12.18. ðŸ¤” Turtles and Strings and LSystemsÂ¶
This section describes a much more interested example of string iteration and the accumulator pattern. Even though it seems like we are doing something that is much more complex, the basic processing is the same as was shown in the previous sections.
In 1968 Aristid Lindenmayer, a biologist, invented a formal system that provides a mathematical description of plant growth known as an Lsystem. Lsystems were designed to model the growth of biological systems. You can think of Lsystems as containing the instructions for how a single cell can grow into a complex organism. Lsystems can be used to specify the rules for all kinds of interesting patterns. In our case, we are going to use them to specify the rules for drawing pictures.
The rules of an Lsystem are really a set of instructions for transforming one string into a new string. After a number of these string transformations are complete, the string contains a set of instructions. Our plan is to let these instructions direct a turtle as it draws a picture.
To begin, we will look at an example set of rules:
A  Axiom 
A > B  Rule 1 Change A to B 
B > AB  Rule 2 Change B to AB 
Each rule set contains an axiom which represents the starting point in the transformations that will follow. The rules are of the form:
left hand side > right hand side
where the left hand side is a single symbol and the right hand side is a sequence of symbols. You can think of both sides as being simple strings. The way the rules are used is to replace occurrences of the left hand side with the corresponding right hand side.
Now letâ€™s look at these simple rules in action, starting with the string A:
A
B Apply Rule 1 (A is replaced by B)
AB Apply Rule 2 (B is replaced by AB)
BAB Apply Rule 1 to A then Rule 2 to B
ABBAB Apply Rule 2 to B, Rule 1 to A, and Rule 2 to B
Notice that each line represents a new transformation for entire string. Each character that matches a lefthand side of a rule in the original has been replaced by the corresponding righthand side of that same rule. After doing the replacement for each character in the original, we have one transformation.
So how would we encode these rules in a Python program? There are a couple of very important things to note here:
 Rules are very much like if statements.
 We are going to start with a string and iterate over each of its characters.
 As we apply the rules to one string we leave that string alone and create a brand new string using the accumulator pattern. When we are all done with the original we replace it with the new string.
Letâ€™s look at a simple Python program that implements the example set of rules described above.
Try running the example above with different values for the numIters
parameter. You should see that for values 1, 2, 3, and 4, the strings generated follow the
example above exactly.
One of the nice things about the program above is that if you want to implement a different set of rules, you donâ€™t need to rewrite the entire program. All you need to do is rewrite the applyRules function.
Suppose you had the following rules:
A  Axiom 
A > BAB  Rule 1 Change A to BAB 
What kind of a string would these rules create? Modify the program above to implement the rule.
Now letâ€™s look at a real Lsystem that implements a famous drawing. This Lsystem has just two rules:
F  Axiom 
F > FF++FF  Rule 1 
This Lsystem uses symbols that will have special meaning when we use them later with the turtle to draw a picture.
F  Go forward by some number of units 
B  Go backward by some number of units 
  Turn left by some degrees 
+  Turn right by some degrees 
Here is the applyRules
function for this Lsystem.
def applyRules(ch):
newstr = ""
if ch == 'F':
newstr = 'FF++FF' # Rule 1
else:
newstr = ch # no rules apply so keep the character
return newstr
Pretty simple so far. As you can imagine this string will get pretty long with a few applications of the rules. You might try to expand the string a couple of times on your own just to see.
The last step is to take the final string and turn it into a picture. Letâ€™s
assume that we are always going to go forward or backward by 5 units. In
addition we will also assume that when the turtle turns left or right weâ€™ll
turn by 60 degrees. Now look at the string FF++FF
. You might try to
use the explanation above to show the resulting picture that this simple string represents. At this point its not a very exciting
drawing, but once we expand it a few times it will get a lot more interesting.
To create a Python function to draw a string we will write a function called
drawLsystem
The function will take four parameters:
 A turtle to do the drawing
 An expanded string that contains the results of expanding the rules above.
 An angle to turn
 A distance to move forward or backward
def drawLsystem(aTurtle,instructions,angle,distance):
for cmd in instructions:
if cmd == 'F':
aTurtle.forward(distance)
elif cmd == 'B':
aTurtle.backward(distance)
elif cmd == '+':
aTurtle.right(angle)
elif cmd == '':
aTurtle.left(angle)
Here is the complete program in activecode. The main
function first creates the
Lsystem string and then it creates a turtle and passes it and the string to the drawing function.
Feel free to try some different angles and segment lengths to see how the drawing changes.
Here is a dragon curve. Use 90 degrees.:
FX
X > X+YF+
Y > FXY
Here is something called an arrowhead curve. Use 60 degrees.:
YF
X > YF+XF+Y
Y > XFYFX
The Sierpinski Triangle. Use 60 degrees.:
FXFFFFF
F > FF
X > FXF++FXF++FXF
12.18.1. LSystems and ListsÂ¶
Letâ€™s return to the Lsystems and introduce a very interesting new feature that requires the use of lists.
Suppose we have the following grammar:
X
X > F[X]+X
F > FF
This Lsystem looks very similar to the old Lsystem except that weâ€™ve added
one change. Weâ€™ve added the characters â€˜[â€˜ and â€˜]â€™. The meaning of these
characters adds a very interesting new dimension to our LSystems. The â€˜[â€˜
character indicates that we want to save the state of our turtle,
namely its position and its heading so that we can come back to this position
later. The â€˜]â€™ tells the turtle to warp to the most recently saved position.
The way that we will accomplish this is to use lists. We can save the
heading and position of the turtle as a list of 3 elements. [heading x
y]
The first index position in the list holds the heading,
the second index position in the list holds the x coordinate,
and the third index position holds the y coordinate.
Now, if we create an empty list and every time we see a â€˜[â€˜ we append the
list that contains [heading, x, y]
we create a history of saved places
the turtle has been where the most recently saved location will always be at
the end of the list. When we find a â€˜]â€™ in the string we use the pop
function to remove the the most recently appended information.
Letâ€™s modify our drawLsystem
function to begin to implement this new
behavior.
When we run this example we can see that the picture is not very interesting, but notice what gets printed out, and how the saved information about the turtle gets added and removed from the end of the list. In the next example weâ€™ll make use of the information from the list to save and restore the turtleâ€™s position and heading when needed. Weâ€™ll use a longer example here so you get an idea of what the kind of drawing the LSystem can really make.
Rather than use the inst
string supplied here, use the code from the string examples above (eg. ActiveCode: 3 (act_lsys_1)), and write your own applyRules function to implement this Lsystem.
This example only uses 6 expansions. Try it out with a larger number of
expansions. You may also want to try this example with different values for
the angle and distance parameters.
Here are the rules for an Lsystem that creates something that resembles a common garden herb. Implement the following rules and try it. Use an angle of 25.7 degrees.
H
H > HFX[+H][H]
X > X[FFF][+FFF]FX
Here is another LSystem. Use an Angle of 25 and see what you get.
F
F > F[F]F[+F]F
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