discussion somewhat, and provide better examples. If there are more examples of stuff that is hard to do, please keep posting them. kind regards -Thorsten