Program Design: Pseudocode and Flowcharts

Designing a Program

• Programs must be designed before they are written
• Program development cycle involves designing a program, writing code, correcting syntax and logic errors, and testing
• Design constitutes the most important part of the program development cycle
• Understanding the program's task involves working with customers, asking questions, and outlining software requirements
• Steps to perform a task include breaking down the required task into a series of steps and creating an algorithm

Pseudocode

• Pseudocode translates to fake code
• Pseudocode is an informal language with no syntax rules
• It's not meant to be compiled or executed, but to create a model program
• Focus is on program design, as there are no concerns over syntax errors
• Pseudocode can be translated directly into actual code in a programming language

Flowcharts

• A flowchart is a diagram that depicts the steps in a program graphically
• Ovals are used as terminal symbols
• Parallelograms signify input and output
• Rectangles represent processing
• Arrows connect symbols, representing program flow

Input, Processing, and Output

• Computers generally perform a three-step process
• Input is received, or any data the program gets while running
• Processing occurs, such as a mathematical calculation
• Output is produced

Displaying Output with Print Function

• A function is a prewritten piece of code that performs an operation
• The print function displays output on the screen
• An argument is data passed to a function, such as data printed to the screen
• Program statements execute in the order they appear, from top to bottom

Strings and String Literals

• A string constitutes a sequence of characters used as data
• A string literal appears in actual program code
• String literals must be in single (') or double (") quotes
• Triple quotes (""" or ''') may enclose string literals
• Enclosed strings can contain single and double quotes and span multiple lines

Comments

• Comments are notes of explanation within a program, which the Python interpreter ignores
• Comments are intended for people reading the program's code
• Begin comments with a # character
• End-line comments appear at the end of a line of code
• End-line comments typically explains the line's purpose

Variables

• A variable represents a value stored in computer memory
• Variables are used to access and manipulate that data
• The variable references the value it represents
• An assignment statement creates a variable and makes a reference to data
• General format: variable = expression
• Example: age = 29
• The equal sign (=) is the assignment operator

Variables (continued)

• In an assignment statement, the variable receiving a value has to be on the left side
• A variable can be passed as an argument to a function
• Variable names should never be enclosed in quotes
• You can only use a variable if a value has been assigned to it

Variable Naming Rules

• Rules for naming variables in Python
• Variable name cannot be a Python key word
• Variable name cannot contain spaces
• First character must be a letter or an underscore
• After the first character, you can use letters, digits, or underscores
• Variable names are case sensitive
• Variable names should reflect its purpose

Displaying Multiple Items with the print Function

• Python can display multiple items with a single call to print
• Commas separate items when passed as arguments
• Arguments are displayed in the order they are passed to the function
• Items are automatically separated by a space when displayed

Variable Reassignment

• Variables can reference different values while a program runs
• Garbage collection removes variables that are no longer referenced
• Garbage collection is carried out by the Python interpreter
• A variable can refer to items of any type
• A variable can be assigned to one type and reassigned to another type

Numeric Data Types, Literals, and the str Data Type

• Data types categorize values in memory
• int is for an integer
• float is for a real number
• str is for storing strings
• A numeric literal is a number written in a program
• No decimal point means it’s considered int
• A decimal point means it’s considered float
• Some operations behave differently depending on data type

Reassigning a Variable to a Different Type

• A variable in Python can refer to items of any type

Reading Input from the Keyboard

• Most programs read from the user
• The built-in input function reads from the keyboard
• The input function returns the data as a string
• Format: variable = input(prompt)
• prompt is a string that instructs users tp enter a value
• A space is not automatically displayed after prompts

Reading Numbers with the input Function

• The input function always returns a string
• Built-in functions can convert between data types
• int(item) converts the item to an int
• float(item) converts the item to a float
• A nested function call has the general format: function1(function2(argument))
• the value returned by function2 is passed to function1
• Type conversion works only if the item is a valid numeric value; otherwise, it throws an exception

Performing Calculations

• Math expressions perform calculations and give a value
• Math operators perform calculations
• Operands are values surrounding the operator
• variables can be used as operands
• Resulting values are typically assigned to a variable
• There are two types of division: / and //
• / performs floating point division
• // performs integer division
  • positive results are truncated, negative results are rounded away from 0

Operator Precedence and Grouping with Parentheses

• Python operator precedence:

1. Operations enclosed in parentheses

• enforces operations to be performed before others

2. Exponentiation (**)
3. Multiplication (*), division (/ and //), and remainder (%)
4. Addition (+) and subtraction (-)

• Higher precedence is performed first
• Same precedence operators execute from left to right

The Exponent Operator and the Remainder Operator

• Exponent operator (**) raises a number to a power
• Syntax: x ** y = x^y
• Remainder operator (%) performs division and returns the remainder
• also known as modulus operator
• Example: 4%2=0, 5%2=1
• Used to convert times and distances and to detect odd or even numbers

Converting Math Formulas to Programming Statements

• An operator is required for any mathematical operation
• When converting mathematical expressions to programming statements:
• Add multiplication operators where necessary
• Insert parentheses where necessary

Mixed-Type Expressions and Data Type Conversion

• The type of data resulting from a math operation depends on the data types of operands
• Two int values: result is an int
• Two float values: result is a float
• int and float: int's temporarily converted to float's, result is a float
• mixed-type expression
• Conversion from float to int can cause truncation of the fractional part

