5. Turtle square progressions

The code progressions below draw a square.

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.

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5.1. Sequencing: steps to draw a square

The code below uses sequencing only.

The code below draws a square of side length 50 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).

Penup, t.pu(), and pendown, t.pd(), are used either side of t.goto(20, 30) to avoid line drawing when repositioning the turtle.

A line is drawn forwards, in the direction the turtle is heading, by: t.fd(50).

The turtle then turns to the left by: t.lt(90).

The t.fd(50) and t.lt(90) are then repeated 3 more times for the other three sides.

import turtle

s = turtle.Screen()
s.bgcolor("white")
s.title("Grsquareid")
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(50)
t.lt(90)
t.fd(50)
t.lt(90)
t.fd(50)
t.lt(90)
t.fd(50)
t.lt(90)

s.exitonclick()

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Tasks

1. From the code above, list the 8 lines that do the actual drawing.

2. From the 8 lines, list the simplest amount of code that is repeated.

3. Using sequencing only, draw a square of side length 500 at (-250, -250).

Q1

From the code above, list the lines that do the actual drawing.

t.fd(50)
t.lt(90)
t.fd(50)
t.lt(90)
t.fd(50)
t.lt(90)
t.fd(50)
t.lt(90)

Q2

From the 8 lines, list the simplest amount of code that is repeated.

t.fd(50)
t.lt(90)

Q3

Using sequencing only, draw a square of side length 500 at (-250, -250).

import turtle

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

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

t.pu()
t.goto(-250, -250)
t.pd()
t.seth(0)

t.fd(500)
t.lt(90)
t.fd(500)
t.lt(90)
t.fd(500)
t.lt(90)
t.fd(500)
t.lt(90)

s.exitonclick()

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5.2. Iteration: using a for-loop to draw a square

The code below uses iteration to reduce code duplication that was present when only sequencing was used.

The code below draws a square of side length 50 at coordinates (20, 30).

The t.fd(50) and t.lt(90) are placed in a for-loop with 4 repeats for the 4 sides.

The iterator used is “_”. This is the standard choice in python when the iterator is not referenced in the for-loop block.

import turtle

s = turtle.Screen()
s.bgcolor("white")
s.title("square")
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(4):
    t.fd(50)
    t.lt(90)

s.exitonclick()

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Tasks

1. Identify the lines of code that replaced the 8 steps: t.fd(50); t.lt(90); t.fd(50); t.lt(90); t.fd(50); t.lt(90); t.fd(50); t.lt(90)

Q1

Identify the lines of code that replaced the 8 steps.

for _ in range(4):
    t.fd(50)
    t.lt(90)

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5.3. Definitions: using a def block to draw a square

The code below uses a definition block to draw a square.

The function has parameters to specify the side length and the starting position of the bottom left vertex.

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.

The initial heading defaults to 0, which is acrosss to the right.

square(t, length=50, start_pos=(0, 0), start_h=0)

t - the turtle object to draw the square

length - side length; default 50

start_pos - start position; default (0, 0)

start_h - start heading; default 0

In the code below, square(t) draws a default square.

The square function’s parameters that have default values don’t need to be passed as arguments when the the function is called.

square(t, length=50, start_pos=(20, 30)) draws a square of length 50 at (x=20, y=30).

square(t, length=250, start_pos=(-300, -200), start_h=20) draws a square of length 250 at (x=-300, y=-200) angled 20 degrees.

import turtle

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

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

def square(t, length=50, start_pos=(0, 0), start_h=0):
    t.pu()
    t.goto(start_pos)
    t.pd()
    t.seth(start_h)
    for _ in range(4):
        t.fd(length)
        t.lt(90)


square(t)
square(t, length=50, start_pos=(20, 30))
square(t, length=250, start_pos=(-300, -200), start_h=20)

s.exitonclick()

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Tasks

1. Modify square(t, length=50, start_pos=(0, 0), start_h=0) to draw a square of length 200 at (-300, -100).

2. Modify square(t, length=50, start_pos=(0, 0), start_h=0) to draw a square of length 30 at (70, 100) with heading 30 degrees.

Q1

Modify square(t, length=50, start_pos=(0, 0)) to draw a square of length 200 at (-300, -100).

square(t, length=200, start_pos=(-300, -100))

Q2

Modify square(t, length=50, start_pos=(0, 0), start_h=0) to draw a square of length 30 at (70, 100) with heading 30 degrees.

square(t, length=30, start_pos=(70, 100), start_h=30)

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5.4. Adding pen colour and fill colour parameters

The syntax below adds parameters for pen and fill colours and pen size.

square(t, length=50, start_pos=(0, 0), start_h=0, penw=1, penc='black', fillc=None)

t - the turtle object to draw the square

length - side length; default 50

start_pos - start position; default (0, 0)

start_h - start heading; default 0

penw - pensize; default 1

penc - pencolor; default “black”

fillc - fillcolor; default None

In the code below, square(t, length=250, start_pos=(-100, -150), start_h=0, penw=2, penc="black", fillc="light green") draws a square of length 250 at (x=-100, y=-150) with a black pencolor, a light green fillcolor, with a pensize of 2.

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

import turtle

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

t = turtle.Turtle()
t.speed(0)


def square(t, length=50, start_pos=(0, 0), start_h=0, penw=1, penc="black", fillc=None):
    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(4):
        t.fd(length)
        t.lt(90)

    if fillc is not None:
        t.end_fill()


square(t, length=250, start_pos=(-100, -150), start_h=0, penw=2, penc="black", fillc="light green")

s.exitonclick()

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Tasks

1. Modify square(t, length=50, start_pos=(0, 0), start_h=0, penw=1, penc="black", fillc=None) to draw a square of length 200 at (-300, -100) with red outline of thickness 5.

2. Modify square(t, length=50, start_pos=(0, 0), start_h=0, penw=1, penc="black", fillc=None) to draw a square of length 30 at (70, 100) with green outline of thickness 3 and a yellow fill.

Q1

Modify square(t, length=50, start_pos=(0, 0), start_h=0, penw=1, penc="black", fillc=None) to draw a square of length 200 at (-300, -100) with red outline of thickness 5.

square(t, length=200, start_pos=(-300, -100), start_h=0, penw=5, penc="red", fillc=None)

Q2

Modify square(t, length=50, start_pos=(0, 0), start_h=0, penw=1, penc="black", fillc=None) to draw a square of length 30 at (70, 100) with green outline of thickness 3 and a yellow fill.

square(t, length=30, start_pos=(70, 100), start_h=0, penw=3, penc="green", fillc="yellow")

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5.5. Practice Questions

Exercises

1. Using sequencing only, draw a square of side length 500 at (-250, -250).

2. Using a repeat loop (without a function), draw a square of side length 50 at (-25, -25).

3. Use the definition provided above to draw a square of length 400 at (x=-200, y=-200) with a purple pencolor, a bisque fillcolor, and a pensize of 10.