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，但我无法理解为什么与 find 相比，equal_range 需要这么长时间。
我有一个排序的向量，我计算了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，但我期待随着数据量的增长，在向量上迭代会比二分搜索更糟糕，但这并没有发生。
我的代码非常简单，但我“怀疑”这里有问题，我不知道是什么。
-- 编辑 -- 我在 VS2012 中做这些测试 --
#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;
}
这次运行的输出是
-- 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