C++_STL_Tools - RicoJia/notes GitHub Wiki

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1. cmath

   • ceil, floor
   • cos, sin

2. std::equal() 比较两个vector

3. limits

   #include <limits>
   cout << numeric_limits<double>::min() << endl;
   cout << numeric_limits<double>::max() << endl;
   double a = std::numeric_limits<double>::infinity(); //  > or < both gives you false!

   • != is false, == is true.
   • (for int you don't have a true infinity std::numeric_limits<int>max(); )
   • isinf:

     #include <cmath>
     isinf(double)

   • std::numeric_limits<double>::min()): this is 10 e^-308

4. std::abs() in only takes in integers, but in it takes in floats. you can use std::fabs() for floating point uses.

   • std::abs vs std::fabs
     • cmath's abs(-2.5) outputs 2. std::abs will output 2.5, because it has type overload.;
     • abs and fabs are there for backward compatibility. So just use std::abs., can be used directly

5. std::count(beg_itr, end_itr, 'target');

6. std::isnan()

7. std::numeric_limits::quiet_NaN();

8. std::plus

   #include <functional>   //std::plus
   //Definition
   template <typename T>
   struct plus{
       T operator(const T& x, const T& y){
           return x + y; 
         }
     }
   
   // Example
   std::transform(arr1, arr1 + 4, arr2, results, std::plus<int>);  //C++ 11
   std::transform(arr1, arr1 + 4, arr2, results, std::plus<>);  //C++ 14

   • standard operator's type can be omitted in C++14, so std::plus<int> -> std::plus<>

9. sqrt(1.5) 直接用

1. std::norm(complex<double>{motion_vector.at(i), motion_vector.at(i+1)}); finding squared norm.

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1. std::accumulate acceptable forms:
   1. simple summing

      #include <numeric>
      sum = std::accumulate(vec.begin(), vec.end(), 0.0); 

      • Note: sum will be casted to int, even if std::vector<double> vec :

      sum = std::accumulate(vec.begin(), vec.end(), 0); 

   2. Other accumulative tasks:

      T accumulate(Iterator first, Iterator Last, T init, Binary_Operation op){
        for(; first != last; ++first){
          init = op(std::move(init), *first); 
        }
      }

      • E.g,

      std::accumulate(vec.begin(), vec.end(), 1, [](int product, int b){return product*b}); 

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======================================================================== 2. max_element

#include <algorithm>
std::max_element(begin_itr, end_itr, policy)
**注意， 如果vector 自己是空的话，max_element 本身只是返回vec.end(), 但是你要dereference 的话，那么你就会有error！**

3. std::min()

   // find min between 2 nums
   #include <algorithm>
   std::min(a,b, [](A& a, B&b){return a.something \<\ b.something}; ); 
   // find min in a list
   std::min({a,b,c...} compare)  // returns the smallest in the initializer list!

   • std::min(unsigned int, int) is not allowed!

4. *在用partition, transform 等时，你的predicate 的argument是 (itr), 而不是iterator 本身

5. Transform

   1. unary operations

   // Definition
   transform(Iterator inputBegin, Iterator inputEnd, 
            Iterator OutputBegin, unary_operation) 
   // example: increment a number in an array
   int arr[] = {1,2,3,4}; 
   n = sizeof(arr)/sizeof(int); 
   std::transform(arr, arr+n, arr, [](int x){return x+1;})

   2. binary operation

   // Definition
   transform(Iterator inputBegin1, Iterator inputEnd1, Iterator inputBegin2, 
             Iterator outputBegin, binary_operation)
   // Example: arr_1 - arr 2 
   int arr_1[] = {1,2,3,4}; 
   int arr_2[] = {1,2,3,4}; 
   int result[5]; 
   std::transform(arr_1, arr_1+4, arr_2, result, [](int i1, i2){return i1 - i2; }); 

   • std::transform(It1 it1_begin, It1, it1_end, It2 it2_end, It3 out_begin, [](const Type1& a, const Type2&b){return something})

6. std::swap() in algorithm

7. For each

   • std::for_each

   #include<algorithm>
   std::for_each(begin(numbers), end(numbers), [](int num){do stuff});

   • for each loop: array will also return a reference/copy!

       int arr[] {1,2,3};
       for (int x : arr) cout<<x;

   • std::for_each

     #include <algorithm>
     void func (const int& i){};
     int main(){
       std::vector<int> vec {1,2,3};
       std::for_each(vec.begin(), vec.end(), &func);
     }

8. std::find_if

   #include <algorithm>
   std::vector<int>::iterator it = std::find_if(vec.begin(), vec.end(), if_true); 

9. std::partition: reorder elements [beg, end), elements satify the predicate will be put left (inside order NOT preserved). returns the first element of the second group.

