C++ STL cheatsheet for competitive progrmming

cheatsheet.cpp

/*
This a header file that includes every standard library.
You can use it to save time.
NOTE: This header file may not be recognized by compilers
other than gcc.
*/
#include <bits/stdc++.h>

/*
//Use this if the above header file doesn't work.

#include <iostream>

#include <algorithm>
#include <list>
#include <map>
#include <priority_queue>
#include <stack>
#include <string>
#include <vector>
*/

using namespace std;


// These will be used later
struct Person {
	string name;
	int age;
} p1, p2, p3;

struct is_older {
	bool operator()(struct Person p1, struct Person p2) {
    	return p1.age > p2.age;
    }
};

bool compare_names(struct Person p1, struct Person p2) {
    	return p1.name < p2.name;
}

bool way_to_sort(int i, int j) { return i > j; }


int main() {

	/*
	========
	 STACK
	========
	*/

	stack <string> distros; //Create a stack of strings.

	distros.push("Ubuntu");  //Pushing elements into the stack.
	distros.push("Mint");

	cout << "Number of distros in the stack are " << distros.size() << endl;
	cout << "Distro on the top is " << distros.top() << endl;

	distros.pop();
	cout << "The top of the stack is now " << distros.top() << endl;

	/*
	========
	 VECTOR
	========
	*/

	vector <int> numbers;

	if (numbers.empty()){ //check if the vector is empty?
		cout << "The vector is empty :(" << endl;
	}

	for(int i=0; i<100; i+=10){ //Add some values to the vector
		numbers.push_back(i);
	}

	cout << "Size of the vector is " << numbers.size() << endl;

	// iterating over the vector, declaring the iterator
	vector <int>::iterator it;

	cout << "The vector contains: ";
	for (it=numbers.begin(); it!=numbers.end(); it++) {
		cout << "  " << *it;
	}

	// getting value at particular position
	int position = 5;
	cout<<"\nVector at position "<<position<<" contains "<<numbers.at(position)<<endl;

	// deleting an element at position
	numbers.erase(numbers.begin() + position);
	cout<<"Vector at position "<<position<<" contains "<<numbers.at(position)<<endl;

	// deleting a range of elements, first two elements
	// NOTE: You may expect elements at 0, 1 and 2 to be deleted
	// but index 2 is not inclusive.
	numbers.erase(numbers.begin(), numbers.begin()+2);
	cout << "The vector contains: ";
	for (it=numbers.begin(); it!=numbers.end(); it++) {
		cout << "  " << *it;
	}

	// Clearing the vector
	numbers.clear();
	if (numbers.empty()){
		cout << "\nThe vector is now empty again :(";
	}


	/*
	=========
	 HASHMAP
	=========
	*/

	// Declaration <key type, value type>
	map <string, string> companies;

	companies["Google"] = "Larry Page";
	companies["Facebook"] = "Mark Zuckerberg";

	// insertion can also be done as
	companies.insert(pair<string, string> ("Xarvis Tech", "xarvis"));
	// or
	companies.insert(map<string,string>::value_type("Quora", "Adam D'Angelo"));
	// or even
	companies.insert(make_pair(string("Uber"), string("Travis Kalanick")));

	// Iterating the map
	map<string, string>::iterator itz;
	cout << "\n\nCompanies and founders" << endl;
	for (itz=companies.begin(); itz!=companies.end(); itz++){
		cout << "Company: " << (*itz).first << "\t Founder: " << itz->second <<endl;
	}

	itz = companies.find("Google");
	cout << itz->second;
        
        /* Check if key exists!

        if ( m.find("f") == m.end() ) {
              // not found
        } else {
             // found
        }
        */


    /*
    ==============
    LINKED LISTS
    ==============
    */
    list<int> mylist;
    list<int>::iterator it1,it2,itx;

    // set some values:
    for (int i=1; i<10; ++i) mylist.push_back(10*i);

                                // 10 20 30 40 50 60 70 80 90
    it1 = it2 = mylist.begin(); // ^^
    advance (it2,6);            // ^                 ^
    ++it1;                      //    ^              ^

    it1 = mylist.erase (it1);   // 10 30 40 50 60 70 80 90
                                //    ^           ^

    it2 = mylist.erase (it2);   // 10 30 40 50 60 80 90
                                //    ^           ^

