# The Hogg Blog

Envisaging the Future by Reflecting on the Past

# Turtle Graphics Update

### Turtle Graphics Update

It is that time of the year again – thanksgiving. So that of course means I need to update the version of Turtle Graphics that I posted a year ago to test out Oslo. Turtle Graphics was a Logo like programming language created in the early 1980's as a means of introducing novices and children to programming on the BBC Microcomputer. Turtle graphics helped introduce people to programming using a simple functional language that provided instant visual gratification allowing the user to move a triangular turtle across the screen using simple commands such as forward, backward, right turn and left turn.

In addition to updating the Turtle Graphics sample so it doesn’t offend any more Americans (my American friend Archie was horrified by the original flag which can be found here) due to an incorrect number of stripes, I have now added the following features:

-          A graphical turtle – with each move and rotate command the turtle now moves and rotates – making it much simpler to know where you are on the drawing surface

-          Functions – you can now create programmatic functions that allow you to centralize logic

-          Ability to execute commands immediately – or by running batch programs

-          Error handling – syntax errors will now be called out properly

-          And as you can see below - color!

The screenshot above shows the output from this program. The complete logo program listing is also included below so you can see how complex objects can be created using a simple grammar comprising of just 8 commands:

Turtle Graphics Syntax

Standard Commands

-          MoveTo <x, y> - Move the turtle to absolute position x, y

-          Move <x, y> - Move the turtle relative to the current position

-          Draw <n> - Draw a line from the current position n pixels

-          Rotate <n> - Rotate the turtle n degrees, where n is between -360 and +360

-          Color <color> - Select a color: Red, Blue, Green, Yellow, White, Black

Loops

-          Loop <n> { <Program> } - Run the Logo program n times

Function

-          Function <FunctionName> { [Program] } -

-          Call <FunctionName> - Calls the previously declared function called FunctionName

I have attached an MSI that installs the Turtle Graphics application. I have also included the Turtle Graphics program listing for the American Flag below so you can get a sense for how this simple grammar works.

I am thinking of setting up a CodePlex site and publishing the Source Code if there is enough interest - please post a response if you would like access to the Source code or if you would be interested in helping me fix any number of bugs that surely exist.

I talk a little more about the design of the application (Oslo Mgrammar and use of WPF) in the original blog post.

Enjoy!!! And (to those in America) Happy Thanksgiving...

// Turtle Graphics Sample Application

// United States Flag

// This function draws each star

Function DrawStar

{

// Draw five points to make each star

Loop 5

{

Rotate 54

Draw 5

Rotate -126

Draw 5

}

}

// This function draws a row of stars

Function DrawRowOfStars

{

// Draw ten columns of stars

Loop 10

{

Call DrawStar

Move 20 -1

}

}

// This function draws 5 rows of stars

Function DrawRowsOfStars

{

// Draw the 50 stars of the United States

MoveTo 27,30

Rotate 90

Color White

// Draw five rows of stars

Loop 5

{

Call DrawRowOfStars

Move -210,20

}

}

// This function draws a line on the flag

Function DrawLine

{

Loop 14

{

Draw 410

Move -410,1

}

}

// This function draws 13 lines

Function DrawLines

{

// Draw the 13 lines of the US flag alternating red and white

MoveTo 20,25

Rotate -90

Loop 6

{

// Lines are 14 pixels high

Color Red

Call DrawLine

Color White

Call DrawLine

}

// Draw the final red line

Color Red

Call DrawLine

}

// This function draws the blue background

Function DrawBlueBackground

{

// Draw the blue background for under the stars

Color Blue

MoveTo 20,25

Loop 97

{

Draw 210

Move -210,1

}

}

// Draw The American Flag by Calling The Appropriate Functions

Call DrawLines

Call DrawBlueBackground

Call DrawRowsOfStars

Attachment: TurtleGraphics.zip