Lab 4: Drawing with Loops and Turtles

CSCI 141 
Lab 4: Drawing with Loops and Turtles
Introduction and Setup
This lab gives you practice with Python loops and Turtle graphics. You’ll complete three
programs that draw pictures: the first two are so-called “ASCII art”, which refers to pictures
made from text characters; the third one is made using a turtle. If you have questions, be
sure to ask the TA: your TA is there to help you! Log into your operating system of choice
and make a lab4 directory in your N drive or home directory. You will write three programs:
triangle.py, triangle.py, and turtledraw.py.
1 triangle.py
Your first task is to write a program that asks the user for a width, then prints a sideways
isosceles triangle made of asterisks (“*”) with the given maximum width. For example, a width
of 3 would print:
*
**
***
**
*
A width of 8 would print:
1
*
**
***
****
*****
******
*******
********
*******
******
*****
****
***
**
*
A triangle of width 1 would print a single asterisk, and width 0 should print no asterisks.
Your solution should use at least one for loop. Here’s a suggested way to approach this problem:
1. Create an empty file triangle.py in your lab4 directory. Write a comment at the top
listing the code’s author, date and a short description of the program.
2. Write a code snippet that prints a given number of asterisks in a row, followed by a
newline. Hint: there are ways to do this with or without using a loop.
3. Draw the top half of the triangle by putting the code you wrote in Step 2 in the body of
a loop that changes the number of asterisks drawn.
4. Draw the bottom half of the triangle using a similar approach.
5. Test your code on widths 2 and 3 first, then check 0 and 1, then try a larger number such
as 8.
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2 flag.py
The purpose of this program is to print an ASCII art approximation of the American flag.
Figure 1: The true American flag
Some things to notice:
• We have to get a bit approximate with ASCII art, because the rows of stars in the true
flag don’t line up with the stripes. Our version lines them up, and thus has only 4 stripes
below the stars, instead of 6 as in the true flag.
• The whole flag has 13 rows.
• The whole flag is 55 characters wide.
• The 9 rows of stars alternate between having 6 and 5 per row, with two spaces in between.
The 6-star rows have no spaces at the beginning and two before the stripes begin; the
5-star rows have two spaces at the beginning and thee spaces before the stripes begin.
Here are some guidelines and suggestions:
• You must control structures (loops, if statements, etc.) to print the rows of the flag.
• You may use no more than 6 print function calls
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* * * * * * ======================================
* * * * * ======================================
* * * * * * ======================================
* * * * * ======================================
* * * * * * ======================================
* * * * * ======================================
* * * * * * ======================================
* * * * * ======================================
* * * * * * ======================================
========================================================
========================================================
========================================================
========================================================
Figure 2: An ASCII art approximation of the American flag.
• All print function calls must be inside the body of a loop.
• The characters must be printed exactly as displayed in the Figure above.
• Try to write this program as concisely as possible. I (Scott Wehrwein) wrote a solution
that has 7 lines of code, not counting comments. If you’re looking for a challenge, see
if you can fit your program in a space smaller than the flag it prints (13 lines long, 55
characters wide).
3 turtledraw.py
In this section, you’ll write a program turtledraw.py that creates a picture like the one shown
in Figure 3. This may seem intimidating at first! But the code you’ll need to write isn’t actually
that complicated. There’s a lot of repetition in the picture, and you’ll use loops to perform this
repetition effortlessly.
You’ll also get some practice looking at the documentation for a module; the turtle module
has tons of functionality, most of which you won’t use or understand—that’s fine! The ability
to sift through unfamiliar documentation and find out how to use the pieces relevant to solving
your problem is a valuable skill.
Each pattern is identical, and is simply composed of many squares with side length 100, each
drawn with one corner in the center of the pattern, but at different angles.
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Figure 3: The result of running turtledraw.py
3.1 Drawing a Square
Start by writing code to draw a square with side length 100. Recall that you saw an example
like this in lecture. After this step, your program should generate the picture shown in Figure
4.
3.2 Drawing one pattern with many squares
Next, you’ll create a single one of the patterns. To draw 60 squares that go all the way around
a circle (360 degrees), you’ll need each one to be different by 6 degrees from the previous one.
Put the code for drawing a square inside a for loop that draws all 60 squares. When it works
correctly, you should get a picture like the one in Figure 5.
3.3 Speeding up the drawing
At this point, you’re probably sitting around waiting for the turtle to finish drawing, and
thinking “wow, CS is boring”. Most programs we write run very quickly, but you can see that
loops make it easy to write short programs that still take a long time to finish! Fortunately,
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Figure 4: Drawing a square with side length 100
the turtle module has some features to speed up the drawing. Start by looking in the turtle
module’s documentation (https://docs.python.org/3.3/library/turtle.html). There’s a
lot there, but don’t worry about all the stuff you don’t understand! Find the section on the
speed method, and read the description to see if you can figure out how it’s used. In your code
above where your turtle starts drawing anything, call the speed method on your turtle object
(e.g., scott.speed(arg )) with the correct argument to make the turtle move as quickly as
possible. Try running your program again - do the squares draw faster now?
