C++ programming and STL facts

Assign value by a pair of {} to a container

// Lots of programmers write code  
// like this one: 
pair<int, int> p = make_pair(3, 4); 
  
// while you can just do this :  
pair<int, int> p = { 3, 4 }; 
  
// even a more complex pair 
pair<int, <char, int> > p = { 3, { 'a', 10 } }; 

As we are familiar with the pair there is also one thing known as tuples.

// A tuple can hold elements of different types
tuple t = {3, 4, 5, 'a'};

Get rid of those includes!

// Simply use below, we do not need to

// include any other library such as

// iostream, vector, string, cmath and so on.

#include <bits/stdc++.h>

/div>
Hidden function (not really hidden but not used often)

__gcd(value1, value2)
You don’t need to code Euclidean Algorithm for a gcd function, from now on we can use. This function returns gcd of two numbers.
e.g. __gcd(18, 45) = 9.

#include <bits/stdc++.h>

using namespace std;

int main()

{

int r = __gcd(10, 15);

cout << r;

}
Output:
```
5
```
Note that this works only in GCC.
Convert integer to string directly in c++ using direct command
to_string();

#include <bits/stdc++.h>

using namespace std;

int main()

{

int a = 2665;

string t = to_string(a);

}
Output:
Convert from string to integer
stoi(t);

#include <bits/stdc++.h>

using namespace std;

int main()

{

string a = "2665";

int t = stoi(a);

}
Output:
A set always keeps elements in ascending order:

set s;

// For descending order

set <int, greater> s;

Every variable declared outside of functions are static and have the default value of 0

#include <bits/stdc++.h> 
using namespace std; 
  
int a[5]; 
int main() 
{ 
    // Values in a[] are 0 
    for (int i = 0; i < 5; i++) 
        cout << a[i] << " "; 
  
    // Values in b[] are garbage 
    cout << endl; 
    int b[5]; 
    for (int i = 0; i < 5; i++) 
        cout << b[i] << " "; 
} 

Output:

0 0 0 0 0 
4196880 0 4196368 0 -731594144

If you declare an array inside a function the value of its elements are garbage, but if you write TYPE a[n] = { }; all elements will be zero by default.

#include <bits/stdc++.h> 
using namespace std; 
  
int main() 
{ 
    // Values in a[] are 0 
    int a[5] = {}; 
    for (int i = 0; i < 5; i++) 
        cout << a[i] << " "; 
  
    // Values in b[] are garbage 
    cout << endl; 
    int b[5]; 
    for (int i = 0; i < 5; i++) 
        cout << b[i] << " "; 
} 

Output:

0 0 0 0 0 
4196896 0 4196368 0 354948080

Number of set bits in binary representation of a number x is
__builtin_popcountll(x)

#include <bits/stdc++.h>

using namespace std;



int main()

{

    int x = 5;

    cout << __builtin_popcountll(x);

}
Output:
```
2
```
Note : This works only in GCC.

memset sets the byte values (or unsigned char) of a block of memory. We can use memset to initialize all elements of an integer array to 0 or -1 but not the other values

#include <bits/stdc++.h> 
using namespace std; 
  
int main() 
{ 
    int a[5]; 
  
    // all elements of A are zero 
    memset(a, 0, sizeof(a)); 
    for (int i = 0; i < 5; i++) 
        cout << a[i] << " "; 
    cout << endl; 
  
    // all elements of A are -1 
    memset(a, -1, sizeof(a)); 
    for (int i = 0; i < 5; i++) 
        cout << a[i] << " "; 
    cout << endl; 
  
    // Would not work 
    memset(a, 5, sizeof(a)); // WRONG 
    for (int i = 0; i < 5; i++) 
        cout << a[i] << " "; 
} 

Output:

0 0 0 0 0 
-1 -1 -1 -1 -1 
84215045 84215045 84215045 84215045 84215045

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