# Writing code faster during Competitive Programming in C++

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


## 1. No more nested `min(x, min(y, ...))`
Use initializer list and `std::min` and `std::max` to make life easy
```cpp
small = min(x, min(y, min(z, k))); // the old way
small = min({x, y, z, k}); // life is easy
```

## 2. JavaScript like Destructuring using Structured Binding in C++
```cpp
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
```


----------------


## 3. 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.

```cpp
#define debug(x) cout << #x << " " << x 
int ten = 10;
debug(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.

```cpp
#define debug(x) cout << #x << " " << x 
#define debug2(x) cout << #x << " " << x << " "  << #y << " " << y 
#define debug3(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,

```cpp
#define debug(...) 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;
debug(xx); // prints "xx = 3"
debug(xx, yy, xxyy); // prints "xx, yy, xxyy = 3, 10, 103"
```


----------------


## 4. Generic Reader and Writer for multiple variables and containers
```cpp
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
```cpp
// 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]
	*/
```


----------------


## 5. Decorators in C++ and Multiple Parameters


### Printing as many variables in one line
```cpp
template<typename ...T>
void printer(T&&... args) {
    ((cout << args << " "), ...);
}

int age = 25;
string name = "Apurv";
printer("I am", name, ',', age, "years old"); 
// ^ This prints the following
// I am Apurv, 22 years old
```

### Powerful decorator functions in C++
```cpp
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.
```cpp
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
// ========
```
It's 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.

----------------

## 6. Using a Template

Using a template is one of the best ideas to speed up your implementation. Keep your template ready that has your macros written in them already. This helps in reducing a lot of your time which otherwise would have gone in writing the whole code from scratch.
I use the following template, you can use one that suits you.
```
#include<bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> pi;

#define F first
#define S second
#define PB push_back
#define MP make_pair
#define REP(i,a,b) for (int i = a; i < b; i++)
#define fastIO() ios_base::sync_with_stdio(0), cin.tie(0)

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");
}

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

const int MAX = 5E5 + 5;
const int MOD = 1E9 + 7;


void solve(){
    int tt;
    cin>>tt;
    while(tt--){
        // Your code goes here
    }
}
int main(){ 
   fastIO();
   solve();
}
```

Thank you for reading, please let me know if you have some possible improvements or addition.


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