to Python Code CS 1111 September 4, 2019 Logistics You do not need - - PowerPoint PPT Presentation

Going From Psuedocode to Python Code

CS 1111 – September 4, 2019

Logistics

• You do not need to do Lab02 Counting, nor can you get credit for it
• The labs are intended primarily for 1110
• We will do roughly every other lab
• Our first Lab will be Lab03, which we will do in-class next Wednesday
• You must be in class to get credit
• You are only required to attend on lab days
• Labs are never HOMEWORK, therefore, you don’t have to do them ahead
• First programming assignments are
• PA1 greeting – Due Wednesday September 18 at 9a.m.
• PA2 nonsense – Due Wednesday September 18 at 9a.m.
• You have two weeks for this, but should be able to do PA1 after today if you wish

Last Time: Psuedocode

• Psuedocode is on of the methods that can be used to represent /

describe an algorithm

• The syntax is not rigid
• But the problem solving IS detailed, executable, and unambiguous
• Psuedocode can be easily translated into a high-level programming

language

• Include a specific sequence of actions that a program will take
• Each line does a single, clear, unambiguous, executable thing

Control Structures

• What control structures did we discuss last time?

Control Structures

• Sequence
• A series of actions performed one after another
• Condition (if)
• To decide which of two or more different statements to execute depending
• n a certain condition
• Repetition (loop)
• Repeat a certain step while a certain condition is true
• Subprogram / named action
• A small part of another program used to solve a certain problem

Example

• Write pseudocode to solve the following problem:
• A company is trying to have a storewide 20% off sale
• State sales tax is 5%
• A cashier should enter the list price which does NOT include the 20% off or

sales tax.

• The system applies the discount, then the sales tax to the list price to

calculate the final total.

• The system then displays the final total

Design

Psuedocode Flow Chart

• 1. Get item price
• 2. Subtract 20% discount
• 3. Add 5% sales tax
• 4. Display final sale price

Get item price Subtract 20% discount Add 5% sales tax Display final sale price Start End

Is this good enough?

• Robustness testing
• Consider inputs that are semantically meaningless, but syntactically valid
• Syntactically valid means the types of the inputs are correct, so the pseudocode

is executable with that input

• Semantically meaningless means that the input wouldn’t make sense from an

external perspective (buying -3 gallons of gas, or saying your birthday is 13/45/3000)

• Price is a number
• Numbers can be 0
• Numbers can be negative
• This one specifically wouldn’t make sense
• How can we adjust our algorithm to consider robustness?

Design

Psuedocode Flow Chart

• 1. Get item price
• 2. If item price < 0

1. Repeat step 1

• 3. Subtract 20% discount
• 4. Add 5% sales tax
• 5. Display final sale price

Get item price Subtract 20% discount Add 5% sales tax Display final sale price Start End

false

item price < 0

true

Python

• Interpreted Programming Language (does not compile)
• Has simple syntax – easy to read
• Has most of the features of traditional programming languages
• Supports a wide range of applications
• Games, web, apps, system administration
• Widely used
• Google, IBM, Disney, EA
• Open Source

Psuedocode to Python

Psuedocode

• 1. Set price = 0
• 2. If item price <= 0

1. Get price from the user

• 3. Subtract 20% discount
• 4. Add 5% sales tax
• 5. Display final sale price

How to Program

• Programmer uses an editor (such as PyCharm) to edit the source

code (.py file)

• When the programmer hits run, the interpreter turns the program

into byte code (.pyc file)

• The Bytecode is translated into machine instructions

z = 0 x = 3 y = x while x != 0: z = z + y x = x – 1 y = z ADD R3 R2 R3 SUB R0 R0 R1 BZERO 4 BRANCH 0 MOVE R2 R3 HALT

High-Level Language Assembly Language Machine Language

Compiler/ Interprette r Assembler

10100001001011 01010001111011 01110000110101 00100110101010 01110010101101 10111101011111 11111111111

PyCharm

• You should have installed this last week (if you haven’t, do Lab0
• n your own ASAP!!!)

Print()

• print() is a Subprogram that prints to the console (bottom part
• f the PyCharm screen)
• You can print:
• Text
• Numbers
• Any variable
• Test
• print("Hello World")
• print()
• print(5)

Comments

• To help ensure our code is not just readable, but understandable,

we use comments

• Comments are ignored by the interpreter, and are there to explain our code
• We use the ‘#’ symbol to start a comment.
• The interpreter ignores all text after the #

Block comments

• We may have a large section of comments (note that because I

have no #, the entire program won’t compile!)

Block comments

• I could just simply add # before every line

Block comments

• Or, I could use block comment

1. Select all lines of code to be commented 2. If you have windows

1. Hit CTRL + /

3. If you have Mac

1. Hit Command + /

Indentation

• Python requires correct indentation
• Below does not compile because of incorrect indentation
• Notice the red squiggly line
• You must indent for the loop, everything indented is INSIDE the

loop

• 4 spaces

Continuing statements over multiple lines

• Two ways to continue a line of code
• "/n" is used to “force” a new line (think of it like hitting enter)

# 1. Implicit continuation: divide statements after parentheses, brackets, and braces # and before or after a plus operator print("My name is Will" + " and my favorite courses are " + "\nCS 1111" + "\nCS 2110" + " and \nCS 2501 SDE") # 2. Explicit continuation: use the \ character to divide statements anywhere on a line t = "My name is Will" + " and my favorite courses are " \ + "\nCS 1111" \ + "\nCS 4640" \ + " and \nCS 2501 SDE" print(t)

Printing quotation marks

• “Wait, if we use quotation marks to surround text, how do we

print quotation marks?”

print("My favorite courses are : " + "'CS 1111'") # use different quatations print("My favorite courses are : " + "\'CS 1111\'") # use an escape character print("My favorite courses are : " + "\"CS 1111\"") # use an escape character

Variables

• A variable is a stored value that can change as a program executes
• “Able to vary”
• Initialize a variable with an assignment statement
• Assignment states use the = sign
• Set the variable on the left side to be equal to value of the right side

x = 5 # 5 is a literal y = 3 # 3 is a literal

• The variables value can be changed later

x = 7 # x can be changed to another literal x = y # variable can also be changed to other variables x = x + 1 # variable can also be set to the value of an expression

Type

• There are different types of information we could want to store
• Therefore, there are different types of variables
• int
• 1, 2, 3, … , 0, -1, -2
• string
• “hi”, “Will”, “There once was a man from Nantucket”
• float
• 3.14, 0.07, -0.3, 5.0
• bool
• True, False

Python Variables

• Python variables can change their data type whenever needed
• The below code is completely valid
• quantity1 is at first an int, then a float, then finally a string
• After this code block, quantity1 is “15”
• There’s no way to find out what quantity1 used to be.

quantity1 = 10 # set quantity1 to an int value of 10 quantity1 = 5.0 # set quantity1 to an int value of 5.0 quantity1 = "15" # set list_price to a str value of "15", not an int of 15

Python Variable Naming

• Python variable names are case sensitive
• Variables names can start with letter or underscore
• Not numbers or symbols
• Variables names can contain letters or underscores or numbers
• Not any other special characters

r = 5 # Create variable r y = R # Error because R is not a variable, but r is y = r # Valid

Good Variable Naming Conventions

• Start with a lowercase letter
• Use underscore notation for variable name with multiple words
• variable_name_with_multiple_words
• Don’t use camelCase: variableNameWithMultipleWords
• This is a Java-esque style: every language has it’s own style
• Use meaningful, easy to remember names
• At least two characters long
• Avoid built-in words (we’ll cover these as we go through)