linux數組長度

A. Linux求數組長度為10000的浮點數(精確小數點右4位)計算值

既然題主沒有說要求用什麼語言，那我就用c++11實現了。

#include<iostream>

#include<random>

#include<thread>

#include<chrono>

#include<algorithm>

#include<iomanip>

usingnamespacestd;

constintsize=10000;

floattable[size];

intmain(){

random_deviceengine;

uniform_real_distribution<float>dist(0,1);

floatsum;

for(auto&i:table){

i=dist(engine);

}

autot_start=chrono::system_clock::now();

sum=accumulate(table,table+size,0.0);

autot_end=chrono::system_clock::now();

autoration=std::chrono::ration_cast<std::chrono::microseconds>(t_end-t_start).count();

cout<<"sumofthemainthread:"<<fixed<<setprecision(4)<<sum<<endl;

cout<<"timeelapsed:"<<ration<<"microseconds"<<endl;

floatsum_child[4];

autofun=[&](intindex){

sum_child[index]=accumulate(table+index*size/4,table+(index+1)*size/4,0.0);

};

t_start=chrono::system_clock::now();

threadthrd_table[4]={

thread(fun,0),thread(fun,1),thread(fun,2),thread(fun,3)

};

for(auto&thrd:thrd_table){

thrd.join();

}

sum=0;

sum=accumulate(sum_child,sum_child+4,0.0);

t_end=chrono::system_clock::now();

ration=std::chrono::ration_cast<std::chrono::microseconds>(t_end-t_start).count();

cout<<"sumofchildthreads:"<<fixed<<setprecision(4)<<sum<<endl;

cout<<"timeelapsed:"<<ration<<"microseconds"<<endl;

return0;

}

編譯：

g++-std=c++11test.cc-lpthread-otest

運行：

./test

結果：

sumofthemainthread:4976.8721

timeelapsed:0ms

sumofchildthreads:4976.8721

timeelapsed:0ms

由於隨機性每次加和的數值不同，但是精確到毫秒時，時間測出來妥妥的都是零。就是數據量太小，實際運行時間在微秒量級，當然看不出來。

精度改為微秒以後：

sumofthemainthread:4957.9878

timeelapsed:113microseconds

sumofchildthreads:4957.9878

timeelapsed:560microseconds

多線程反而比單線程慢，因為啟動線程本身也需要時間。

數據量再增大1000倍：

sumofthemainthread:4999892.0000

timeelapsed:25313microseconds

sumofchildthreads:4999892.0000

timeelapsed:8986microseconds

這回看著正常多了吧