7. Turtle star progressions

The code progressions below draw a star.
With each version, an improvement in code structure is made.
Firstly, only sequencing is used, with no iteration.
Secondly, iteration, using a for-loop, reduces code duplication.
Thirdly, a definition block with parameters allows for code reuse via the use of arguments.

For star polygrams, made of a polygon and isosceles triangles see: https://www.desmos.com/calculator/hpglbai0jg

7.1. Sequencing: steps to draw a star

The code below uses sequencing only.
The code below draws a star with diagonals of 120, at coordinates (20, 30).
The start direction, eastwards, is set by: t.seth(0)
The start position, at (20, 30), is set by: t.goto(20, 30).
t.pu() and t.pd() are used either side of it to avoid line drawing when repositioning the turtle.
A line is drawn forwards by: t.fd(120).
The turtle then turns to the left by: t.lt(144).
Then these 2 steps are repeated until the start is fully drawn.

Code Completion: star steps

Q

Complete the code to draw a 5 pointed star with diagonals of 120 at (20, 30), by replacing the XXXs.

import turtle

s = turtle.Screen()
s.bgcolor("white")
s.title("star")
s.setup(width=800, height=600, startx=0, starty=0)

t = turtle.Turtle()
t.speed(5)

t.pu()
XXX(20, 30)
t.pd()
t.seth(0)

t.fd(120)
t.lt(144)
t.fd(XXX)
t.lt(XXX)
t.fd(XXX)
t.lt(XXX)
t.fd(XXX)
t.lt(XXX)
t.fd(XXX)
t.lt(XXX)

s.exitonclick()

Ans

Completed code to draw a 5 pointed star with diagonals of 120 at (20, 30).

import turtle

s = turtle.Screen()
s.bgcolor("white")
s.title("star")
s.setup(width=800, height=600, startx=0, starty=0)

t = turtle.Turtle()
t.speed(5)

t.pu()
t.goto(20, 30)
t.pd()
t.seth(0)

t.fd(120)
t.lt(144)
t.fd(120)
t.lt(144)
t.fd(120)
t.lt(144)
t.fd(120)
t.lt(144)
t.fd(120)
t.lt(144)

s.exitonclick()

Tasks

From the code above, list the 10 lines that do the actual drawing.
From the 10 lines, list the simplest amount of code that is repeated.

7.2. Iteration: using a for-loop to draw a star

The code below uses iteration to reduce code duplication that was present when only sequencing was used.
The code below draws a star of diagonal 120 at coordinates (20, 30).
Firstly, 2 lines are drawn, then this is repeated.
The iterator used is “_”. This is the standard choice in python when the iterator is not referenced in the for-loop block.

Code Completion: star for loops

Q

Complete the code to draw a star with diagonals of 120 at (20, 30), by replacing the XXXs.

import turtle

s = turtle.Screen()
s.bgcolor("white")
s.title("star")
s.setup(width=800, height=600, startx=0, starty=0)

t = turtle.Turtle()
t.speed(5)

t.pu()
XXX(20, 30)
t.pd()
t.seth(0)

for _ in range(XXX):
    t.fd(XXX)
    t.lt(XXX)

s.exitonclick()

Ans

Completed code to draw a 5 pointed star with diagonals of 120 at (20, 30).

import turtle

s = turtle.Screen()
s.bgcolor("white")
s.title("star")
s.setup(width=800, height=600, startx=0, starty=0)

t = turtle.Turtle()
t.speed(5)

t.pu()
t.goto(20, 30)
t.pd()
t.seth(0)

for _ in range(5):
    t.fd(120)
    t.lt(144)

s.exitonclick()

Tasks

Modify the code above to draw a 9 pointed star with diagonals of 120 at (20, 30), by turning 160 degrees at the points of the stars.

7.3. Definitions: using a def block to draw a star

The code below uses a definition block to draw a star.
The function has parameters to specify the diagonal length and the starting position.
The function also requires the turtle to be passed as an argument so it can be referred to.
Before the for-loop, the turtle is repositioned without drawing the movement; penup and pendown are needed for that.

star(t, length=50, points=5, start_pos=(0, 0), start_h=0): t - the turtle object to draw the star

length - diagonal length; default 50

points - number of points in the star, odd integer >=5; default 5

start_pos - start position; default (0, 0)

start_h - start heading; default 0 degrees

The formula to calculate the turning angle of the turtle is (360 * ((points-1)/2))/points.

e.g for 5 points the angle is (360 * (5-1)/2)/5 which is 720/5 which is 144.

