Circle in Turtle Geometry
1sep12

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\maketitle

\section{Introduction}
This elementary lesson on Python with turtle geometry motivates the definition
of a circle as a plane curve with constant curvature. It includes a detailed
reading of a Python journal which created this figure.

\section{Python Journaling}
Here is a (somewhat edited) transcript,
circle-journal  an immediate mode
Python session. Such a \texti{Python Journal} is a recording of what
was typed and entered into a Python session. You can just follow the
steps in the circle-journal yourself. And you can read the following
lesson if you aren't sure you understand what you're doing.

\$python Python 2.4.3 (#1, Jan 10 2007, 12:05:23) [GCC 4.0.1 (Apple Computer, Inc. build 5250)] on darwin Type "help", "copyright", "credits" or "license" for more information. > > > import turtle There is no response by Python to this lasst command. You have told Python to use the turtle libraries (in Pythonspeak, this is called the "turtle module"). If do get a reply from Python here, then Python cannot find this module. And you cannot continue this lesson, until you install this module. This means, your current Python needs to be augmented with software you can obtain elsewhere. But that is another story not told here.. To see the entire "vocabulary" of the turtle-module, enter the following command. Note that the name \texttt{dir}, which stands for "directory", is the name of a function, and therefore it must be followed by the parentheses proper to functional notation in mathematics. The vocabulary only tells you the names. It does not tell you how to use these. For example, \texttt{cos} obviously stands for the cosine function, and so you would type \texttt{cos(42)}. But you still not know whether this cosine fuction is in degrees or radians. You can find out, but how will be told later. For the time being, use functions you are told about here. >>> dir(turtle) ['Error', 'Pen', 'RawPen', 'Tkinter', '__builtins__', '__doc__', '__file__', '__name__', '_canvas', '_getpen', '_pen', '_root', 'acos', 'asin', 'atan', 'atan2', 'backward', 'ceil', 'circle', 'clear', 'color', 'cos', 'cosh', 'degrees', 'demo', 'down', 'e', 'exp', 'fabs', 'fill', 'floor', 'fmod', 'forward', 'frexp', 'goto', 'heading', 'hypot', 'ldexp', 'left', 'log', 'log10', 'modf', 'pi', 'position', 'pow', 'radians', 'reset', 'right', 'setheading', 'setx', 'sety', 'sin', 'sinh', 'sqrt', 'tan', 'tanh', 'tracer', 'up', 'width', 'window_height', 'window_width', 'write'] It is neither necessary nor advisable to understand all of these. You can, of course, experiment with any of the words which don't begin with a double underscore. The ones we're interested for this lesson are are \texttt{forward, right, reset}. \texttt{ >>> turtle.forward(50) } The turtle module now opens a screen on your computer. That is, it invokes another module called "Tkinter" which "knows" how to draw in the plane. Moreover, we want to abbreviate the command to something simpler. We can write an alias for the words, but not for the parentheses. The latter indicate the the words refer to a function, and the input to the function, in this case, was 50. >>> \texttt{ fd = turtle.forward} \n >>> \texttt{ fd(50) }\n >>> \texttt{ turtle.right(90)} \n >>> \texttt{ fd(50)} \n >>>\texttt{ rt=turtle.righ)} \n >>> \texttt{ rt(90)} \n >>> \texttt{ fd(50)} \n >>> \texttt{ rt(90)} \n >>> \texttt{ fd(50}) Well, wasn't that fun? But tedious. Let's check the directory for a nother useful function, one that puts everything back to the beginning. \texttt{>>> dir(turtle)} Same output as before. \texttt{>>> turtle.reset()} Aha, that's what we want. Let's call it "zap" for short. >>> \texttt{zap=turtle.reset} We next "define" our own function to draw a square. We'll call it \texttt{ square} and we insert a placeholder for the side-length of the square. The word \texttt{ dis} can be anything you like, but best not to confuse Python by using one of the words already in its vocabulary. When you end the line with a colon, Python presents you with another prompt, the tripple of dots. There you MUST indent a number of spaces. Although you may, you should ot use (TAB). Here I used my usual two spaces. One space is too short to see, more spaces is a waste of effort. To tell Python you're done writing, enter a blank line. Python will return to the standard >>> prompt. \texttt{ >>> def square(dis): }\n \texttt{ ... \ \ fd(dis) }\n \texttt{ ... \ \ rt(ang) }\n \texttt{ ... \ \ fd(dis) }\n \texttt{ ... \ \ rt(ang) }\n \texttt{ ... \ \ fd(dis) }\n \texttt{ ... \ \ rt(ang) }\n \texttt{ ... \ \ fd(dis) }\n \texttt{ ... \ \ rt(ang) }\n \texttt{ ... }\n \texttt{ >>> square(96) } Always check that something you've just defined does what you expect it to do. We now speed up. Here is a loop. Note that loop counter, a dummy variable here called "i" because it's easy to type, takes on five values: 0,1,2,3,4. We do not use it for anything. We just want five loops. Clearly 4 would have sufficed. \texttt{ >>> for i in range(5): }\n \texttt{ ... \ \ fd(dist) }\n \texttt{ ... \ \ rt(ang) }\n \texttt{ ... } Now we are ready to do some serious programming. \texttt{ >>> zap() } A polygon has three attributes, number of sides, lenght of side, and angle. \texttt{ >>> def poly(nn, dis, ang): }\n \texttt{ ... \ \ for i in range(nn): }\n \texttt{ ... \ \ \ \ fd(dis) }\n \texttt{ ... \ \ \ \ rt(ang) }\n \texttt{ ... }\n Can you write out in words what this program does? It's easier to see it do it, isn't it? \texttt{ >>> poly(4,96,90) }\n \texttt{ >>> poly(8,43,45) }\n You are welcome to double the number of sides, and halve the side lenght and turning angle. But you'll get a cramp typing. Let's do this: \texttt{ >>> num=4 }\n \texttt{ >>> dist=96 }\n \texttt{ >>> angl=90 }\n \texttt{ >>> zap() }\n \texttt{ >>> poly(num,dist,angl) }\n \texttt{ >>> poly(2*num,dist/2,angl/2) }\n \texttt{ >>> poly(5*num,dist/5,angl/5) }\n What's going on here? Write a short essay into your journal on this experiment. \section{Mathematics} Ever since Euclid in the year -300 school children were taught that a circle is the locus of points equidistant from its center point. But every child knows that a circle is a curve which is round, like a wheel. How to reconcile this? The above experiment suggest a way. Note that in each iteration, although we halve the distance moved,$ds$, we also halve the angle turned,$d\phi$. Thus the ratio,$\frac{d\phi}{ds} =$is kept constant. In the limit, as$ds \rightarrow 0\$,
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