Here I will add some interesting C++/Python tricks I learn from around the internet (ex - cppcon, and pycon)

• Links are valid cpp comments

  void z_function() {
    https://cp-algorithms.com/string/z-function.html
    // ^ here https acts as a empty goto line and rest followed by // is a comment
  }
  

• The snippet

  int main()
  {
    int a = 42;
    printf(“%d\n”, a);
  }
  

  May compile in C (without explicit declaration of #include <stdio.h>). A proper C compiler will create an implicit declaration for printf() and compile this into an object file. When linked with Standard library, it will find a definition of printf() that accidentally matches the implicit declaration.

A C++ compiler will refuse to compile this code as the language requires explicit declaration of all functions. From Deep C and Deep C++

• The program

  #include <stdio.h>
  struct mystruct {
    int enabled:1;
  }
  int main() 
  {
    struct mystruct s;
    s.enabled = 1;
    if (s.enabled == 1) {
        printf("Is enabled \n");
    } else {
        printf("Is disabled\n");
    }
  }
  

  will print Is disabled!. See explanation

• Short and concise way to map count of characters in a string str

  map<char, int> count;
  for (auto ch : str) {
    count[ch]++;
  }
  for (auto e : count) {
    cout << e.first << " " << e.second << "\n";
  }
  

• C++17 supports init statements inside if and switch. For example

  set<int> input = {1, 5, 3, 6};
  if (auto it = input.find(7); it == input.end()) {
    cout << "7 not found!\n";
  } else {
    cout << "7 found!\n";
  }
  

• Using tie to give multiple variable values

  vector<tuple<int, int, string>> aa;
  for (int i = 0; i < 5; ++i) {
    aa.push_back(make_tuple(-i, i*i, "str-" + to_string(i)));
  }
  sort(begin(aa), end(aa));
  int u, v;
  string name;
  for (auto e : aa) {
    tie(u, v, name) = e;
    cout << u << " " << v << " " << name << "\n";
  }
  

• for loops can have an else in Python. It is slightly weird construct, and not very recommended to use

  found_obj = None
  for obj in objects:
    if obj.key == search_key:
        found_obj = obj
        break
  else:
    print('No object found.')
  

• valarray<> can be used to write and execute numerical computation code efficiently. Please ignore the // lines. Apparently the code highlighter has issues processing blank lines in code.

  #include <iostream>
  #include <vector>
  #include <functional>
  #include <valarray>
  //
  using namespace std;
  //
  int main() {
  valarray<double> w{1, 3, 0, -1, 0.2};
  valarray<double> x{2, 2.5, 3, 0.1, 1./3};
  valarray<double> b{0, 0, -1, -1, 1};
  w *= 0.5; // 2, 6, 0, -2, 0.4
  b = 1.0 - b; // 1, 1, 2, 2, 0
  //
  valarray<double> logits = w*x + b;
  valarray<double> explogits = exp(logits);
  double z = explogits.sum();
  valarray<double> softmax = explogits / z;
  softmax[softmax > 0.5] = 1;
  softmax[softmax < 0.01] = 0;
  //
  //0.0533795 1 0.0533795 0.0507762 0
  for (auto e : softmax) {
    cout << e << " ";
  }
  cout << "\n";
  //
  return 0;
  }
  

• Finding route through grid
• CGAL Linear programming example

• SVD Demo LAPACK

• Cppcon talks