C++ Hacks for Competitive programming

There are lot of C++ Hacks you can use to save time in competitive programming. Here are few of them.

• Getting rid of “includes”: Just use
  #include <bits/stdc++.h>
• Useful inbuilt functions:
  • __gcd(a, b): Returns the GCD of a and b
  • __builtin_popcount(x) : Returns the number of set bits in x
• Initializer Lists: These make STL containers easy to initialize, for eg:
  • vector<int> odd_nos = {1, 3, 5, 7, 9};
  • pair<int, string> p = {1, “Hi”}; //Equiv. to p=make_pair(1, "Hi")
  • map<int, string> m = { {1, “This”}, {2, “is”}, {3, “awesome”} };
• Finding min and max of multiple variables:

//Long Way:
 
int max_of_3 = max(a, max(b, c));
int max_of_4 = max(max(a, b), max(c, d));
 
//Easier Way - Can be extended to any number of variables:
 
int max_of_3 = max({a, b, c});
int max_of_4 = max({a, b, c, d});

• Range-based for loops: Makes it much simpler to iterate through containers.

//Old Way:
for(auto it=container.begin(), it!=container.end(); it++)
	cout<<*it<<" ";
 
//Alternatively:
for(int i=0;i<container.size();i++) //If the container is a vector
	cout<<container[i]<<" ";
 
//Easier Way:
for(auto &it:container) //Using & also allows us to modify the elements
	cout<<it<<" ";

• Tie and Swap:
  • Tie makes it easier to initialize multiple variables in a single line
  • Swap enables swapping of variables, and even containers with a single statement

//Initializes a with -1, b with 1, etc
 tie(a, b, c, d) = make_tuple(-1, 1, -2, 2);

//x, y can be two integers, or two vectors, or any two containers
swap(x, y);

• Macros:
  • If you are tired of typing some statement like push_back again and again, you can just use #define pb push_back , and type pb in your code.
  • Getting the name of the argument in macro using #, and using it to debug your program:

#define trace(x) cerr<<#x<<": "<<x<<" "<<endl;
 
int main()
{
	int a=5;
	trace(a); //prints: "a: 5" to stderr stream 
}

• Variadic Function Templates:

• C++ allows templates with variable number of arguments. If you are tired of writing trace1, trace2, trace3, etc for different number of variables you want to print in a line, then you can read about Variadic Functions Templates here.
• Code for debugging variable number of parameters: Here.