The turtle moves on a square canvas of size 600 pixels. The lower left
corner of the canvas is located at (-300,-100) and the upper right
corner is located at (300,500). The turtle is initially located at
(0,0) with heading 90 degrees. The initial pen state is down.
- <![CDATA[(forward <num expr>)]]>
Evaluate <num expr>. Call the result d.
Move the turtle forward a distance d pixels. Example 1:
<![CDATA[
(forward 50)
]]>
moves the turtle forward a distance of 50.
- <![CDATA[(right <num expr>)]]> Evaluate <num expr>. Call
the result theta. Turn the turtle clockwise theta
degrees. Example 2:
<![CDATA[
((forward 50)
(right 90)
(forward 50)
)
]]>
moves the turtle forward 50 pixels, right 90 degrees, then forward 50
pixels. Note the final orientation of the turtle.
- <![CDATA[(left <num expr>)]]> Evaluate <num expr>. Call the
result theta. Turn the turtle counter-clockwise theta
degrees. Example 3:
<![CDATA[
((forward 50)
(left 90)
(forward 50)
)
]]>
turns the turtle counter-clockwise by 90 degrees.
- <![CDATA[(backward <num expr>)]]>
Evaluate <num expr>. Call the result d. Move the turtle
backward a distance d pixels. Example 4:
<![CDATA[
((forward 50)
(right 90)
(backward 50)
)
]]>
moves the turtle forward 50, right 90 degrees, then backward 50. Note
the final orientation of the turtle.
- <![CDATA[(penup)]]> Raises pen, i.e., changes pen to the "up" state.
Note: The turtle's path is only drawn when the pen is in
the "down" state.
- <![CDATA[(pendown)]]> Lowers pen, i.e., changes pen to the "down"
state. Example 5:
<![CDATA[
(repeat 5
(penup)
(forward 5)
(pendown)
(forward 5)
)
]]>
draws a dashed line.
- <![CDATA[(stop)]]>
Terminates program.
- <![CDATA[(if <bool expr> <then stmt block>)]]> Evaluates <bool
expr>. If <bool expr> evaluates to true, executes
<then stmt block> The <then stmt block> can be either
a single statement, <stmt>, or a list of statements enclosed
in parentheses, (<stmt 1> <stmt 2> ... <stmt
N>). Example 6:
<![CDATA[
(if (= d 0)
(stop)
)
]]>
executes (stop) if d = 0. Example 7:
<![CDATA[
(if (< c 10)
((pendown)
(forward 100)
(stop)
)
)
]]>
executes ((pendown) (forward 100) (stop)) if c < 10.
- <![CDATA[(if <bool expr> <then stmt block> <else stmt block>)]]>
Evaluates <bool expr>. If <bool expr> evaluates to #t,
executes <then stmt block> otherwise executes <else stmt
block> Both <then stmt block> and <else stmt
block> can be either a single statement, <stmt>, or a list
of statements enclosed in parentheses, (<stmt 1> <stmt
2> ... <stmt N>). Example 8:
<![CDATA[
(if (< c 10)
((pendown)
(forward 100)
(stop)
)
((penup)
(backward 100)
(stop)
)
)
]]>
- <![CDATA[(repeat <num expr> <stmt 1> <stmt 2> ...<stmt N>)]]>
Evaluate <num expr>. Call the result m.
Executes <stmt 1> <stmt 2> ... <stmt N>
m times. Example 9:
<![CDATA[
(repeat 5
(forward 50)
(right (/ 360 5))
)
]]>
draws a polygon with 5 sides.
- <![CDATA[(to <proc> (<var 1> <var 2>...<var N>) <stmt 1> <stmt 2>...<stmt M>)]]>
Defines a procedure with name, <proc1>, and
parameters/local variables, <var 1> <var 2>...<var
N>. Example 10:
<![CDATA[
((to square (side)
(repeat 4
(forward side)
(right 90)
)
)
(square 50)
)
]]>
defines and calls a procedure which draws a square.
- <![CDATA[(<proc> <expr 1> <expr 2>...<expr N>)]]>
Evaluates <expr 1> <expr 2> ... <expr N> and passes the
values as arguments to the procedure named, <proc 1>.
Example 11:
<![CDATA[
(square 100)
]]>
draws a square with sides of length 100.
- <![CDATA[(make <var> <expr>)]]>
Within the body of a procedure, <proc>, evaluate
<expr> and assign the value to <var>. Note that
<var> must be in the parameter/local variable list for
<proc>. Example 12:
<![CDATA[
((to spiral(side angle max count)
(repeat max
(forward (* side count))
(right angle)
(make count (+ count 1))
)
)
(penup)
(forward 70)
(pendown)
(spiral 0.05 10 180 0)
)
]]>
increments count by 1 within the body of procedure, spiral.
- <![CDATA[(color <num expr>)]]>
Evaluate <num expr>. Call the result hue. Change the
current pen color to hue where hue is an angle between 0 and
359 degrees. Red is 0 degrees, green is 120 degrees and blue is 240
degrees. Example 13:
<![CDATA[
((right 30)
(color 60)
(forward 100)
(right 120)
(color 300)
(forward 100)
(right 120)
(color 180)
(forward 80)
)
]]>
- <![CDATA[(setxy <num expr 1> <num expr 2>)]]>
Evaluate <num expr 1> and <num expr 2>.
Call the results X and Y. Move the turtle to
location (X,Y). Example 14:
<![CDATA[
((penup)
(setxy 50 50)
)
]]>
Example 15:
<![CDATA[
((to circle (h r)
(repeat 90
(color h)
(make r (* (/ h 360) 2 3.1416))
(setxy (* (cos r) 50) (+ (* (sin r) 50) 50))
(make h (+ h 4))
)
)
(penup)
(setxy 50 50)
(pendown)
(circle 0 0)
)
]]>
- <![CDATA[(push)]]>
Save the turtle's current state on the turtle stack.
The state consists of the turtle's location, heading,
pen state, and color.
- <![CDATA[(pop)]]>
Restore the turtle's state to the last state which was pushed (but has
not yet been popped) onto the turtle stack. Example 16:
<![CDATA[
((color 200)
(forward 25)
(push)
(color 0)
(right 45)
(forward 50)
(pop)
(forward 25)
)
]]>
- <![CDATA[<num expr>]]>
Any Scheme expression which evaluates to a number.
- <![CDATA[<bool expr>]]>
Any Scheme expression which evaluates to a boolean.
