Writing C/C++ code efficiently in Competitive programming

First of all you need to know about Template, Macros and Vectors before moving on the next phase! 

• Templates are the foundation of generic programming, which involve writing code in a way that is independent of any particular type.
• A Macro is a fragment of code which has been given a name. Whenever the name is used, it is replaced by the contents of the macro.
• Vectors are same as dynamic arrays with the ability to resize itself automatically when an element is inserted or deleted, with their storage being handled automatically by the container.

So we can use these powerful tools for writing our code in a effective way.
Some of the cool tricks that may be used in Competitive programming are given as follows: 

• Using Range based for loop: This is very cool feature in C++11 and would be considered best if you want to iterate from begins to end. This code shows how to use ranged for loops to iterate through an array and a vector: 

// C++ program to demonstrate range based for
// loops for accessing vector and array elements
#include<iostream>
#include <vector>
using namespace std;

int main()
{
    // Create a vector object that
    // contains 5 elements
    vector<int> vec = {0, 1, 2, 3, 4};

    // Type inference by reference using auto.
    // Range based loops are preferred when no
    // modification is needed in value
    for (const auto &value : vec)
        cout << value << ' ';

    cout << '\n';

    // Basic 5 element integer array
    int array[]= {1, 2, 3, 4, 5};
    for (const auto &value: array)
        cout << value << " ";

    return 0;
}

Output:

0 1 2 3 4
1 2 3 4 5

• Initializer list: This type is used to access the values in a C++ initialization list. Here the objects of this type are automatically constructed by the compiler from initialization list declarations, which is a list of comma-separated elements enclosed in braces. 

#include<iostream>

template<typename T>
void printList(std::initializer_list<T> text)
{
    for (const auto & value: text)
        std::cout << value << " ";
}

// Driver program
int main()
{
    // Initialization list
    printList( {"One", "Two", "Three"} );
    return 0;
}

Output: 

One Two Three

• Assigning Maximum or Minimum value: This one is useful to avoid extra effort in writing max() or min() function. 

#include<iostream>

// Call by reference is used in x
template<typename T, typename U>
static inline void amin(T &x, U y)
{
    if (y < x)
        x = y;
}

// call by reference is used in x
template<typename T, typename U>
static inline void amax(T &x, U y)
{
    if (x < y)
        x = y;
}

// Driver program to find the Maximum and Minimum value
int main()
{
    int max_val = 0, min_val = 1e5;
    int array[]= {4, -5, 6, -9, 2, 11};

    for (auto const &val: array)

        // Same as max_val = max (max_val, val)
        // Same as min_val = min (min_val,val)
        amax(max_val, val), amin (min_val, val);


    std::cout << "Max value = " << max_val << "\n"
              << "Min value = " << min_val;
    return 0;
}

Output:

Max value = 11
Min value = -9

• Fast Input/Output in C/C++: In Competitive programming, you must read Input/Output as fast as possible to save valuable time. 

#include <bits/stdc++.h>

template<typename T> void scan(T &x)
{
    x = 0;
    bool neg = 0;
    register T c = getchar();

    if (c == '-')
        neg = 1, c = getchar();

    while ((c < 48) || (c > 57))
        c = getchar();

    for ( ; c < 48||c > 57 ; c = getchar());

    for ( ; c > 47 && c < 58; c = getchar() )
        x= (x << 3) + ( x << 1 ) + ( c & 15 );

    if (neg) x *= -1;
}

template<typename T> void print(T n)
{
    bool neg = 0;

    if (n < 0)
        n *= -1, neg = 1;

    char snum[65];
    int i = 0;
    do
    {
        snum[i++] = n % 10 + '0';
        n /= 10;
    }

    while (n);
    --i;

    if (neg)
        putchar('-');

    while (i >= 0)
        putchar(snum[i--]);

    putchar('\n');
}

// Driver Program
int main()
{
    int value;

    // Taking input
    scan(value);

    // Printing output
    print(value);
    return 0;
}

Input:  756
Output: 756

To know more about fast input and output Read this article . 

• Using Macros as for loop: Perhaps, it would not be good to use such macros as it would reduce the readability of code but for writing fast code you can take that risk! 

#include <bits/stdc++.h>
using namespace std;

#define rep(i,n) for (i = 0; i < n; ++i)
#define REP(i,k,n) for (i = k; i <= n; ++i)
#define REPR(i,k,n) for (i = k; i >= n; --i)


// Driver program to test above Macros
int main()
{
    int i;
    int array[] = {4, 5, 6, 9, 22, 11};
    int size= sizeof(array)/sizeof(array[0]);
    
    // Default 0 index based loop
    rep(i, size)     
        cout << array[i] << " ";
    cout<<"\n";
    
    // Starting index based loop
    REP(i, 1, size-1)     
        cout << array[i] << " ";
    cout<<"\n";
    
    // Reverse for loop
    REPR(i, size-1,0)     
        cout << array[i] << " ";
    return 0;
}

Output 

4 5 6 9 22 11
5 6 9 22 11
11 22 9 6 5 4

• Using "bits/stdc++.h": Instead of adding tons of #include lines, just use #include<bits/stdc++.h> The files includes all the header files you'll need in competitive programming, saving a lot of your time.
• Containers: Using various containers like vector, list, map etc enables one to use the pre-defined functions and reduces the size of code considerably (more often than not)
• Fast cin and cout: If you use cin and cout for I/O, just add the following line just after the main(). 

std::ios_base::sync_with_stdio(false);

• auto: Using auto to declare datatypes can save lot of time during programming contests. When a variable is defined as auto, compiler determines its type during compile-time.
• Libraries and pre-defined functions: Using builtin functions such as __gcd(A,B), swap, _builtin_popcount(R), _builtin_clz(R) etc wherever that can be applied. Try to learn different functions available in algorithm library of C++.They are useful most of the times in programs

Ultimately, by using these smart tricks you can easily write code in a minimum amount of time and words.