C Plus Plus

Why use CPP?

Because it allows us to OOP in Cpp

C Plus plus is quite similar to C mit etwas Pro und Kontra. All C programme will run in CPP. But it also has some distinct features. Such as:

Working with input-output

1. iostream cin, cout
2. freeopen “r” and “w” mode and link stdin or stdout

   	freopen(input_file.c_str(),"r",stdin);
   	string s;
   	getline(cin, s);
   	freopen(out_file.c_str(),"w",stdout);
   	cout << s;

3. `fstream` that lets you open multiple file and can write based on need.
```cpp
    int x;
    double d;
    char ch;

    ifstream inputf;
    inputf.open(INPUT_FILE_PATH.c_str());
    ofstream of;
    of.open(OUTPUT_FILE_PATH.c_str());
    inputf >> x >> d >> ch;
    of << x << d << ch;
    inputf.close();
    of.close();

Understanding namespace

Namespace allows you to use different variable, function of a same name. How?

int main() {
	std::cout << "This is a string";
}
 

Vs

using namespace std;
int main() {
	cout << "This is a string";
}

So it allows us to define a set of variable and functions in a separate scope.

namespace Calculate {
	int function add_number (int x, int y) {
		return x + y; 	
	}
	int function multiply_number (int x, int y) {
		return x * y; 	
	}
}
 
int main() {
	int x = Calculate::add_number(10+20);
	int y = Calculate::multiply_number(10+20);
 
	return 0;
}

Understanding conditions and loop

int main() {
	if (expression1) {
		cout << "This is true" << endl;
	}
	else if (expression2) {
		cout << "This is true" << endl;
	}
	else {
		cout << "This is true" << endl;
	}
}

int main () {
	for (int i = 0; i < 10; i++) {
		if (i == 2) {
			continue;	
		}
		cout << i << endl;
		if (i == 9) {
			break;
		}
	}
}

Understanding Array

int main() {
	int n;
	cin >> n;
	int arr[n];
 
	for (int i =0; i < n; i++) {
		cin >> arr[i];
	}
 
	int sum = 0;
	for (int i =0; i < n; i++) {
		sum = sum + arr[i];
	}
 
	int mult = 1;
	for (int i =0; i < n; i++) {
		mult = mult * arr[i];
	}
	return 0;
}

Understanding String

int main() {
	int n;
	cin >> n;
	int arr[n];
 
	for (int i =0; i < n; i++) {
		cin >> arr[i];
	}
 
	int sum = 0;
	for (int i =0; i < n; i++) {
		sum = sum + arr[i];
	}
 
	int mult = 1;
	for (int i =0; i < n; i++) {
		mult = mult * arr[i];
	}
	return 0;
}

Understanding function

pass by reference and pass by value.

void Swap_values (int &x , int &y){
	int z = x;
	x = y;
	y = z;
}
 
int main () {
	int x = 5, y = 10;
	swap(x,y);
	cout << "X : " << x << ", Y: " << y << endl;
	return 0;
}

Understanding Pointer

int main () {
	int x = 10;
	int *y = &x;
 
	cout << x << endl;
	cout << y << endl;
	cout << *y << endl;
 
	x = 100;
	cout << x << endl;
	cout << y << endl;
	cout << *y << endl;
}

Dynamic Allocation

We can take memory space from heap dynamically instead of taking it from memory. This is the concept of Dynamic allocation.

void dynamic_alloc() {
 
    int *x = new int;
    *x = 10;
    cout << "The pointer is: " << x << "\n" << "The Value is: "<< *x << endl;
    delete x;
 
/*  For Array  */
    int n;
    cin >> n;
    int *y = new int[n];
    for (int i = 0; i<n ; i++) {
        cin >> y[i];
    }
    for (int i = 0; i<n ; i++) {
        cout << y[i] << " ";
    }
    cout << "The pointer of Array is: " << y << endl;
    cout << "The second pointer and value is: " << y + 1 << ", " << *y + 1 << endl;
    delete [] y;
}

Dynamic Vector

Whenever we need dynamic array, vector is the solution. It allows us to dynamically increase the size of the vector.

void dynamic_vector(){
	vector<int>a = {1,2,3,4,5};
	a.resize(3);
	for (int i=0; i <a.size() ; i++) {
		cout << a[i] << endl;
    }
    a.insert(a.begin(), 10);
    cout << "A Begin Position: " << a << endl;
    a.erase(a.begin() + a.size() - 1 );
    for (int i=0; i <a.size() ; i++) {
	    cout << a[i] << endl;
	}
}

Here a.begin() is not a pointer its an iterator. What is an iterator?

Practice Some problem

contest Link: https://vjudge.net/contest/535482 password: helloworld

Some tips for contest.

1. cin and cout these std functions is usually slow. So, use these line to make the commands faster.

    ios_base::sync_with_stdio(0);
    cin.tie(0);

• ios_base::sync_with_stdio(0); turns off the overall synchronization between C++ and C-style I/O.
• cin.tie(0); specifically unties cin from cout, meaning flushing cout won’t necessarily trigger a flush on cin anymore.

2. avoid using endl. Because it forces a flush. That makes the code slower. So use "\n" instead.