Modern Cpp Tricks Save

Introduction

This is the list of modern CPP tricks often used in Coding Interviews and Competitive Programming.
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Contents:

• No more nested min(x, min(y, ...))
• JavaScript like Destructuring using Structured Binding in C++
• Powerful Logging and Debugging
  • How debug macros work?
  • The Problem with this macro - its not scalable
  • Solution using a powerful macro
• Generic Reader and Writer for multiple variables and containers
  • Usage
• Decorators in C++ and Multiple Parameters
  • Live Demo on YouTube
  • Printing as many variables in one line
  • Powerful decorator functions in C++
  • Exploiting decorators by nesting them
• Sets and Maps Tricks
  • s.contains vs s.find(...) != s.end
  • multiset.extract vs multiset.erase(multiset.find(...))

No more nested min(x, min(y, ...))

Use initializer list and std::min and std::max to make life easy

small = min(x, min(y, min(z, k))); // the old way
small = min({x, y, z, k}); // life is easy

JavaScript like Destructuring using Structured Binding in C++

pair<int, int> cur = {1, 2};
auto [x, y] = cur;
// x is now 1, y is now 2
// no need of cur.first and cur.second

array<int, 3> arr = {1, 0, -1};
auto [a, b, c] = arr;
// a is now 1, b is now 0, c is now -1

Powerful Logging and Debugging

How debug macros work?

Straight to the point, I have often used the debug macro which stringifies the variable names and their values.

#define deb(x) cout << #x << " " << x 
int ten = 10;
deb(ten); // prints "ten = 10"

This is often useful in debugging.

The Problem with this macro - its not scalable

However, when you have multiple variables to log, you end up with more deb2 and deb3 macros.

#define deb(x) cout << #x << " " << x 
#define deb2(x) cout << #x << " " << x << " "  << #y << " " << y 
#define deb3(x, y, z) cout << #x << " " << x << " "  << #y << " " << y << " "  << #z << " " << z 

This is not scalable.

Solution using a powerful macro

Here is the solution using variadic macros and fold expressions,

#define deb(...) logger(#__VA_ARGS__, __VA_ARGS__)
template<typename ...Args>
void logger(string vars, Args&&... values) {
    cout << vars << " = ";
    string delim = "";
    (..., (cout << delim << values, delim = ", "));
}

int xx = 3, yy = 10, xxyy = 103;
deb(xx); // prints "xx = 3"
deb(xx, yy, xxyy); // prints "xx, yy, xxyy = 3, 10, 103"

Generic Reader and Writer for multiple variables and containers

template <typename... T>
void read(T &...args) {
    ((cin >> args), ...);
}

template <typename... T>
void write(string delimiter, T &&...args) {
    ((cout << args << delimiter), ...);
}

template <typename T>
void readContainer(T &t) {
    for (auto &e : t) {
        read(e);
    }
}

template <typename T>
void writeContainer(string delimiter, T &t) {
    for (const auto &e : t) {
        write(delimiter, e);
    }
    write("\n");
}

Usage

// Question: read three space seprated integers and print them in different lines.
	int x, y, z;
	read(x, y, z);
	write("\n", x, y, z);
	
// even works with variable data types :)
	int n;
	string s;
	read(n, s);
	write(" ", s, "has length", n, "\n");
	
// Question: read an array of `N` integers and print it to the output console.
	int N;
	read(N);
	vector<int> arr(N);
	readContainer(arr);
	writeContainer(" ", arr); // output: arr[0] arr[1] arr[2] ... arr[N - 1]
	writeContainer("\n", arr);
	/**
	* output:
	* arr[0]
	* arr[1]
	* arr[2]
	* ...
	* ...
	* ...
	* arr[N - 1]
	*/

Decorators in C++ and Multiple Parameters

Live Demo on YouTube

• Check the online demo on YouTube here on Rachit's channel.

Printing as many variables in one line

template<typename ...T>
void printer(T&&... args) {
    ((cout << args << " "), ...);
}

int age = 25;
string name = "Rachit";
printer("I am", name, ',', age, "years old"); 
// ^ This prints the following
// I am Rachit, 25 years old

Powerful decorator functions in C++

template<typename F>
auto debug_func(const F& func) {
    return [func](auto &&...args) { // forward reference
        cout << "input = ";
        printer(args...);
        auto res = func(forward<decltype(args)>(args)...);
        cout << "res = " << res << endl;
        return res;
    };
}

debug_func(pow)(2, 3);
// ^ this automatically prints
// input = 2 3 res = 8

Exploiting decorators by nesting them

Lets define another decorator beautify as follows.

template<typename F>
auto beautify(const F& func) {
    return [func](auto &&...args) { // forward reference
        cout << "========" << endl;
        func(forward<decltype(args)>(args)...);
        cout << "========" << endl;
    };
}

beautify(debug_func(pow(2, 3)));
// ^ this now prints
// ========
// input = 2 3 res = 8
// ========

Its amazing how much you can do by writing such generic decorators and nest them.
Think about decorators like log_time that calculates the time taken for a given function.

Sets and Maps Tricks

s.contains vs s.find(...) != s.end

set<int> example{1, 2, 3, 4};
example.find(3) != example.end() // true
example.contains(3) // true

Note that this works with map as well as set.

multiset.extract vs multiset.erase(multiset.find(...))

multiset<int> mset{1, 1, 2, 2, 3, 3};
mset.erase(1); // {2, 2, 3, 3} deleted all 1s
mset.erase(mset.find(2)) // {2, 3, 3} need to use erase + find
mset.extract(3) // {2, 3} simple and clean

• multiset::extract helps remove single instance naturally.
• Note this also works with normal set and map.
• It returns a node handle