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

bool isPlace(vector<int>&, int, int);
void nqueen(vector<int>&, int, int, int&);
void prettyPrint(vector<int>&);

int main() {
    /**
     * In order to get a plausible configuration of placing n queens on the
     * board we place a queen on a row then move on to place a queen in the
     * next row. We continue this, till we encounter a error, in which case
     * we backtrack and change positions of previous queens till an optimal
     * placement grid is achieved
     */ 
    cout << "\nThis program finds out positions of N - Queens.\n" << endl;
    int size, queens;
    cout << "Enter the size of chess board : ";
    cin >> size;
    cout << "Enter the number of queens to consider : ";
    cin >> queens;

    vector<int> board(size, 0);
    
    int count = 0;

    nqueen(board, 0, queens, count);
    
    cout<<"\n\nThere are "<<count<<" possible solutions.";

    return 0;
}

void nqueen(vector<int> &board, int current, int queens, int &count) {
    // Here we traverse through all rows of the board
    for (int col = 0; col < board.size(); col += 1) {
        if (isPlace(board, current, col)) {
            // If this column can accept a queen
            // NOTE - The number of the queen represents its row,
            // valuea at that index represents the col it is placed in
            board[current] = col;

            if (current == queens-1) {
                prettyPrint(board);
                count++;
                cout << endl;
            } else nqueen(board, current + 1, queens, count);
        }
    }
}

bool isPlace(vector<int> &board, int current, int col) {
    // Here we traverse through all columns of the given row
    for (int j = 0; j < current; j += 1) {
        // This ensures that not only are there no queens below or above
        // But also queens are absent from diagonals
        if (board[j] == col || abs(board[j] - col) == abs(j - current)) return false;
    }
    return true;
}

void prettyPrint(vector<int> &board) {
    for (int i = 0; i < board.size(); i += 1) {
        cout << endl;
        for (int j = 0; j < board.size(); j += 1) {
            if (board[i] == j) cout << "Q" << " ";
            else cout <<  "* ";
        }
    }
}
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Read inputs from stdin

OneCompiler's C++ online compiler supports stdin and users can give inputs to programs using the STDIN textbox under the I/O tab. Following is a sample program which takes name as input and print your name with hello.

#include <iostream>
#include <string>
using namespace std;

int main() 
{
    string name;
    cout << "Enter name:";
    getline (cin, name);
    cout << "Hello " << name;
    return 0;
}

About C++

C++ is a widely used middle-level programming language.

  • Supports different platforms like Windows, various Linux flavours, MacOS etc
  • C++ supports OOPS concepts like Inheritance, Polymorphism, Encapsulation and Abstraction.
  • Case-sensitive
  • C++ is a compiler based language
  • C++ supports structured programming language
  • C++ provides alot of inbuilt functions and also supports dynamic memory allocation.
  • Like C, C++ also allows you to play with memory using Pointers.

Syntax help

Loops

1. If-Else:

When ever you want to perform a set of operations based on a condition If-Else is used.

if(conditional-expression) {
   //code
}
else {
   //code
}

You can also use if-else for nested Ifs and If-Else-If ladder when multiple conditions are to be performed on a single variable.

2. Switch:

Switch is an alternative to If-Else-If ladder.

switch(conditional-expression){    
case value1:    
 // code    
 break;  // optional  
case value2:    
 // code    
 break;  // optional  
......    
    
default:     
 code to be executed when all the above cases are not matched;    
}

3. For:

For loop is used to iterate a set of statements based on a condition.

for(Initialization; Condition; Increment/decrement){  
  //code  
}

4. While:

While is also used to iterate a set of statements based on a condition. Usually while is preferred when number of iterations are not known in advance.

while (condition) {  
// code 
}

5. Do-While:

Do-while is also used to iterate a set of statements based on a condition. It is mostly used when you need to execute the statements atleast once.

do {  
 // code 
} while (condition);

Functions

Function is a sub-routine which contains set of statements. Usually functions are written when multiple calls are required to same set of statements which increases re-usuability and modularity. Function gets run only when it is called.

How to declare a Function:

return_type function_name(parameters);

How to call a Function:

function_name (parameters)

How to define a Function:

return_type function_name(parameters) {  
 // code
}