Rock Paper Scissors
Python Lab Project
Dan McElroy
This presentation is offered under the (CC BY-NC-SA 4.0)
Creative Commons Attribution Non-Commercial Share license.
Hello everyone.
Welcome to an introduction to programming.
I am using a Python program that plays the Rock Paper
Scissors game for this discussion.
This discussion covers how to use a development system to
enter and run a Python program.
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The Rock Paper Scissors Game
Rock-Paper-Scissors is also known as Roshambo.
How the game is played.
Two people play against each other by raising their arms
slightly and as they each bring one hand down on the count
of three.
Each person makes a sign that indicates Rock with a fist,
Paper with a flat palm and fingers together, or Scissors with
two fingers sticking out.
Both people need to make their sign known at the same
time.
You can't change your sign once you start seeing your
opponent's sign when you think you might lose.
If both people make the same sign, it's a tie.
Otherwise, rock crushes scissors and wins.
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Paper covers rock and wins.
Scissors, cuts paper and wins.
It's kind of difficult to play this game by yourself.
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Rock Paper Scissors Project Discussion
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The Rock Paper Scissors Game
The onlineGDB development system
Python code for the game
Random numbers, arrays, if…elif…else statements
How to enter and run the program
Document the project in the lab report
For your lab assignment, you just need to type in the Python
program, make it work and complete the lab report. I am
providing a fairly detailed description of the inner workings
of the program to give you an idea of how programs work
and how programmers design a solution to a problem.
I have provided similar projects to students in programming
classes with a partial solution to help get started.
Some students came up with a solution that was completely
different from the sample solution.
Wow! Their solution was totally cool, super short to write
but very abstract.
This presentation covers.
The Rock Paper Scissors Game.
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The onlineGDB development system.
Python code for the game.
Random numbers, arrays, if…elif…else statements.
How to enter and run the program.
Document the project in the lab report.
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Creative Commons License
The solution provided here is a modification of the Rock
Paper Scissors program at
https://TheHelloWorldProgram.com/python/pythongame-rock-paper-scissors/
A big thank you to the people at
"TheHelloWorldProgram.com" for sharing their work
through the Creative Commons Non-Commercial ShareAlike
4.0 International license.
This license permits people to modify the code for the
program provided that
appropriate credit is given, people can't make money off it
and any modifications
must be shared using the same license.
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Other types of software distribution include.
Freeware - free to use and distribute, but you don't get a
copy of the source code.
Shareware - you get a free copy of the software, but you are
expected to pay if you choose to use it.
Open Source - the source code is available. Sometimes
restrictions exist that you are required to pay if you use it.
Propriety - source code is not provided, it is protected by
copyright and remains the property of the creator.
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Run the Rock Paper
Scissors Game
I ran the Rock Paper Scissors program and played five times.
The first time I chose Rock and the computer chose Paper.
I lost, because paper covers rock.
I played again and this time chose Paper.
So did the computer and it was a tie game.
Then I chose Scissors and so did the computer.
It was another time game.
I chose Scissors again the next time, but the computer chose
Rock.
Rock crushes scissors and I lost.
Then I tried to be fancy and choose rock but didn't capitalize
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the R.
It didn't like my spelling.
Stupid game.
It looks like capitalization matters when I make my choice.
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Rock Paper Scissors Project
Write a program to play the rock-paper-scissor game.
The user chooses either Rock, Paper or Scissors, and
the computer chooses Rock, Paper or Scissors.
The program announces either a tie or the winner
and the basis for determining the outcome.
Paper covers rock, Rock breaks scissors,
Scissors cut paper, or a tie game.
The program should include a loop that lets the
user play again and again until program is stopped.
Here is the project definition.
Write a program to play the rock-paper-scissor game.
The user chooses either Rock, Paper or Scissors, and the
computer chooses Rock, Paper or Scissors.
The program announces either a tie or the winner and the
basis for determining the outcome.
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Paper covers rock, Rock breaks scissors, Scissors cut paper,
or a tie game.
The program is to include a loop that lets the user play again
and again until program is stopped.
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Truth-Table
A Truth-Table is used to simplify the design and to
make sure that all conditions are tested.
A Truth-Table lists each possible combination for player and
computer.
Possible entries for player are along the left side.
Possible choices for the computer are along the top.
This is a 3 by 3 grid with 9 possible results.
Entries are placed in the truth table to show the result for
each combination of Player and Computer.
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List All Possible Combinations
Option 1 – Test Each Individually
There are nine possible results when playing the game.
The program can be written to individually test the
possibility of each outcome.
This would mean nine separate tests.