    ++it1;                      //       ^        ^
    --it2;                      //       ^     ^

    mylist.erase (it1,it2);     // 10 30 60 80 90

    cout << "\nmylist contains:";
    for (itx=mylist.begin(); itx!=mylist.end(); ++itx)
        cout << ' ' << *itx;
    cout << '\n';

    // NOTE: it1 still points to 40, and 60 is not deleted
    cout << endl << *it1 << "\t" << *it2 <<endl;

    // This will print an unexpected value
    it1++;
    cout << *it1;

    cout << "\nmylist now contains:";
    for (it1=mylist.begin(); it1!=mylist.end(); ++it1)
        cout << ' ' << *it1;
    cout << '\n';

    /*
    =======
    HEAPS
    =======
    */

    // Creates a max heap
    priority_queue <int> pq;

    // To create a min heap instead, just uncomment the below line
    // priority_queue <int, vector<int>, greater<int> > pq;

    pq.push(5);
    pq.push(1);
    pq.push(10);
    pq.push(30);
    pq.push(20);

    // Extracting items from the heap
    while (!pq.empty())
    {
        cout << pq.top() << " ";
        pq.pop();
    }


    // creating heap from user defined objects
    // Let's initialize the properties of `Person` object first
    p1.name = "Linus Torvalds";
    p1.age = 47;

    p2.name = "Elon Musk";
    p2.age = 46;

    p3.name = "Me!";
    p3.age = 19;

    // Initialize a min heap
    // Note: We defined a comparator is_older in the beginning to
    // compare the ages of two person.
    priority_queue <struct Person, vector<struct Person>, is_older> mh;
    mh.push(p1);
    mh.push(p2);
    mh.push(p3);

    // Extracting items from the heap
    while (!mh.empty())
    {
    	struct Person p = mh.top();
        cout << p.name << " ";
        mh.pop();
    }

    /*
    =========
    SORTING
    =========
    */

    // The following list type initialization is only supported after C++11
    //vector<int> int_vec = {56, 32, -43, 23, 12, 93, 132, -154};

    // If the above style of initialization doesn't work, use the following one
    static int arr[] = {56, 32, -43, 23, 12, 93, 132, -154};
    int arr_len = sizeof(arr) / sizeof(arr[0]);
    vector <int> int_vec(arr, arr + arr_len);

    cout << endl;
    // Default: sort ascending
    // sort(int_vec.begin(), int_vec.end());
    // To sort in descending order:
    // Do not include the () when you call wayToSort
    // It must be passed as a function pointer or function object
	sort(int_vec.begin(), int_vec.end(), way_to_sort);
    for (vector <int>::iterator i = int_vec.begin(); i!=int_vec.end(); i++)
        cout << *i << " ";
    cout << endl;

    // sorting the array
    sort(arr, arr + arr_len);
    for (int i=0; i < arr_len; i++) {
    	cout << arr[i] << " ";
    }

    // Sorting user-defined objects
    static struct Person persons[] = {p1, p2, p3};
    sort(persons, persons+3, compare_names);

    // This will printout the names in alphabetical order
    for (int i=0; i < 3; i++) {
    	cout << persons[i].name << " ";
    }

	return 0;
}

cool~

Thanks . This is what we need. Thank you for your blog article, it helped us lot.

thanks

Thanks for providing this

Thanks

Not only this:
// This will print an unexpected value
it1++;
cout << *it1;
But also this, will cause crash:
// NOTE: it1 still points to 40, and 60 is not deleted
cout << endl << *it1 << "\t" << *it2 <<endl;

After using iterators in the list.erase() method, they are invalidated and we shouldn't use them.

Aweskm😀😃 Please provide output as comment in code.

Thank you

You completely forgot about strings. Nut rest of all the things are good.

Not only this: // This will print an unexpected value it1++; cout << *it1; But also this, will cause crash: // NOTE: it1 still points to 40, and 60 is not deleted cout << endl << *it1 << "\t" << *it2 <<endl;

After using iterators in the list.erase() method, they are invalidated and we shouldn't use them.

std::list::erase returns a new valid iterator pointing to the element immediately following the erased element