   #include <algorithm>
   // Bad Example: 
   auto first_it = std::partition(ls.begin(), ls.end(), [&pivot](const int& i){return i < pivot; }); // is first_it the pivot? Not necessarily, because the order of the right-hand side might be screwed up!

   1. std::partition: all members in that range [) wil be modified, so don't change that range. Internally, it's also blocking.

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1. std::advance
   1. iterator only? yes.

   #include <iterator>
   std::advance(it, distance);

   2. what if out of limits?
   • if distance < 0, decrement
   • If distance out of bounds,undefined behavior

• increment for loop not by ++

  for(auto start = id; start < test_vec.size(); start += num_threads){}

2. reminder: for(auto i: map) returns a std::pair, not iterator! If returning iterator, then you need *it.

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1. std::hash: #include <functional>
   • works well with prime nums?

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1. Chrono lib addresses different systems have different clocks, and precision might be different. So it separates duration, and point of time from specific clocks.

   • is both the header file and the namespace
   • Contains all time-handling facilities used by thread library
   • Wall time vs CPU time, and a super good doc

2. clock:

   1. has:
      • std::chrono::steady_clock: cannot be adjusted? ticks at mono-rate, like a stop-watch
      • std::chrono::system_clock: same as the one in tool bar, so affected by "day-light saving"
      • std::chrono::high_resolution_clock: smallest possible tick period, high_resolution_clock may be a synonym for steady_clock and system_clock
      • except for steady_clock, adjustments are applied to accomodate local clock drift. So a call now() may return a value earlier than a previous now(). (think about multi-threaded case)
   2. class members:
      • now (time_point): time_point <system_clock> start = system_clock::now();, a static function
      • period: a ratio, tick period

        cout<<std::chrono::system_clock::period::num<<" | "<<std::chrono::system_clock::period::den<<endl;    // see 1 | 1000000000, 
        // std::ratio must be printed this way

        • Except for std::chrono::steady_clock, this period is not guaranteed to be the steady period!
      • is_steady

        cout<<std::chrono::system_clock::is_steady<<endl;

3. duration:

   1. std::chrono::duration<double>
   2. std::chrono::milliseconds, std::chrono::seconds, std::chrono::hours
      • are duration</*signed integer type of at least 45 bits*/, std::milli>
   3. Define your own type: - ratio is respective to second

      using short_min = std::chrono::duration<short, std::ratio<60,1>>    // a min in short int
      using double_ms = std::chrono::duration<double, std::ratio<1,1000>>    // a millisecond in double
      
      short_min sm(2);
      cout <<"sm count "<<sm.count()<<endl;     // see 2

   4. class members:
      1. count(): double for duration, int for milliseconds and others

         milliseconds m(30); 
         cout<<m.count()<<endl;    //30

         • time_since_epoch.count(): returns the number of periods passed, not in seconds!

           std::cout << "periods: " << dtn.count() << std::endl;
           std::cout << "seconds: " << dtn.count() * system_clock::period::num / system_clock::period::den;
           // periods: 1338280396212871
           // seconds: 1338280396

      2. period, which is a std::ratio, can be used for conversion

         m.count() * std::chrono::milliseconds::period::num / std::chrono::milliseconds::period::den   //(num for numerator, den for denominator, ALL IN INT, here this ratio is 1:1000, so RELATIVE TO SECONDS)

      3. arithmatic:

         30*std::chrono::seconds(1);  //equivalent to std::chrono::seconds(30)
         minutes(1) - seconds(55);     // get seconds(5)

4. conversions:

   • seconds -> timepoint: std::chrono::system_clock::time_point tp{std::chrono::seconds{timeSeconds}};
   • hours -> seconds, implicit, no explicit conversion required.
   • timepoint cast:

     auto start_ms = std::chrono::time_point_cast<std::chrono::milliseconds>(start_).time_since_epoch().count();