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List All Possible Combinations
Option 2 – Three groups of three
The program could also be written by dividing the results
into three groups of three combinations.
If the player selects Rock, then what are the three possible
results for the computer's selections.
Similar for when the player selects Paper or Scissors.
The program code could be organized this way, but there
would still be nine tests.
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List All Possible Combinations
Option 3 – Test for TIE, then others
The code provided by TheHelloWorldProgram.com has the
program organized this way.
First, the code checks to see if the player and computer
selected the same choice.
If this is true, then it is a tie game.
If not, keep going down the list.
If the player selected Rock, then check the computer's
selection for Paper or Scissors.
We don't need to check the computer for Rock, because we
already know that if both are the same, that condition has
already been tested.
If the computer selected Paper, then the computer wins.
The only other possible computer selection is Scissors.
If that happened, then the player won because Rock breaks
Scissors.
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Do something similar for when the player selects Paper or
Scissors.
Just make sure that the winning and losing combinations are
listed correctly.
If the program is coded with this combination of tests, then
there are only 7 tests necessary, not 9.
When the program runs, you will probably never notice any
difference how it works or how fast it runs.
So! Which is the best way to code the program?
It really doesn't matter as long as the program works and
produces the correct results.
It's like if I asked two people to write an essay on the exact
topic.
The essays may cover the same information, but they would
be written differently.
The same thing happens when programmers write code.
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Python Code for the Game
Download an image of the code
from Canvas or the web
https://program-info.net/
PythonRockPaperScissors.png
Description of the program
Arrays
Loops
Random Numbers
if…elif…else statements
print statements
Get a copy of the code either from the class Canvas website,
or directly from the web.
An image of the code and the lab report form are available
at
https://program-info.net/PythonRockPaperScissors.png
The code for the project is a modification of the code
provided by TheHelloWorldProgram.com
You need to enter the code that I am providing, not the
code available from TheHelloWorldProgram.com
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Literals and Variables
List of variables in the program
possibleSelection - an array that holds "Rock", "Paper", "Scissors"
playerSelection - holds the player's selection from the keyboard
computerSelection - holds the computer's random selection
from the possibleSelection array.
Literals and variables
print("Computer selects", computer)
A literal is a notation for representing a fixed value in source
code.
Literals can be numeric values such as 57, -3, etc. or a string
of characters such as "Hello".
A variable sets aside a place in memory to hold data that
can be changed while the program is running.
Variables are given names by the programmer to make
programming easier.
Here is a list of the variables used in the Rock Paper Scissors
program.
possibleSelection - an array that holds "Rock", "Paper",
"Scissors"
playerSelection - holds the player's selection from the
keyboard
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computerSelection - holds the computer's random selection
from the possibleSelection array, "Rock", "Paper", or
"Scissors".
This print statement contains both a literal, "Computer
selects" and a variable named computer.
If the computer selection was "Paper" then the print
statement combines the literal and variable together and
displays, "Computer selects Paper".
Python uses the word print to display messages to the video
console.
Early computers used a printing Teletype device as the
console and the word Print has stuck around since then.
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Arrays
The position in an
array is called an index
An array contains a list of similar data items. This
program uses an array of three elements to hold the
character strings: "Rock", "Paper" and "Scissors". The
name of the array is possibleSelection. This array is
declared as
possibleSelection = ["Rock", "Paper", "Scissors"]
An array contains a list of similar data items.
This program uses an array of three elements
to hold the character strings: "Rock", "Paper" and "Scissors".
The name of the array is possibleSelection. This array is
declared as
possibleSelection = ["Rock", "Paper", "Scissors"]
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Arrays
possibleSelection = ["Rock", "Paper", "Scissors"]
array name
index
Access the contents of an array by its name and index value
possibleSelection[1] This is equal to "Paper"
The position in an array is called the index.
The first element in an array is at index position 0.
This array has three elements and index values from 0 to 2.
The contents of an array can be accessed by using the name
of the array,
an open square-bracket [
the index value, and a closing square-bracket.
For example, possibleSelection[1] contains the value
"Paper"
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Structured Programming
Sequence
Selection
Repetition
Process
Decision
Loop?
No
Yes
Process
Process
Process
Process
Process
The concept of structured programming states that all
programs can be created with only three constructs:
Sequence, Selection and Repetition or a combination of any
of them. Sequence is the easiest to understand. Program
steps are executed sequentially, one after another. Many
times, the order in which the steps are executed is very
important. Other times it does not matter. For example, if I
were baking a cake, it may not matter very much whether I
put the egg in the bowl and then the flour or reversed the
two before mixing them with water. However, it would make
a lot of difference if I put the cake batter in a 9 x 13 baking
dish and placed it in an oven for 1 hour and 30 minutes,
took it out and then turned the oven on to 350 degrees.