The code to draw a star with an odd number of points from 5 upwards is below.

Code Completion: star definition

Q

Complete the star definition by replacing the “XXX”s.

def star(t, length=50, points=5, start_pos=(0, 0), start_h=0):
    ang = (360 * ((points-1)/2))/points
    t.pu()
    t.goto(start_pos)
    t.pd()
    t.seth(start_h)
    for _ in range(points):
        t.fd(length)
        t.lt(ang)

Ans

Completed star definition.

def star(t, length=50, points=5, start_pos=(0, 0), start_h=0):
    ang = (360 * ((points-1)/2))/points
    t.pu()
    t.goto(start_pos)
    t.pd()
    t.seth(start_h)
    for _ in range(points):
        t.fd(length)
        t.lt(ang)

Tasks

Use the code above to draw a star with 7 points and diagonal length of 300, at (-100, 0).

7.4. Adding pen colour and fill colour parameters

The syntax below adds parameters for pen and fill colours.

star(t, length=50, points=5, start_pos=(0, 0), start_h=0, penw=1, penc='black', fillc=None): t - the turtle object to draw the star

length - diagonal length; default 50

points - number of points in the star, odd integer >=5; default 5

start_pos - start position; default (0, 0)

start_h - start heading; default 0 degrees

penw - pensize; default 1

penc - pencolor; ; default “black”

fillc - fillcolor; default None

In the code below, star(t, length=300, points=7, start_pos=(-100, 0), start_h=0, penw=3, penc="grey90", fillc="yellow") draws a 9 pointed star at (x=-100, y=0) with a grey90 pencolor, a yellow fillcolor, using a pensize of 3.

The code needs to check the fillc argument since setting a fillcolor to None will throw an error.

Code Completion: star definition

Q

Complete the code to draw a 9 pointed star at (x=-100, y=0) with a grey90 pencolor, a yellow fillcolor, using a pensize of 3, by replacing the XXXs.

import turtle


def star(t, length=50, points=5, start_pos=(0, 0), start_h=0, penw=1, penc="black", fillc=None):
    ang = (360 * ((points-1)/2))/points
    t.pu()
    t.goto(start_pos)
    t.pd()
    t.seth(start_h)

    t.pensize(penw)
    t.pencolor(penc)

    if fillc is not None:
        t.fillcolor(fillc)
        t.begin_fill()

    for _ in range(points):
        t.fd(length)
        t.lt(ang)

    if fillc is not None:
        t.end_fill()


s = turtle.Screen()
s.bgcolor("white")
s.title("star")
s.setup(width=800, height=600, startx=0, starty=0)

t = turtle.Turtle()
t.speed(9)


star(t, length=XXX, points=XXX, start_pos=(XXX), start_h=0, penw=XXX, penc="XXX", fillc="XXX")

t.ht()
s.exitonclick()

Ans

Completed code to draw a 9 pointed star at (x=-100, y=0) with a grey90 pencolor, a yellow fillcolor, using a pensize of 3.

import turtle


def star(t, length=50, points=5, start_pos=(0, 0), start_h=0, penw=1, penc="black", fillc=None):
    ang = (360 * ((points-1)/2))/points
    t.pu()
    t.goto(start_pos)
    t.pd()
    t.seth(start_h)

    t.pensize(penw)
    t.pencolor(penc)

    if fillc is not None:
        t.fillcolor(fillc)
        t.begin_fill()

    for _ in range(points):
        t.fd(length)
        t.lt(ang)

    if fillc is not None:
        t.end_fill()


s = turtle.Screen()
s.bgcolor("white")
s.title("star")
s.setup(width=800, height=600, startx=0, starty=0)

t = turtle.Turtle()
t.speed(9)


star(t, length=300, points=7, start_pos=(-100, 0), start_h=0, penw=3, penc="grey90", fillc="yellow")

t.ht()
s.exitonclick()

7.5. Practice Questions

Exercises

Draw a 5 pointed star with an orange fill colour.
Draw a 7 pointed star with a red pen and gold2 fill colour.
Draw a random number, between 10 and 20, of 5 pointed stars, of random size, between 10 and 100, at random positions.