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# Turtle Graphics (optional)

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**CS1302 Introduction to Computer Programming**
___

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In this notebok, we will demonstrate the benefit of code reuse with [`turtle`](https://docs.python.org/3/library/p.t.html), which is a module for drawing graphics.

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## Start to move the turtle

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Because Jupyter notebook does not fully support `turtle` (the [`Tkinter` GUI toolkit](https://docs.python.org/3/library/tkinter.html)), we will use `Thonny` under the Linux `Desktop` instead.

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**How to start a python script in `Thonny`?**

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- Open a `Desktop` from the Launcher.
- Open `Thonny` via `Application->Development->Thonny` from the menu at the top left corner.
- Create a new Python script via `File->New`.
- Save the file using `File->Save as...` to `cs1302/Lab3b/myscript.py` under your home directory.

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````{note}

- You may also use Xpra on Windows/Mac/Linux, in which case you can run Thonny from the menu item `Start->Applications->Thonny`. For mobile devices or devices with limited memory, `Desktop` is recommended.
- A python script ends with the extension `.py`.
- `myscript.py` should be placed in the same folder as your Lab3b for the later code to work.

````

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**How to move the turtle?**

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Type the following in your python script to import a turle and instruct it to move forward `100` steps:

```Python
import turtle as t

t.forward(100)
t.exitonclick()
```

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To get the turtle moving, press `F5` or click the green play button in the toolbar:

![Forward](images/forward.dio.svg)

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To exit, simply click on the display window.

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## Draw polygons

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**How to have the turtle draw a triangle?**

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The following code draws a red equilateral triangle:

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```Python
t.fillcolor('red')
t.begin_fill()
                 
t.forward(100)
t.left(120)
t.forward(100)
t.left(120)
t.forward(100)
t.left(120)
                 
t.end_fill()
t.exitonclick()
```

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`t.left(120)` rotates the turtle to the left by `120` degrees.

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**Exercise** (Optional) The code above is quite *repetitive*. Shorten the code using a for loop and run it using your python script.

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**How to draw a square, a regular pentagon, or even any regular polygon?**

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Instead of writing a new script for each polygon, we can write a function:

```Python
def polygon(n, edge, color):
    """Draw a colored polygon.

    A function using turtle to draw a polygon 
    with arbitrary color and number of edges.

    Parameters
    ----------
    n: int
        number of edges.
    edge: int
        length of each edge.
    color: string
        color of the polygon
    """
    ...
```

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**Exercise** (Optional) Complete the function `polygon` and call it to draw a `'blue'` pentagon with length `50`.

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````{hint}

For a $n$-sided polygon, the turtle should turn by an angle of $\frac{360}{n}$ at each corner of the polygon. E.g., for the equilateral triangle with `n=3`, the turning angle should be $\frac{360}3=120$.

````

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## Tesselation

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**Tessellation problem**

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[Tessellation](https://en.wikipedia.org/wiki/Tessellation) refers to the problem of using geometric shapes to tile a flat plane. E.g., [rhombitrihexagonal tiling](https://en.wikipedia.org/wiki/Rhombitrihexagonal_tiling) is a beautiful tessellation often used by temples and museums. Each hexagon is surrounded by 6 triangles and 6 squares.

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<center><figure>
<a title="Unsplash" href="https://commons.wikimedia.org/wiki/File:Tiling_small_rhombi_3-6_simple.svg"><img width="400" alt="Rhombitrihexagonal tiling" src="https://upload.wikimedia.org/wikipedia/commons/thumb/a/a0/Tiling_small_rhombi_3-6_simple.svg/900px-Tiling_small_rhombi_3-6_simple.svg.png"></a>
  <figcaption>Rhombitrihexagonal tiling</figcaption>
</figure>
</center>

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The following code draws:

![Rhombitrihexagon](images/rhombitrihexagon.dio.svg)
 
``` Python
from polygons import *

t.speed('fast')
edge = 50

t.left(180)
hexagon(edge, 'red')
t.right(180)

for _ in range(6):
    t.left(30)
    triangle(edge, 'yellow')
    t.right(90)
    square(edge, 'purple')
    t.forward(edge)
    
t.hideturtle()
t.exitonclick()
```

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````{tip}

- The module `polygons` implements the functions `triangle`, `square`, `hexagon`, as well as the more general function `polygon`.
- `t.speed('fast')` speeds up the turle. You can also use `'fastest'`.
- `t.hideturtle()` hides the turtle.

````

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**Exercise** Modify the above code to draw the following pattern:

![Rhombitrihexagons](images/rhombitrihexagons.dio.svg)

Challenge yourself making the code as efficient as possible, e.g., by avoiding double drawing.