Yuck.
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The selection structure is commonly implemented with
if…else if…else statements. The decision is made with a
logical expression or test that evaluates to true or false. In
Python, the else…if words are combined into elif.
The repetition structure gives the ability for the program to
repeat a sequence of statements over and over again. The
test for looping can be either at the beginning or the end of
code that belongs to the loop. In most cases, something in
the code attached to the loop should cause the loop to end.
We need to look at the Rock Paper Scissors code to see if it
will have a condition to cause the loop to end.
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Use a Loop to Repeat the Game
Line 11 in the code has the while has a block of code
attached that will loop as long as the condition while
playerChoice is equal to False.
The double == operator is a test for equality to see if both
sides of == are the same.
We can see on line 9 that playerChoice is set to False and
line 11 says to loop if playerChoice is False, so it loops.
All of the code after the while statement is indented, which
means that this is the block of code that will loop.
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Input the User's Selection
playerChoice = input("Rock, Paper, Scissors? ")
On the screen when the program runs
Prompt message
User Input
saved in playerChoice
Several things happen with this statement on line 14.
The single equal sign = is the assignment operator.
Everything on the right side of the = is evaluated first
then the result is placed in the variable on the left side of
the = operator.
input("Rock, Paper, Scissors? ") does two things.
The text inside the double-quotes " is the prompt message
which is displayed on the screen asking the user to input
something.
The input(. . .) function collects the keys typed at the
keyboard.
The assignment operator takes the result from the input(. .
.) function
and places it in the playerChoice variable.
Putting all this together, a message is displayed, "Rock,
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Paper, Scissors? "
and waits for the user to type something.
If the user types, "Paper", then the word "Paper" is saved in
the playerChoice variable.
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Random Number Generator
The program is using Python's random number generator
to choose a selection for the computer.
The first executable line in the program is
from random import randint
This gives the program the ability to use the randint( )
function
which is available from Python's library of functions.
Line 15 has
computerChoice = possibleSelection[randint(0,2)]
The first thing executed is randint(0,2) which randomly
chooses a number from 0 to 2.
Which means the values 0, 1 or 2 will be chosen.
The program then uses that choice to pick either Rock,
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Paper or Scissors from the possibleSelection array.
The assignment operator = then saves that value in the
computerChoice variable.
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Flowchart to Determine a Tie or Winner
START
while
Loop
Player
"Rock"
No
Input from
user
Yes
Computer
selection
Computer
"Paper"
Tie?
Yes
Tie
"Tie Game"
No
Yes
No
Lose
Win
Paper covers Rock breaks
Rock
Scissors
Player
"Paper"
Yes
No
Computer
"Scissors"
Yes
Lose
Scissors
cut Paper
No
Win
Paper
covers Rock
Player
Scissors
Yes
No
X
Computer
"Rock"
Yes
Lose
Rock breaks
Scissors
No
Win
Scissors
cut Paper
Here is a flowchart giving a visual presentation of how the
program works.
Actually, the while statement is coded to cause the program
to loop forever.
You need to click the STOP button at the top of the
development system to stop the program.
Later in this presentation, we can examine possible
enhancements on stopping the program a different way.
The code flows down sequentially to get the user's input,
a random selection for the computer,
then test to see if they are both the same.
If the user's and the computer's selection are the same,
then it is a TIE game.
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A message is displayed, then the program goes back to the
top of the loop.
If it isn't a tie game, the program checks to see if the player
entered "Rock"
If not, see if the player entered "Paper"
If not, see if the player entered "Scissors"
For any of those three entries by the user, drop down and
determine
a winner or loser based on what the computer's random
selection was.
If the player entered something other than Rock, Paper or
Scissors,
then display an error message and ask the user to re-enter
their choice.
I ran out of room on the page when that happens,
So, do me a favor and pretend that it's there.
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Determine a Tie, Winner or Loser
Use a combination of if…elif…else statements to
determine if it is a tie, the user wins or the computer
wins the game.
In Python, the if statement needs a condition that evaluates
to TRUE or FALSE.
If the condition is true, do one thing, otherwise, do
something different.
For example, if I was writing a program for an ATM machine,
if withdrawal request is less than the balance on the
account, give the money,
else display a message, "insufficient funds for the
transaction".
Program complexity can increase with a series of if. . .elif. .
.elif. . .else statements.
Each of the if and elif statements need to have a condition
that evaluates to TRUE or FALSE.
If any of them evaluate to TRUE, the its block of code is
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executed and the rest of the elif and else statements are
skipped.
