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cpp_tips_and_tricks.md

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C++ Tips and Tricks 😀

General ⚙️

  • tie(w,x,y,z) = make_tuple(10,20,30,40) :- w=10,x=20,y=30,z=40
  • unordered_set,unordered_map are faster than set and maps.
  • set uses self balancing binery search trees therefore O(nlogn).
  • unordered_set uses hashing therefore O(1) same for the map and unordered_map.
  • auto keyword can be used for declaring variables, it automatically set the type of that variable.
  • lower_bound : find >= elem , upper_bound : find > element
  • when we want to find element equal to we use lower_bound else upper_bound
  • is_sorted : Returns true if the container is sorted else false.
  • In Global we can declare array of bool upto 10^8 or int array upto 10^7.
  • In Local we can declare array of bool upto 10^7 or int array upto 10^6.

Faster operations ⏩⏩

  • a>>1 == divide a by 2.
  • a<<1 == multiply a with 2.
  • if (a&1)==0 == number is even.
  • if (a&1)==1 == number is odd, make sure you use the brackets.
  • swap numbers == a=a^b;b=a^b;a=a^b;
  • We should use emplace_back to push pair in vector insteed of pb(mp()) or pb({}).

Bit magic / Bit mask 0️⃣1️⃣

  • __builtin_popcount(n) == returns no. of set bits in n.
  • find i'th bit --> (n & (1<<i))==0 then ith bit is 0 else 1.
  • set i'th bit --> ( n | (1<<i)) then ith bit set to 1.
  • clear i'th bit --> (n & ~(1<<i)) then ith bit set to 0
  • mask = &(1<<i) for find.
  • mask = |(1<<i) for set-1.
  • mask = & ~(1<<i) for set-0.
  • (n&(n-1)) used in many ways :
    1. It reduces last set bit to 1. ex.1101->1100->1000->0000 then we can count no of set bits by count.
    2. we can find if a number is power of 2 or not -->if (n&(n-1))==0 then it is power of two else not.

std::string 🧵

  • temp.find(B) != string::npos, find B is in string or not.
  • s.insert(pos, string), insert string at pos position.

std::vector 🔼

  • v.erase(iterator) to delete elem while traversing in vector. Remember to do it--.
  • auto it = lower_bound(v.begin(), v.end(),elem);
  • int index = it - v.begin(); OR index = it - v;

std::set 🍁

  • auto it = s.lower_bound(elem);
  • int elem = *it;
  • s.count() returns 1 if element is present in set else 0.
  • NOTE: for vector and other containers which contains duplicate elements count returns count of the number.

std::map 🗺️

  • mp.count() returns 1 if element is present in map else 0.
  • NOTE: for vector and other containers which contains duplicate elements it returns count of the number.

Other tips 🍹

  1. We can use dec,hex,oct to print numbers in different bases.

    // bitset<bitcnt>(num)
cout << bitset<12>(n) << "\n"; // Binary
cout << oct << n << "\n"; // Octal
cout << hex << n << "\n"; // Hexadecimal
cout << dec << n << "\n"; // Decimal-normal

  2. std::minmax_element == returns pair{minelement, maxelement}

    vector<int> x = {1, 2, 3, 4};
auto it = minmax_element(all(x));
cout << *it.first << ' ' << *it.second << endl;
    output:
1 4

  3. is_palindrome function using std::equal.

    bool is_palindrome(string s) {
    return equal(all(s) - s.size() / 2, s.rbegin());
}

©️ prepared by chetan6780