C++ tricks for competitive programming (for C++ 11)

We have discussed some tricks in the below post. In this post, some more tricks are discussed. Writing C/C++ code efficiently in Competitive programming Although, practice is the only way that ensures increased performance in programming contests but having some tricks up your sleeve ensures an upper edge and fast debugging. 

1) Checking if the number is even or odd without using the % operator: Although this trick is not much better than using the % operator but is sometimes efficient (with large numbers). Use & operator: 

CPP

if (num & 1)
   cout << "ODD";
else
   cout << "EVEN";

Example: num = 5 Binary: “101 & 1” will be 001, so true num = 4 Binary: “100 & 1” will be 000, so false. 

2) Fast Multiplication or Division by 2 Multiplying by 2 means shifting all the bits to left and dividing by 2 means shifting to the right. Example : 2 (Binary 10): shifting left 4 (Binary 100) and right 1 (Binary 1) 

CPP

// Multiply n with 2
n = n << 1;

// Divide n by 2
n = n >> 1;

3) Swapping of 2 numbers using XOR: This method is fast and doesn’t require the use of 3rd variable. 

CPP

// A quick way to swap a and b
a ^= b;
b ^= a;
a ^= b;

4) Avoiding the use of strlen(): 

CPP

//  Use of strlen() can be avoided by:
for (i=0; s[i]; i++)
{
}
// loop breaks when the character array ends.

5) Use of emplace_back() (Discussed here, here and here) Instead of push_back() in STL emplace_back can be used because it is much faster and instead of allocating memory somewhere else and then appending it directly allocates memory in the container. 

6) Inbuilt GCD function: C++ has an inbuilt GCD function and there is no need to explicitly code it. Syntax: __gcd(x, y); 

7) Using Inline functions: We can create inline functions and use them without having to code them up during the contest. Examples: (a) function for sieve, (b) function for palindrome. 

8) Maximum size of the array: We must be knowing that the maximum size of the array declared inside the main function is of the order of 10^6 but if you declare the array globally then you can declare its size up to 10^7. 

9) Calculating the most significant digit: To calculate the most significant digit of any number log can be directly used to calculate it. We can calculate the number of digits and then divide the number by 10^(number of digits-1)

Suppose the number is N then 
Let int K = log10(N); // number of digits in a number=log10(N)+1
And int X = pow(10, K);
Then int ans=N/X will be the most significant digit

C++

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

int MSD(int n){
      if(n == 0)
        return 0;
      int k = log10(n);
    int x = pow(10,k);
    int ans = n/x;
      return ans;
}

int main() {
    int n;
    cin >> n;
    cout << MSD(n) << endl;
    return 0;
}

10) Calculating the number of digits directly: To calculate the number of digits in a number, instead of looping you can efficiently use log :

Number of digits in N =floor(log10(N)) + 1;  

11) Efficient trick to know if a number is a power of 2 using the normal technique of division the complexity comes out to be O(logN), but it can be solved using O(v) where v is the number of digits of the number in binary form. 

CPP

/* Function to check if x is power of 2*/
bool isPowerOfTwo (int x)
{
  /* First x in the below expression is
    for the case when x is 0 */
  return x && (!(x&(x-1)));
}

12) C++11 has inbuilt algorithms for the following:

       // are all of the elements positive?
       all_of(first, first+n, ispositive()); 

      // is there at least one positive element?
      any_of(first, first+n, ispositive());

      // are none of the elements positive?
      none_of(first, first+n, ispositive()); 

Please refer Array algorithms in C++ STL (all_of, any_of, none_of, copy_n and itoa) for details. 

13) Copy Algorithm: used to copy elements from one container to another.

int source[5] = {0, 12, 34, 50, 80};
int target[5];
// copy 5 elements from source to target
copy_n(source, 5, target);

Please refer Array algorithms in C++ STL (all_of, any_of, none_of, copy_n and itoa) for details. 

14) The Iota Algorithm The algorithm iota() creates a range of sequentially increasing values, as if by assigning an initial value to *first, then incrementing that value using prefix ++. In the following listing, iota() assigns the consecutive values {10, 11, 12, 13, 14} to the array arr, and {‘a’, ‘b’, ‘c’} to the char array c[]. 

CPP

int a[5] = {0};
char c[3] = {0};

// changes a to {10, 11, 12, 13, 14}
iota(a, a+5, 10);
iota(c, c+3, 'a'); // {'a', 'b', 'c'}

Please refer Array algorithms in C++ STL (all_of, any_of, none_of, copy_n and itoa) for details. 

15) Initialization in Binary form: In C++ 11 assignments can also be made in binary form. 

CPP

// C++ code to demonstrate working of
// "binary" numbers
#include<iostream>
using namespace std;
int main()
{
    auto number = 0b011;
    cout << number;
    return 0;
}

Output :

3

16) Use of “and” Though not a very productive one, this tip helps you to just use conditional operator and instead of typing &. 

CPP

// C++ code to demonstrate working of "and"
#include<iostream>
using namespace std;
int main()
{
    int a = 10;
    if (a < 20 and a > 5)
      cout << "Yes";
    return 0;
}

Output :

Yes