The else statement does test for any condition.
The else statement is at the end of the sequence
and executes as a default if nothing else passed.
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Input Validation
Your program still needs to verify that only selections
of Rock, Paper or Scissors were entered by the user.
Display a message indicating illegal input if anything
except those three selections were entered.
Remember the red 'X' on the flowchart where I ran out of
room?
This is covered by the last else statement and processes the
default
when the user entered anything except "Rock", "Paper" or
"Scissors".
When that happens, the program displays a message,
"That's not a valid play. Check your spelling!"
This will happen even if the user types "rock" without the
capital-R.
If you are the programmer, make your error messages polite
and don't insult the intelligence of the people using the
software.
Maybe, that error message wasn't very polite.
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The onlineGDB.com development system
• What is
onlineGDB.com
• Starting
onlineGDB.com
• Select the
language
• Adjusting
window sizes
Writing the code is only the first step.
You also need a compiler or interpreter program to actually
run the program.
One of the best tools I have found is onlineGDB.com
It can run C, C++, HTML, Python and code from several
other programming languages.
You don't need to install any other applications on your
computer to use onlineGDB.com
It runs on the cloud, somewhere in the world.
Use any browser and put onlineGDB.com in the address bar.
When it starts, select Python 3 as the language.
Enter the program in the code window.
After the code is entered, click the RUN button at the top of
the window.
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If there are any errors, go back and fix them.
Error messages will appear at the bottom in the console
window which has white letters on a black background.
The error messages indicate the line where the error was
detected.
Sometimes the actual error occurred on a previous line but
was only detected later.
Be careful when typing and make sure that capital letters
and small letters are entered correctly.
For example, if a variable is declared as playerChoice with a
capital-C and one time in the program
a small-C was entered, Python will won't link these two
names together and instead create
two separate variables causing the program to not produce
correct results.
The console window is at the bottom.
You can resize this window by grabbing the top of the
window with your mouse or touch-pad and moving it up or
down.
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Indent Using the Tab Key
Use the TAB key to indent
a Python program. Don't
use the space bar.
Use Shift+TAB to indent
to the left.
No indentation. Level 0
Indentation Level 1
Indentation Level 2
Indentation Level 3
Some programming languages use words like "Begin" and
"End" or open and close curly-braces { and } to identify a
block of code. Python does neither. It uses indentation to
space in blocks of code that belong to a higher level of code.
For example, lines 10 through 36 are indented to level-1
indicating a block of code belonging to the while statement
on line 9.
The print statement on line 16 is indented to level-2
because it belongs to the if statement on line 15.
The print statement on line 19 is indented to level-3
because it belongs to the if statement on line 18.
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For best results when entering a Python program, use the
TAB key to indent to the right for each level of code. Use
Shift+TAB to move the indentation to the left.
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Possible Enhancements
• Ignore case during comparisons ("Rock" vs. "rock")
• Game counter
• Keep track of win/lose percentage
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Comments
Comments in Python start with the hash-mark # and stop
at the end of the line. Comments are provided as a way of
documenting the program and helping the next person
who reads the code to understand it easier.
For full credit on the project, you also need to enter all of
the comments as shown on the provided code.
Comments in Python start with the hash-mark # and stop at
the end of the line. Comments are provided as a way of
documenting the program and helping the next person who
reads the code to understand it easier.
For full credit on the project, you also need to enter all of
the comments as shown on the provided code.
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Save a Copy of Your Program
After you have the program working correctly, make sure you
save a copy to your computer and in the lab report.
a) click the cursor in the code window,
copy to the clipboard (Ctrl+A, Ctrl+C on Windows)
or (Command+A, Command+C on macOS)
then (Ctrl+V or Command+V) to paste it in your
lab report.
b) Use the download button
on onlineGDB. This
should save the code in the Downloads directory of your
computer in a file named main.py
After you have the program working correctly, make sure
you save a copy to your computer and in the lab report.
a) click the cursor in the code window,
copy to the clipboard (Ctrl+A, Ctrl+C on Windows)
or (Command+A, Command+C on macOS)
then (Ctrl+V or Command+V) to paste it in your
lab report.
b) Use the download button on onlineGDB. This
should save the code in the Downloads directory of your
computer in a file named main.py
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Document the Project in the Lab Report
• Insert screenshots
• Insert the Python code
Instructions in the lab report have instructions on how
to get a screenshot of your console window.
Insert screenshots
Insert the Python code
Instructions in the lab report have instructions on how to
get a screenshot of your console window.
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And again, a great big welcome to the wonderful world of
programming using Python.
Dandalf signing off for now.
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