That helps some, but it still takes a few seconds to draw 60 squares! The reason for this is that
Python is re-drawing the entire picture every time the turtle moves. You can think of this as
Python having to loop over all 120,000 pixels and re-color each one. We can speed things up
even more by telling the turtle not to re-draw between moves, and to draw once at the end.
Near the top of your code, before your turtle starts drawing, add the following line:
turtle.tracer(0, 0)
Notice that this a turtle module function, not a method of your turtle object. This disables
re-drawing after each move. Next, at the end of your program, add the following line:
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Figure 5: Drawing a single radial pattern.
turtle.update()
This tells Python to re-draw the picture to the screen. Now we’re not wasting effort re-drawing
each time the turtle moves! When debugging your code, you may find it helpful to keep the
animations to see the sequence of moves the turtle makes. Make sure that the submitted verision
of your code has the above lines and draws the whole picture in less than a couple seconds.
3.4 Repeating the pattern four times
The last thing we need to do is draw the pattern four times, near each corner of the window. In
particular, draw the pattern centered at all four corners 200 units away from the middle of the
screen (at coordinates (0,0), where the turtle starts). Start by looking at the documentation
for the penup, pendown, and goto methods. Start by using these methods to move your turtle
to position (-200, -200) before drawing the pattern. Make sure the turtle doesn’t draw a line
when it’s moving from the middle of the screen to the corner.
Finally, we need to move the turtle to each of the four corners and repeat the same pattern.
You could copy/paste the same code four times and change the coordinates, but that wouldn’t
be ideal if we wanted to draw 400 of these patterns instead. So let’s use loops instead!
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The coordinates where we want to draw the pattern are:
(-200, -200)
(-200, 200)
( 200, -200)
( 200, 200)
Notice that this is just all possible ordered pairs of -200 and 200. You saw an example of how
to print all possible pairs of 1 through 6 in lecture; this is similar, but with -200 and 200 instead
of 1 through 6, and instead of printing them, you’re moving the turtle to those coordinates and
drawing the pattern. In class we used while loops, but for loops could also work (and might
be a more natural choice). You may choose whichever loop you like. The pseudocode I’d write
for this is something like:
for i = each of {-200, 200}:
for j = each of {-200, 200}:
move to (i,j) without drawing a line
draw the pattern
3.5 Color the drawing (optional)
Look up the turtle’s color method and figure out how to make your drawing more colorful.
You can color the drawing however you want, but don’t change the pattern drawn. An example
coloring is shown in Figure 6.
Preparing your files for submission: creating a zip file
For this lab and future labs and assignments with multiple files to submit, you will submit a
single zip file containing all your programs. Instructions on how to do this depend on your
platform; here are the steps for Linux, Windows, and Mac:
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Figure 6: Sample colored pattern.
Linux
Using the terminal (command line):
1. Navigate to the directory containing your files:
cd lab4
2. run the zip command, giving it the zip file name as the first argument, and all the files
you want to submit as the reamining arguments. For example:
zip lab4.zip triangle.py flag.py turtledraw.py
3. Verify that the zip file (e.g., lab4.zip) has been created by listing the files in the directory:
ls
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You can also use the graphical file manager. Navigate to the directory with your submission
files, select all the files you wish to submit (hold CTRL and click on each file to select multiple
files). Right click on one of the selected files and select Compress.... Name the zip file and
make sure to select the .zip option.
3.6 Windows
Navigate to the directory containing your code using the file explorer. Select all the files you
wish to submit (hold CTRL and click on each file to select multiple files). Right-click on one
of the files, hover over Send to, and choose Compressed (zipped) folder. Rename the
resulting zip file to your desired filename (e.g., lab4.zip).
3.7 Mac
Follow the terminal (command line) instructions under Linux.
Alternatively, navigate to your code in Finder and select all the files you wish to submit. Select
multiple files by holding down Command and clicking each file. Right click one of the files
and select Compress 3 items (the number will depend on how many files you have selected).
Rename the resulting Archive.zip to your desired filename (e.g., lab4.zip).
Submission
Submit a zip file called lab4.zip containing triangle.zip, flag.zip, and turtledraw.zip
to the Lab 4 assignment on Canvas.
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Rubric
You submitted a single zip file called lab4.zip, containing three Python files
called triangle.py, flag.py, and turtledraw.py
1 point
The top of each program has comments including your name, date, and a short
description of the program’s purpose. Comments placed throughout the code
explain what the code is doing.
3
triangle.py produces the correct output 6
flag.py produces the correct output 5
flag.py uses 6 or less print statements, and no print statements outside a
loop
5
turtledraw.py draws at least one instance of the pattern. 4
turtledraw.py draws four copies of the pattern in the correct corner positions
using loops.
4
turtledraw.py draws the pattern quickly and without animation. 2
Total 30 points
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