   • From seconds to hours, needs explicit conversion using duration_cast, and will be truncated

     std::chrono::milliseconds ms(54802); 
     std::chrono::seconds s = std::chrono::duration_cast<std::chrono::seconds>(ms);
     cout<<s.count()<<endl;      //see 54

   • time_since_epoch -> time in seconds (python's time.time()): seconds from epoch in double

     cout<<std::chrono::duration_cast<seconds>(system_clock::now().time_since_epoch()).count() / 1.0 <<endl;

   • duration -> time in double:

   duration<double> time_stamp = system_clock::now().time_since_epoch(); 
   payload["timestamp"] = time_stamp.count(); 

5. std::chrono::time_point<std::chrono::system_clock>

   • difference must be std::duration<double>

     std::chrono::time_point<std::chrono::system_clock> start, end; 
     start = std::chrono::system_clock::now(); 
     end = std::chrono::system_clock::now(); 
     std::chrono::duration<double> diff1 = end - start; 
     std::chrono::duration<int> diff2 = end - start;     //ERROR: not defined
     cout<<"elapsed time in seconds: "<<diff1.count()<<endl;

   • std::chrono::duration<int> = time_point::time_since_epoch()
   • std::chrono::time_point<std::chrono::system_clock, std::chrono::minutes> returns duration in minutes
   • can do std::chrono::high_resolution_clock::now() + std::chrono::seconds(500)

5. std::time_t: almost (not guaranteed) that will hold number of seconds since epoch

   std::time_t end_time = std::chrono::system_clock::to_time_t(end);     // see 1629678991

   • get formatted time:

   std::string get_formatted_time(const time_point<system_clock>& t) {
       // the format is like 23-Aug-2021-22-41-10
       time_t int_time = system_clock::to_time_t(t);
       tm* local_time = localtime(&int_time);
       char buffer[80];
       strftime(buffer, 80, "%d-%b-%Y-%H-%M-%S", local_time);
       return buffer;
   }

6. std::ctime(std::time_t*)

   cout<<std::ctime(&end_time)<<endl;  //Mon Aug 23 00:53:41 2021

7. there are SI ratios:

   • std::atto 10^-18
   • std::exa 10^18

8. Profile Timer - no "correct way", just what ppl use:

   struct Profiler_Timer{
      Profiler_Timer(const std::string& name = ""): name_(name){
          start_ = std::chrono::high_resolution_clock::now();  
      } 
   
      ~Profiler_Timer(){
         auto end = std::chrono::high_resolution_clock::now();
         auto start_us = std::chrono::time_point_cast<std::chrono::microseconds>(start_).time_since_epoch().count(); 
         auto end_us = std::chrono::time_point_cast<std::chrono::microseconds>(end).time_since_epoch().count(); 
         std::cout<<name_<<"finishes in "<< (end_us - start_us)/1000000.0<<"s"<<endl; 
      }
   
      std::chrono::time_point<std::chrono::high_resolution_clock> start_; 
      std::string name_; 
   };
   

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1. Classic Literals

   42    //int
   2.4   //double
   3.2F  //float
   'w'   //char
   32ULL //unsigned long long 
   0xD0  //Hex
   "cd"  //c-style string 

2. std::literals: No underscores

   • "Hello"s, std::string literal (not c-string literal!, C++14)

     #include <iostream>
     cout<<"Hello"s; 

   • 0b10: a binary number literal

     cout<<0b10; //you get 2;

   • complex 5i

     using namespace std::complex_literals;
     std::complex<double> z4 = 1.0 + 2i;   //ONLY double + 2i, no int!!
     std::cout << z4<< '\n';

   • std::chrono_literals: (C++14), Have to have using namespace std::chrono_literals
     • 5h, std::chrono::hours
     • 5min
     • 5s
     • E.g

       #include<chrono>
       int main(){
         using namespace std::chrono_literals; 
         cout<<1h/30min;       //you'll get 2
         std::this_thread::wait_for(40ms);
       }

3. User Defined Literals: UDL

   long double operator"" _kg(long double x){return 1000*x;} 
   long double operator"" _g(long double x){return x;} 
   long double weight = 3.6_kg; 
   3.6_kg/2_g; 

   • only suffix form is supported for UDL (e.g, _kg)
   • UDL is treated as a call to the literal operator ""