Re: [Boost-users] [foreach][range] How would BOOST_REVERSE_FOREACHbe implemented?

7 Aug 2007

      Hmm  I happen to use the idea of replacing begin with rbegin and iterators
with reverse_iterator, etc Doing this I wrote a simple test file to compare
performance. To my surprise doing std::reverse(v.begin(), v.end()) and than
using BOOST_FOREACH is actually faster than a hard brute force method of
BOOST_FOREACH_REV.

Test ran with mingw compiler 3.4.5

Heres the code.
#include <vector>
#include <iostream>
#include <algorithm>
#include 
#include 
#include 

int main()
{
       const long TOTAL_ELEMENT = 10000;
       std::cout << boost::format("%f\t%f\n") % "BOOST_FOREACH" %
"BOOST_FOREACH_REV";
       boost::timer t;
       std::vector<int> vec_int;
       for (int j = 1; j < 50; ++j)
     {
       vec_int.reserve(TOTAL_ELEMENT*j);
       double t1, t2;
       for (int i = 0; i < TOTAL_ELEMENT*j; ++i)
       {
           vec_int.push_back(i);
       }
       t.restart();
       std::reverse(vec_int.begin(), vec_int.end());
       BOOST_FOREACH(int& i, vec_int)
       {
             i -= 1;
       }
       t1 = t.elapsed();
       t.restart();
       BOOST_FOREACH_REV(int& i, vec_int)
       {
             i -= 1;
       }
       t2 = t.elapsed();
       std::cout << boost::format("%f\t%f\n") % t1 % t2;
    }
}

/*
BOOST_FOREACH    BOOST_FOREACH_REV
0.001000    0.001000
0.002000    0.003000
0.002000    0.005000
0.004000    0.005000
0.005000    0.006000
0.003000    0.009000
0.010000    0.008000
0.008000    0.010000
0.008000    0.010000
0.008000    0.014000
0.009000    0.016000
0.016000    0.014000
0.010000    0.019000
0.011000    0.017000
0.013000    0.017000
0.013000    0.018000
0.014000    0.022000
0.016000    0.020000
0.016000    0.021000
0.017000    0.023000
0.017000    0.025000
0.018000    0.031000
0.018000    0.026000
0.020000    0.034000
0.019000    0.030000
0.022000    0.037000
0.021000    0.033000
0.024000    0.032000
0.023000    0.039000
0.025000    0.033000
0.026000    0.039000
0.026000    0.046000
0.029000    0.040000
0.028000    0.039000
0.030000    0.042000
0.030000    0.051000
0.032000    0.051000
0.035000    0.059000
0.035000    0.054000
0.033000    0.055000
0.035000    0.059000
0.035000    0.119000
0.035000    0.062000
0.037000    0.059000
0.037000    0.057000
0.037000    0.063000
0.072000    0.061000
0.039000    0.062000
0.039000    0.060000
*/

On 8/6/07, chun ping wang  wrote:
...
HI, yeah i wanted to do reverse iteration lookup so I took liberty to edit
the file of foreach.hpp. Its ugly but it "works". I haven't done any
detail testing.
I only test it on basic string.. so i am sure of that.
On 7/1/07, Pete Bartlett  wrote:
...
...
You're not the first to want a REVERSE_FOREACH, but this is the first
legitimate argument I've heard for it. Could you go to
http://svn.boost.org/trac and open a new ticket there. Make the ticket
type "feature request" and the component "foreach". I'll look into it
when I have a chance.
Thanks very much for your interest, Eric.
I've set up a ticket on the issue which you can view at
http://svn.boost.org/trac/boost/ticket/1071
Pete Bartlett
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