我正在使用find，equal_range和我自己的二进制搜索函数进行一些测试，而且我很难理解为什么与find相比，equal_range需要这么长时间。
我有一个有序矢量，我计算搜索操作的持续时间。最初的想法是看看find和equal_range之间的性能差异是什么，但是我期待随着迭代越过向量而增加的数据量将比二进制搜索更糟糕而且没有发生。
我的代码非常简单，但我“怀疑”这里出了问题，我不知道是什么。
- 编辑 - 我在VS2012中进行这些测试 -
#include "stdafx.h"
#include <vector>
#include <algorithm>
#include <chrono>
#include <iostream>
#include <list>
using namespace std;
#define TIMING
#ifdef TIMING
#define INIT_TIMER auto start = std::chrono::high_resolution_clock::now();
#define START_TIMER start = std::chrono::high_resolution_clock::now();
#define STOP_TIMER(name) std::cout << name << ": " << \
std::chrono::duration_cast<std::chrono::nanoseconds>( \
std::chrono::high_resolution_clock::now()-start \
).count() << " ns " << std::endl;
#else
#define INIT_TIMER
#define START_TIMER
#define STOP_TIMER(name)
#endif
template<typename T> int mybsearch(const std::vector<T>& vec, unsigned start, unsigned end, const T& key)
{
// Termination condition: start index greater than end index
if(start > end)
{
return -1;
}
// Find the middle element of the vector and use that for splitting
// the array into two pieces.
unsigned middle = start + ((end - start) / 2);
if(vec[middle] == key)
{
return middle;
}
else if(vec[middle] > key)
{
return mybsearch(vec, start, middle - 1, key);
}
return mybsearch(vec, middle + 1, end, key);
}
template<typename Iterator, typename T> Iterator Tbsearch(Iterator& begin, Iterator& end, const T& key)
{
// Keep halving the search space until we reach the end of the vector
Iterator NotFound = end;
while(begin < end)
{
// Find the median value between the iterators
Iterator Middle = begin + (std::distance(begin, end) / 2);
// Re-adjust the iterators based on the median value
if(*Middle == key)
{
return Middle;
}
else if(*Middle > key)
{
end = Middle;
}
else
{
begin = Middle + 1;
}
}
return NotFound;
}
int _tmain(int argc, _TCHAR* argv[])
{
vector<int> V;
typedef vector<int>::iterator it;
std::pair<it,it> res;
it val;
list<int> L;
typedef list<int>::iterator itL;
std::pair<itL,itL> lpair;
itL lval;
for(int i=0; i<1000000; i++)
{
V.push_back(i+1);
L.push_back(i+1);
}
INIT_TIMER
cout << "-- find --\n";
for(int k=0;k<10;k++)
{
int look = pow(k,k);
START_TIMER
val = std::find(V.begin(),V.end(),look);
STOP_TIMER("find took ")
if(val!=V.end())
cout << look << " found\n";
else
cout << look << " not found\n" ;
}
cout << "-- homemade bsearch (index) --\n";
for(int k=0;k<10;k++)
{
int look = pow(k,k);
START_TIMER
int a = mybsearch(V, 0, V.size()-1, look);
STOP_TIMER("find took ")
if(a>0)
cout << look << " found\n";
else
cout << look << " not found\n" ;
}
cout << "-- homemade bsearch (iterators) --\n";
for(int k=0;k<10;k++)
{
int look = pow(k,k);
START_TIMER
it f = Tbsearch(V.begin(), V.end(), look);
STOP_TIMER("find took ")
if(f!=V.end())
cout << look << " found\n";
else
cout << look << " not found\n" ;
}
cout << "-- equal range --\n";
for(int k=0;k<10;k++)
{
int look = pow(k,k);
START_TIMER
res = std::equal_range(V.begin(),V.end(),look);
STOP_TIMER("equal_range took ")
if(res.first!=res.second)
cout << look << " found\n";
else
cout << look << " not found\n" ;
}
return 0;
}
此次运行的输出是
-- find --
find took : 0 ns
1 found
find took : 0 ns
1 found
find took : 0 ns
4 found
find took : 0 ns
27 found
find took : 0 ns
256 found
find took : 0 ns
3125 found
find took : 0 ns
46656 found
find took : 2000200 ns
823543 found
find took : 2000200 ns
16777216 not found
find took : 2000200 ns
387420489 not found
-- homemade bsearch (index) --
find took : 0 ns
1 not found
find took : 0 ns
1 not found
find took : 0 ns
4 found
find took : 0 ns
27 found
find took : 0 ns
256 found
find took : 0 ns
3125 found
find took : 0 ns
46656 found
find took : 0 ns
823543 found
find took : 0 ns
16777216 not found
find took : 0 ns
387420489 not found
-- homemade bsearch (iterators) --
find took : 0 ns
1 found
find took : 0 ns
1 found
find took : 0 ns
4 found
find took : 0 ns
27 found
find took : 0 ns
256 found
find took : 0 ns
3125 found
find took : 1000100 ns
46656 found
find took : 1000100 ns
823543 found
find took : 0 ns
16777216 not found
find took : 0 ns
387420489 not found
-- equal range --
equal_range took : 683068300 ns
1 found
equal_range took : 681068100 ns
1 found
equal_range took : 681068100 ns
4 found
equal_range took : 679067900 ns
27 found
equal_range took : 679067900 ns
256 found
equal_range took : 680068000 ns
3125 found
equal_range took : 683068300 ns
46656 found
equal_range took : 677067700 ns
823543 found
equal_range took : 680068000 ns
16777216 not found
equal_range took : 678067800 ns
387420489 not found
干杯
答案 0 :(得分：0)
在VS2012的调试版本中我得到了
-- equal range --
equal_range took : 796079600 ns
1 found
equal_range took : 792079200 ns
1 found
equal_range took : 785078500 ns
4 found
equal_range took : 786078600 ns
27 found
equal_range took : 785078500 ns
256 found
equal_range took : 784078400 ns
3125 found
equal_range took : 785078500 ns
46656 found
equal_range took : 786078600 ns
823543 found
equal_range took : 784078400 ns
16777216 not found
equal_range took : 784078400 ns
387420489 not found
而在发布版本中我得到了
-- equal range --
equal_range took : 0 ns
1 found
equal_range took : 0 ns
1 found
equal_range took : 0 ns
4 found
equal_range took : 0 ns
27 found
equal_range took : 0 ns
256 found
equal_range took : 0 ns
3125 found
equal_range took : 0 ns
46656 found
equal_range took : 0 ns
823543 found
equal_range took : 0 ns
16777216 not found
equal_range took : 0 ns
387420489 not found
迭代器检查调试版本会减慢它的速度。