Program to Print Butterfly Pattern (Star Pattern)

Given an integer N, print N rows of Butterfly pattern.

Examples:

Input: 3
Output:
* *
** **
*****
** **
* *

Input: 5
Output:
* *
** **
*** ***
**** ****
*********
**** ****
*** ***
** **
* *

Approach:

The problem can be solved using three nested loops inside an outer loop. The outer loop will run for the rows, the first inner loop will print the stars, the second inner loop will print the spaces and the third inner loop will again print the stars.

Step-by-step algorithm:

• Maintain two variable spaces = 2 * N – 1 and stars = 0 to store the number of spaces and stars for each row.
• Run an outer loop from i = 1 to the number of rows (2 * N – 1).
  • If we are in the upper half of the butterfly, decrease number of spaces by 2 and increase number of stars by 1.
  • If we are in the lower half of the butterfly, increase number of spaces by 2 and decrease number of stars by 1.
  • Run an inner loop from j = 1 to stars.
    • Print an asterisk in each iteration of the inner loop.
  • Run an inner loop from j = 1 to spaces.
    • Print a space in each iteration of the inner loop.
  • Run an inner loop from j = 1 to stars.
    • Print an asterisk in each iteration of the inner loop.
  • Print a newline character (“\n”) to move to the next row.
• After N iterations, we will have the right half pyramid pattern.

Below is the implementation of the above approach:

C++

#include <iostream>
using namespace std;

int main()
{
    // Number of rows
    int N = 5;

    // Variables to store number of spaces and stars
    int spaces = 2 * N - 1;
    int stars = 0;

    // The outer loop will run for (2 * N - 1) times
    for (int i = 1; i <= 2 * N - 1; i++) {
        // Upper half of the butterfly
        if (i <= N) {
            spaces = spaces - 2;
            stars++;
        }
        // Lower half of the butterfly
        else {
            spaces = spaces + 2;
            stars--;
        }
        // Print stars
        for (int j = 1; j <= stars; j++) {
            cout << "*";
        }
        // Print spaces
        for (int j = 1; j <= spaces; j++) {
            cout << " ";
        }
        // Print stars
        for (int j = 1; j <= stars; j++) {
            if (j != N) {
                cout << "*";
            }
        }
        cout << "\n";
    }

    return 0;
}

Java

public class ButterflyPattern {
    public static void main(String[] args) {
        // Number of rows
        int N = 5;

        // Variables to store number of spaces and stars
        int spaces = 2 * N - 1;
        int stars = 0;

        // The outer loop will run for (2 * N - 1) times
        for (int i = 1; i <= 2 * N - 1; i++) {
            // Upper half of the butterfly
            if (i <= N) {
                spaces = spaces - 2;
                stars++;
            }
            // Lower half of the butterfly
            else {
                spaces = spaces + 2;
                stars--;
            }
            // Print stars
            for (int j = 1; j <= stars; j++) {
                System.out.print("*");
            }
            // Print spaces
            for (int j = 1; j <= spaces; j++) {
                System.out.print(" ");
            }
            // Print stars
            for (int j = 1; j <= stars; j++) {
                if (j != N) {
                    System.out.print("*");
                }
            }
            System.out.println();
        }
    }
}
//this code is contributed by Adarsh

Python3

# Number of rows
N = 5

# Variables to store number of spaces and stars
spaces = 2 * N - 1
stars = 0

# The outer loop will run for (2 * N - 1) times
for i in range(1, 2 * N):
    # Upper half of the butterfly
    if i <= N:
        spaces = spaces - 2
        stars += 1
    # Lower half of the butterfly
    else:
        spaces = spaces + 2
        stars -= 1

    # Print stars
    for j in range(1, stars + 1):
        print("*", end="")

    # Print spaces
    for j in range(1, spaces + 1):
        print(" ", end="")

    # Print stars
    for j in range(1, stars + 1):
        if j != N:
            print("*", end="")

    print()  # Move to the next line

C#

using System;

class Program
{
    static void Main()
    {
        // Number of rows
        int N = 5;

        // Variables to store the number of spaces and stars
        int spaces = 2 * N - 1;
        int stars = 0;

        // The outer loop will run for (2 * N - 1) times
        for (int i = 1; i <= 2 * N - 1; i++)
        {
            // Upper half of the butterfly
            if (i <= N)
            {
                spaces = spaces - 2;
                stars++;
            }
            // Lower half of the butterfly
            else
            {
                spaces = spaces + 2;
                stars--;
            }

            // Print stars
            for (int j = 1; j <= stars; j++)
            {
                Console.Write("*");
            }

            // Print spaces
            for (int j = 1; j <= spaces; j++)
            {
                Console.Write(" ");
            }

            // Print stars
            for (int j = 1; j <= stars; j++)
            {
                if (j != N)
                {
                    Console.Write("*");
                }
            }

            Console.WriteLine();
        }
    }
}

Javascript

// Number of rows
const N = 5;

// Variables to store number of spaces and stars
let spaces = 2 * N - 1;
let stars = 0;

// The outer loop will run for (2 * N - 1) times
for (let i = 1; i <= 2 * N - 1; i++) {
    // Upper half of the butterfly
    if (i <= N) {
        spaces = spaces - 2;
        stars++;
    }
    // Lower half of the butterfly
    else {
        spaces = spaces + 2;
        stars--;
    }
    // Print stars
    for (let j = 1; j <= stars; j++) {
        process.stdout.write("*");
    }
    // Print spaces
    for (let j = 1; j <= spaces; j++) {
        process.stdout.write(" ");
    }
    // Print stars
    for (let j = 1; j <= stars; j++) {
        if (j !== N) {
            process.stdout.write("*");
        }
    }
    process.stdout.write("\n");
}

Output

*       *
**     **
***   ***
**** ****
*********
**** ****
***   ***
**     **
*       *

Time Complexity: O(N^2), where N is the number of rows in the pattern.
Auxiliary Space: O(1)