# Turtle Graphics, how does it work?

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• 10-03-2008
freddyvorhees
Turtle Graphics, how does it work?
The Problem:

Code:

``` ( Turtle Graphics ) The Logo language, which is popular among elementary school children, made the concept of turtle graphics famous. Imagine a mechanical turtle that walks around the room under the control of a C++ program.  The turtle holds a pen in one of two positions, up or down. While the pen is down, the turtle traces out shapes as it moves; while the pen is up, the turtle moves about freely without writing anything. In this problem, you will simulate  the operation of the turtle and create a computerized sketchpad as well. Use a 20-by-20 array floor that is initialized to zeros. Read commands from an array that contains them. Keep track  of the current position of the turtle at all times and whether the pen is currently up or down. Assume that the turtle  always starts at position (0, 0) of the floor with its pen up. The set of turtle commands your program must process are shown in Fig. 7.33. Figure 7.33. Turtle graphics commands. 1  Pen up   2  Pen down   3  Turn right   4  Turn left   5,10  Move forward 10 spaces (or a number other than 10)   6  Print the 20-by-20 array   9  End of data (sentinel)   Suppose that the turtle is somewhere near the center of the floor. The following "program" would draw and print a 12-by-12 square and end with the pen in the up position: 2 5,12 3 5,12 3 5,12 3 5,12 1 6 9 As the turtle moves with the pen down, set the appropriate elements of array floor to 1's. When the 6 command (print) is given, wherever there is a 1 in the array, display an asterisk or some other character you choose. Wherever there is a zero, display a blank. Write a program to implement the turtle graphics capabilities discussed here. Write several turtle graphics programs to draw interesting shapes. Add other commands to increase the power of your turtle graphics language.```
I have no idea how it will work. Is it like snake, but instead you have to draw? Or you just have to type the coordinate like in a graph? I just cant visualize it
• 10-03-2008
tabstop
Um.

It's a turtle, you see, holding a pen. You give commands to the turtle and the turtle obeys them. The commands are basically start drawing, stop drawing, turn, and walk. And ... well, that's it.

Your job is keep track of (1) where the turtle is (2) which direction it's facing (3) what's been drawn on the floor.
• 10-03-2008
freddyvorhees
So while using it you cant see what you are drawing until you press 6?
• 10-03-2008
tabstop
Well -- using your system as designed for this assignment, yes. (If I recall the original correctly, you could see what was going on the whole time there, but that's not really relevant to the discussion.)
• 10-03-2008
freddyvorhees
Whats the "pen up"-"pen down" for if you just have to enter the coordinates?
• 10-03-2008
tabstop
Because nowhere do you enter any coordinates. The only coordinate-type thing is "how far to walk" (if the command is 5).
• 10-03-2008
freddyvorhees
Nice. Thank you very much
• 10-03-2008
freddyvorhees
One more question. You can't undo what you've drawn right?
• 10-03-2008
tabstop
That would probably fall under the heading of
Quote:

Add other commands to increase the power of your turtle graphics language.
• 08-28-2009
redsunlee
Thanks for the suggestion, I wish it had worked.

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• 08-28-2009
audinue
By the way, what is this program useful for?

I don't get it why in almost graphics, scripting, GUI or game engine there is turtle graphics.
• 08-28-2009
tabstop
Quote:

Originally Posted by redsunlee
Thanks for the suggestion, I wish it had worked.

So do I. But good job hiding the links to French credit something-or-others! Well done!
Quote:

Originally Posted by audinue
By the way, what is this program useful for?

I don't get it why in almost graphics, scripting, GUI or game engine there is turtle graphics.

Turtle graphics is used to teach people things. It's used to teach 10-year-olds about Cartesian coordinates and the like, and 20-year-olds about how to follow directions.
• 08-28-2009
audinue
How do they teach cartesian coordinates from fourth quadrant??

How the hell it can teach people to follow directions??

Code:

`currdir = turtle.dir`

Code:

`queue.push(turtle.dir)`
???
• 08-28-2009
tabstop
Quote:

Originally Posted by audinue
How do they teach cartesian coordinates from fourth quadrant??

I don't know what you mean here.
Quote:

Originally Posted by audinue
How the hell it can teach people to follow directions??

The point of the learning exercise, for the 20-year-olds, is to figure out how to make the system work -- i.e., given a list, or file, or etc., with instructions in it, they need to write a system to parse the instructions and (yes) follow directions. That was, after all, the assignment that was posted in this thread.
• 08-28-2009
audinue
Quote:

Originally Posted by tabstop
I don't know what you mean here.

I think cartesian coordinates may start with (0, 0). To move left and top by 1 px it's (1, 1).

While in array it's (1, -1).
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