Fun coding challenge: print the American flag

Printing is an important part when programming. Printing the US flag to the system console was a small part of “the shape game” for kids, in which the players will select the correct name for a given shape.

Basic Requirements:

You are required to print the American flag to the console using just “*” and “=”. The number of stars varies depending the line (row). A sample output:

* * * * * * ==================================
 * * * * *  ==================================
* * * * * * ==================================
 * * * * *  ==================================
* * * * * * ==================================
 * * * * *  ==================================
* * * * * * ==================================
 * * * * *  ==================================
* * * * * * ==================================
==============================================
==============================================
==============================================
==============================================
==============================================
==============================================

Fun facts of students’ solutions

Most of the students provided solutions which the flag is hardcoded. It means they fixed (data or parameters) in a program in such a way that they cannot be altered without modifying the program. A typical solution (in JAVA) looks like this:

public class USAFlag {
    public static void main(String[] args)
    {
        System.out.println("* * * * * * ==================================");
        System.out.println(" * * * * *  ==================================");
        System.out.println("* * * * * * ==================================");
        System.out.println(" * * * * *  ==================================");
        System.out.println("* * * * * * ==================================");
        System.out.println(" * * * * *  ==================================");
        System.out.println("* * * * * * ==================================");
        System.out.println(" * * * * *  ==================================");
        System.out.println("* * * * * * ==================================");
        System.out.println("==============================================");
        System.out.println("==============================================");
        System.out.println("==============================================");
        System.out.println("==============================================");
        System.out.println("==============================================");
        System.out.println("==============================================");
    }
}

Or partly hard-coded 😀 . Printing the first few rows are tricky as they are mixed “*” and “=”. Therefore, they decided to store the two lines (of the flag) in a variable.

public static void main(String[] args) {
  String p1 = "* * * * * * ==================================\n * * * * *  ==================================";
  String p2 = "==============================================";
  for (int i = 0; i < 4; i++) {
   System.out.println(p1);
  }
  System.out.println("* * * * * * ==================================");
  for (int i = 0; i < 6; i++) {
   System.out.println(p2);
  }
 }

The advanced requirements:

The previous code works perfectly but it is not flexible. Now let make it a bit more challenging by some new requirements:

1. No more hard-coded
2. Make the flag scalable with the width and height (as integers)
3. Make the code as short as possible.

Give yourself 5 mins before going to the following solution.

Print flag Using Java

We use loops to print line by line. First, we print star patterns, then we print “=”, finally we print new line.

The default width and height are 46 and 15 respectively. The star pattern is 6* on even index lines (if the first line has index 0) and 5* on odd index lines. Between 2 stars, we have a space (” “). Therefore, all the stars are bounded in 9×12 rectangle. We print a star only when the sum of the row and column indices are ODD.

An easy to understand version:

public static void main(String[] args) {
   for (int i = 0; i < 15; i++) {
        for (int j = 0; j < 46; j++) {
          if(i < 9 && j < 12){
              if((i+j)%2 == 0 && j != 11){
                 System.out.print("*");}
              else {
                 System.out.print(" ");}
          } else {
             System.out.print("=");
          }
        }//End for
        System.out.println();
   }//End for
}

A crazy version with only ONE print line.

For scalability, we create a new function, printUSAFlag, that takes 2 input params height and width. The indices i and j are controlled with ratio to the original width and height. Now we try to shorten the code with the same logic by replacing if-statements with the ternary operator. Syntax:

condition ? expr1 : expr2
#condition—An expression that evaluates to true or false
#expr1, expr2—Expressions with values of any type

If condition is true, the value of expr1 is returned; if condition is false, the value of expr2 is returned.

public static void printUSAFlag( int height, int width) {
  for (int i = 0; i < height; i++) {
    for (int j = 0; j < width; j++) {
      System.out.print(((i< 9*height/15 && j < 12*width/46 )?
	 (((i+j)%2==0 && j != 12*width/46 - 1)?"*":" "):"=" )+(j==width-1?"\n":""));
	 }
    }
 }

printUSAFlag(5,16)
#output:
* * *===========
 * * ===========
* * *===========
================
================

Print with Python

With the same logic, we can implement with Python.

def printUSAFlag(height, width):
  s = ""
  for i in range(height):
    for j in range(width):
      s+=(('*' if ((i+j)%2==0 and j!= 12*width/46 -1) else " ") if (i < 9*height/15 and j <12*width/46) else "=") + ('\n' if j==width-1 else '')
  print(s)

Want to play with python a little more with python generator?

def printUSAFlag(height, width):
  print(''.join([(('*' if ((i+j)%2==0 and j!= 12*width/46 -1) else " ")
         if (i < 9*height/15 and j <12*width/46) else "=") +
         ('\n' if j==width-1 else '') for i in range(height) for j in range(width)]))

Print Flag with Kotlin

If you are new to Kotlin, you can read this quick tutorial.

fun printUSAFlag(height: Int, width: Int) {
        for (i in 0 until height) {
            for (j in 0 until width) {
                print(
                    (if (i < 9 * height / 15 && j < 12 * width / 46)
                        if ((i + j) % 2 == 0 && j != 12 * width / 46 - 1) "*" else " "
                    else
                        "=") + if (j == width - 1) "\n" else ""
                )
            }
        }
    }

Now we can test with any width and height.

fun main() {
    printUSAFlag(6, 18)
}

* * ==============
 *  ==============
* * ==============
==================
==================
==================