Hacker Challenge Solution: Christmas Tree

Let's look at one of the solutions for a Christmas tree pattern.

We'll cover the following

Printing the Christmas tree

Your code should print the following Christmas tree:

Playground for your implementation

Write your code here.

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#include <iostream>
using namespace std;
void printASymbolKTimes(char s, int k)
{
    for(int t=1; t<=k; t++)
        cout << s;
}
// Write the code for helping functions here
// Write the code for the main christmasTree function here
void christmasTree(int h)
{
    cout<<"Height:"<<h<<endl; 
    // Write your code here
}
int main()
{
    christmasTree(10);
    return 0;
}

We can divide the Christmas tree into three main parts (from the image above):

  1. The triangles
  2. The trunk
  3. The floor

1. The triangles

Note that we have three triangles in the above image. These three triangles are the same with respect to height, but every part has a different range. The top triangle has a range 11 to 23h\frac{2}{3}h, the second has a range of h4+1\frac{h}{4}+1 to 3×h43\times \frac{h}{4} and the third one with h2+1\frac{h}{2}+1 to hh.

We can make a printing function for the triangle with range passed as parameters along with the height, and then call the same function three times with different ranges to print these three triangles’ portions.

Printing the triangle

Let us make the pattern table for the following figure where h (height) is equal to 10.

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Triangle of height 10
Triangle of height 10

The printTriangle() Function

Line #

Spaces

Dashes (-)

1

h -1

1 * 2 - 1

2

h - 2

2 * 2 - 1

3

h - 3

3 * 2 - 1

...

...

...

ln

h - ln

ln * 2 -1

The implementation of the printTriangle() function will be as follows:

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void printTriangle(int si , int ei, int h)
{
    for(int ln=si; ln<=ei; ln++)
    {
        printASymbolKTimes(' ', h-ln);
        printASymbolKTimes('-', ln*2-1);
        cout << endl;
    }
}

Suppose we call the printTriangle() function with si = 3, ei = 8, and h = 10, then this function will display the following triangle:

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Printing the triangle with different parameters
Printing the triangle with different parameters

Instruction: Add the printTriangle() function in the playground above.

Drawing triangles

To print the above three triangles, we call the printTriangle() function with the following parameters:

  1. The first triangle starts printing from the top to the height of 2*h/3.
    printTriangle(1, 2*h/3,h);
  2. The second triangle starts printing from the 1*h/4+1 to the height of 3*h/4.
    printTriangle(1*h/4+1, 3*h/4,h);
  3. The third triangle from the 1*h/2+1 to the height of h.
    printTriangle(1*h/2+1, h,h);

Instruction: Add these lines in the christmasTree() function above and, execute the code. You should see half of the tree already on the console.

Let’s move toward printing the trunk of the Christmas tree.

2. The trunk

To print the trunk, we first need to figure out the width and height of the trunk. Look at the following figure below to understand the width and the height of a trunk:

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The trunk
The trunk

To print the trunk, we just need to follow these three steps:

  1. Print the (2*h-1)*1/3 spaces.
  2. Print the (2*h-1)*1/3 dashes.
  3. Move to the next line and repeat the above two steps, 2*h/3 times.

The pattern table will look like the following:

The printTrunk() Function

Line #

Spaces

Dashes (-)

1

(2*h-1)/3

(2*h-1)/3

2

(2*h-1)/3

(2*h-1)/3

3

(2*h-1)/3

(2*h-1)/3

...

...

...

2*h/3

(2*h-1)/3

(2*h-1)/3

Instruction: Write a printTrunk(int h) function in the above playground, call it after printing the triangles and execute the code. You should see the entire picture of the tree. Now, we’ll move on to the floor.

3. The floor

As we can see, the last line of the tree is the floor. To print the floor, we just need to print the dashes 2*h-1 times.

The printFloor() Function

Line #

Dashes (-)

1

2*h-1

Instruction: Write a printFloor(int h) function in the above playground, call it after printing the trunk, and execute the playground. You should be able to see the complete required output. Change the passed value to some other heights and see whether the output is correct with respect to every height or not.

Hurrah! You are done with Christmas tree printing.

The complete implementation

Just for reference, the complete implementation will look like the following:

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#include <iostream>
using namespace std;
void printASymbolKTimes(char s, int k)
{
    for(int t=1; t<=k; t++)
        cout << s;
}
void printTriangle(int si , int ei, int h)
{
    for(int ln=si; ln<=ei; ln++)
    {
        printASymbolKTimes(' ', h-ln);
        printASymbolKTimes('-', ln*2-1);
        cout << endl;
    }
}
void printTrunk(int h)
{
    int base_length = 2*h-1;
    for(int ln=1; ln<=2*h/3; ln++)
    {
        printASymbolKTimes(' ', base_length*1/3);
        printASymbolKTimes('-', base_length*1/3);
        cout <<endl;
    }
}
void printFloor(int h)
{
    int base_length = 2*h-1;
    printASymbolKTimes('-', base_length);
}
void christmasTree(int h)
{
    cout<<"Height:"<<h<<endl; 
    printTriangle(1, 2*h/3,h);
    printTriangle(1*h/4+1, 3*h/4,h);
    printTriangle(1*h/2+1, h,h);
    printTrunk(h);
    printFloor(h);
}
int main()
{
    christmasTree(10);
    return 0;
}