ubuntu 命令 cd - 功能
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Ubuntu/Linux File & Directory Commands
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代码：
#include <iostream>
#include <opencv2/core.hpp>
#include <opencv2/videoio.hpp>
#include <opencv2/highgui.hpp>
using namespace std;
using namespace cv;
using namespace std;
int main(int argc, char* argv[])
{
cout<<"OpenCV4 Example."<<endl;
cout<<"OpenCV Version = "<<getVersionString()<<endl;
webcam();
return 0;
}
void webcam()
{
Mat frame;
VideoCapture cap;
int deviceID = 0; // 0 = open default camera
int apiID = cv::CAP_ANY; // 0 = autodetect default API
cap.open(deviceID, apiID);
if (!cap.isOpened()) {
cerr << "ERROR! Unable to open camera\n";
return;
}
cout << "Start grabbing" << endl << "Press any key to terminate" << endl;
for (;;)
{
cap.read(frame);
if (frame.empty()) {
cerr << "ERROR! blank frame grabbed\n";
break;
}
imshow("Live", frame);
if (waitKey(40) >= 0)
break;
}
}运行结果：
./opencv-demo
OpenCV4 Example.
OpenCV Version = 4.5.2
Start grabbing
Press any key to terminate运行效果：
代码：
#include <iostream>
#include <opencv2/core.hpp>
#include <opencv2/videoio.hpp>
#include <opencv2/highgui.hpp>
#include "opencv2/objdetect.hpp"
#include <opencv2/imgproc.hpp>
using namespace std;
using namespace cv;
void face_detect()
{
Mat frame;
VideoCapture cap;
cap.open("http://ivi.bupt.edu.cn/hls/cctv2.m3u8");
if (!cap.isOpened()) {
cerr << "ERROR! Unable to open video\n";
return;
}
double fps = cap.get(cv::CAP_PROP_FPS);
cout << "fps = "<< fps <<endl;
//获取cap视频流的每帧大小
int width = cap.get(cv::CAP_PROP_FRAME_WIDTH);
int height = cap.get(cv::CAP_PROP_FRAME_HEIGHT);
cout<<"width = "<<width <<", height = "<<height<<endl;
Size S(width, height);
Scalar blue(255, 0, 0);
CascadeClassifier classifier;
classifier.load("./xml/haarcascade_frontalface_alt.xml");
std::vector<Rect> detected_objects;
int cnt = 0;
for (;;)
{
cap.read(frame);
if (frame.empty()) {
cerr << "ERROR! blank frame grabbed\n";
break;
}
if(cnt++ % 5 == 0)
{
classifier.detectMultiScale( frame, detected_objects);
for ( size_t i = 0; i < detected_objects.size(); i++ )
{
rectangle(frame, detected_objects[i], blue, 3);
}
}
imshow("Live", frame);
if (waitKey(38) >= 0)
break;
}
}运行结果：
./opencv-demo OpenCV4 Example.
OpenCV Version = 4.5.2
[NULL @ 0x556838ec4680] non-existing SPS 0 referenced in buffering period
[NULL @ 0x556838ec4680] SPS unavailable in decode_picture_timing
[h264 @ 0x556838eed000] non-existing SPS 0 referenced in buffering period
[h264 @ 0x556838eed000] SPS unavailable in decode_picture_timing
fps = 25
width = 720, height = 576
[h264 @ 0x55683916bfc0] error while decoding MB 30 33, bytestream -5
[h264 @ 0x556838f67040] error while decoding MB 23 1, bytestream -15
[h264 @ 0x556838fcca00] error while decoding MB 30 14, bytestream -11
[h264 @ 0x55683916bfc0] error while decoding MB 9 27, bytestream -9运行效果：
<bits/stdc++.h>
万能的头文件bits/stdc++.h,几乎包含所有的可用到的C++库函数。以后写代码就可以只引用这一个头文件。
/** @file stdc++.h
* This is an implementation file for a precompiled header.
*/
文件内容：
// 17.4.1.2 Headers
// C
#ifndef _GLIBCXX_NO_ASSERT
#include <cassert>
#endif
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#if __cplusplus >= 201103L
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdalign>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cuchar>
#include <cwchar>
#include <cwctype>
#endif
// C++
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#if __cplusplus >= 201103L
#include <array>
#include <atomic>
#include <chrono>
#include <codecvt>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#endif
#if __cplusplus >= 201402L
#include <shared_mutex>
#endif
使用万能头文件有利有不利，但优点肯定大于缺点。
优点
缺点
TLE是（time limit exceeded）缩写，超出时间限制。
关于输入输出优化有一条让不要用cin/cout，而是用scanf和printf.
Change methods of Input-Output: You must choose proper input-output functions and data structure that would help you in optimization.
In C++, do not use cin/cout – use scanf and printf instead.
其实在看了分析文章《C++的輸出入cin/cout和scanf/printf誰比較快？》
只需要注意到以下三个地方，就可以愉快的使用cin/cout了。
1：sync_with_stdio 函數：和stdio同步
std::ios_base::sync_with_stdio(false);
2：endl 和 flush 物件：cout的缓冲区优化
简单点就是使用cout<'\n'替换cout<<endl;
3：cin.tie(0)：cin和cout綁定
cin.tie(0);
最后是测试代码
ios_base::sync_with_stdio(false);
cin.tie(0);
for(int i = 0; i < (int)1e7; i++){
cin>>a;
cout<<a+1<<'\n';
}
for(int i = 0; i < (int)1e7; i++){
scanf("%d\n",&a);
printf("%d\n",a+1);
}