C ++:使用向量/数组优化速度? [英] C++: Optimizing speed with vector/array?
问题描述
我有一个嵌套的for循环结构,现在我在每次迭代开始时重新声明向量:
I have a nested for-loop structure and right now I am re-declaring the vector at the start of each iteration:
void function (n1,n2,bound,etc){
for (int i=0; i<bound; i++){
vector< vector<long long> > vec(n1, vector<long long>(n2));
//about three more for-loops here
}
}
这使我能够开始新鲜每次迭代,这是伟大的,因为我的内部操作大部分是vec [a] [b] + =一些值的形式。但我担心大n1或大n2的速度很慢。我不知道向量/数组/等的底层架构,所以我不知道什么最快的方式是处理这种情况。我应该使用数组吗?我应该清除不同?我应该完全不同地处理逻辑?
This allows me to "start fresh" each iteration, which works great because my internal operations are largely in the form of vec[a][b] += some value. But I worry that it's slow for large n1 or large n2. I don't know the underlying architecture of vectors/arrays/etc so I am not sure what the fastest way is to handle this situation. Should I use an array instead? Should I clear it differently? Should I handle the logic differently altogether?
编辑:向量的大小技术上不会改变每次迭代(但它可能会根据函数参数改变)。我只是试图清除它/ etc所以程序是尽可能快的人类可能在所有其他情况下。
The vector's size technically does not change each iteration (but it may change based on function parameters). I'm simply trying to clear it/etc so the program is as fast as humanly possible given all other circumstances.
编辑:
我的不同方法的结果:
Timings (for a sample set of data):
reclaring vector method: 111623 ms
clearing/resizing method: 126451 ms
looping/setting to 0 method: 88686 ms
推荐答案
下面是一些测试几种不同方法的代码。
Here is some code that tests a few different methods.
#include <chrono>
#include <iostream>
#include <vector>
int main()
{
typedef std::chrono::high_resolution_clock clock;
unsigned n1 = 1000;
unsigned n2 = 1000;
// Original method
{
auto start = clock::now();
for (unsigned i = 0; i < 10000; ++i)
{
std::vector<std::vector<long long>> vec(n1, std::vector<long long>(n2));
// vec is initialized to zero already
// do stuff
}
auto elapsed_time = clock::now() - start;
std::cout << elapsed_time.count() << std::endl;
}
// reinitialize values to zero at every pass in the loop
{
auto start = clock::now();
std::vector<std::vector<long long>> vec(n1, std::vector<long long>(n2));
for (unsigned i = 0; i < 10000; ++i)
{
// initialize vec to zero at the start of every loop
for (unsigned j = 0; j < n1; ++j)
for (unsigned k = 0; k < n2; ++k)
vec[j][k] = 0;
// do stuff
}
auto elapsed_time = clock::now() - start;
std::cout << elapsed_time.count() << std::endl;
}
// clearing the vector this way is not optimal since it will destruct the
// inner vectors
{
auto start = clock::now();
std::vector<std::vector<long long>> vec(n1, std::vector<long long>(n2));
for (unsigned i = 0; i < 10000; ++i)
{
vec.clear();
vec.resize(n1, std::vector<long long>(n2));
// do stuff
}
auto elapsed_time = clock::now() - start;
std::cout << elapsed_time.count() << std::endl;
}
// equivalent to the second method from above
// no performace penalty
{
auto start = clock::now();
std::vector<std::vector<long long>> vec(n1, std::vector<long long>(n2));
for (unsigned i = 0; i < 10000; ++i)
{
for (unsigned j = 0; j < n1; ++j)
{
vec[j].clear();
vec[j].resize(n2);
}
// do stuff
}
auto elapsed_time = clock::now() - start;
std::cout << elapsed_time.count() << std::endl;
}
}
更新了代码,以便在方法之间进行更公平的比较。
编辑2 :清理代码有点,方法2或4是要走的路。
Edit: I've updated the code to make a fairer comparison between the methods. Edit 2: Cleaned up the code a bit, methods 2 or 4 are the way to go.
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关键是你应该尝试不同的方法和
The point is that you should try out different methods and profile your code.
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