Print Butterfly Pattern (Star Pattern)

Given an integer N, print a butterfly star pattern with 2N − 1 rows. The number of stars increases from 1 to N in the upper half and then decreases from N − 1 to 1 in the lower half, forming a symmetric butterfly shape.

Examples:

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

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

Using Nested Loops – O(N²) Time and O(1) Space

The butterfly pattern can be printed using nested loops. The outer loop runs for all rows, while three inner loops are used to print the left stars, spaces, and right stars. In the upper half, the number of stars increases and spaces decrease. In the lower half, the stars decrease and spaces increase, forming the butterfly shape.

Step-wise Approach

• Initialize two variables spaces = 2 * N - 1 and stars = 0 to track the number of spaces and stars in each row.
• Run an outer loop from i = 1 to 2 * N - 1 to generate all rows of the butterfly pattern.
• If the current row is in the upper half (i ≤ N), decrease spaces by 2 and increase stars by 1.
• If the row is in the lower half (i > N), increase spaces by 2 and decrease stars by 1.
• Print stars number of * characters using a loop.
• Print spaces number of spaces using another loop.
• Print stars number of * characters again to form the right wing of the butterfly.
• Print a newline character to move to the next row.

//Driver Code Starts
using namespace std;

//Driver Code Ends

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 << "
";
    }

    return 0;
}

//Driver Code Starts
class GFG {

//Driver Code Ends

    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();
        }
    }
}

def main():

    # 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()



#Driver Code Starts
if __name__ == "__main__":
    main()
#Driver Code Ends

//Driver Code Starts
using System;

class GFG
{
//Driver Code Ends

    static void 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++)
            {
                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();
        }
    }
}

function main()
{
    // Number of rows
    let 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
        let line = "";
        for (let j = 1; j <= stars; j++) {
            line += "*";
        }

        // Print spaces
        for (let j = 1; j <= spaces; j++) {
            line += " ";
        }

        // Print stars
        for (let j = 1; j <= stars; j++) {
            if (j != n) {
                line += "*";
            }
        }

        console.log(line);
    }
}


//Driver Code Starts
// driver code
main();
//Driver Code Ends