Breaking Long Statements into Multiple Lines (1 of 2)

• Lines of code that cannot be viewed on screens or printed due to length can be broken up
• The multiline continuation character (\) allows breaking a statement into multiple lines

Breaking Long Statements into Multiple Lines (2 of 2)

• Any part of a statement that is enclosed in parentheses can be broken without the line continuation character

String Concatenation

• String concatenation appends one string to the end of another
• The + operator does string concatenation

Implicit String Literal Concatenation

• String literals written adjacent to each other are implicitly concatenated into a single string

More About the print Function

• The print function displays a line of output
• A newline character is at the end of printed data
• The special argument end='delimiter' causes print to place the delimiter at the end of the data, instead of the newline character
• The print function uses space as an item separator
• The special argument sep='delimiter' causes print to use the delimiter as an item separator
• Special characters appearing in string literals are preceded by a backslash (\)
• Examples: newline (\n), horizontal tab (\t)
• Treated as commands embedded in strings

Displaying Formatted Output with F-strings

• F-strings are string literals prefixed with the letter f
• F-strings support placeholders for variables
• Placeholders can also be expressions evaluated at runtime
• Format specifiers can be used with placeholders
• .2f means round the value to 2 decimal places and display the value as a floating-point number
• The order of designators in a format specifier: [alignment] [width] [,] [.precision] [type]

Magic Numbers

• Magic numbers are numeric values appearing in a program's code with no explanation
• It can be hard to determine its purpose
• Taking a lot of effort to change the number in each location
• Risk of making an error each time you use the magic number

Named Constants

• Name constants should be used instead of magic numbers
• A named constant is a name representing a value that will not change during the program
• Example: INTEREST_RATE = 0.069
• This creates a named constant named INTEREST_RATE assigned the value 0.069
• Named constants can be used instead of magic numbers

Advantages of Using Named Constants

• Named constants make code self-explanatory (self-documenting)
• Make code easier to maintain
• Prevents typographical errors that are common when using magic numbers

Introduction to Turtle Graphics

• Python's turtle graphics system displays a small cursor known as a turtle
• Python statements can be used to move the turtle, drawing lines and shapes

Introduction to Turtle Graphics (2 of 2)

• Import the turtle module with the statement import turtle to use the turtle graphics system
• This loads the turtle module into memory

Moving the Turtle Forward

• Use the turtle.forward(n) statement to move the turtle forward n pixels

Turning the Turtle

• The turtle's initial heading is 0 degrees (east)
• The turtle.right(angle) statement turns the turtle right by the given angle in degrees
• The turtle.left(angle) statement turns the turtle left by the given angle in degrees

Setting the Turtle's Heading

• Use the turtle.setheading(angle) statement to set the turtle's heading to a specific angle

Setting the Pen Up or Down

• When the turtle's pen is down, the turtle draws a line as it moves
• By default, the pen is down
• When the turtle's pen is up, the turtle does not draw as it moves
• Use the turtle.penup() statement to raise the pen
• Use the turtle.pendown() statement to lower the pen

Drawing Circles

• Use the turtle.circle(radius) statement to draw a circle with a specified radius

Drawing Dots

• Use the turtle.dot() statement to draw a simple dot at the turtle's current location

Changing the Pen Size and Drawing Color

• Use the turtle.pensize(width) statement to change the width of the turtle's pen in pixels
• Use the turtle.pencolor(color) statement to change the turtle's drawing color
• A complete list of colors is in Appendix D

Working with the Turtle's Window

• Use the turtle.bgcolor(color) statement to set the window's background color
• A complete list of colors is in Appendix D
• Use the turtle.setup(width, height) statement to set the size of the turtle's window in pixels
• Arguments width and height are width and height in pixels

Resetting the Turtle's Window

• The turtle.reset() statement does the following:
• Erases all drawings in the graphics window
• Resets the drawing color to black
• Resets the turtle to it's original position (center)
• Does not reset the graphics background color
• The turtle.clear() statement does the following:
• Erases all drawings that currently appear in the graphics window
• Does not change the turtle's position
• Does not change the drawing color
• Does not change the graphics window's background color
• The turtle.clearscreen() statement does the following:
• Erases all drawings that currently appear in the graphics window
• Resets the drawing color to black
• Resets the turtle to it's original position (center)
• Resets the graphics window's background color to white.

Working with Coordinates

• The turtle uses Cartesian Coordinates

Moving the Turtle to a Specific Location

• Use the turtle.goto(x, y) statement to move the turtle to a specific location
• The turtle.pos() statement displays the turtle's current X, Y coordinates
• The turtle.xcor() statement displays the turtle's current X coordinate
• The turtle.ycor() statement displays the turtle's current Y coordinate

Animation Speed

• Use the turtle.speed(speed) command to change the turtle's speed of movement
• The speed argument ranges from 0 to 10
• A speed of 0 makes all of the turtle's moves instantly and disables animation

Hiding and Displaying the Turtle

• Use the turtle.hideturtle() command to hide the turtle
• This command does not change the way graphics are drawn. It simply hides the turtle icon
• Use the turtle.showturtle() command to display the turtle

Displaying Text

• Use the turtle.write(text) statement to display text in the turtle's graphics window
• The text argument is a string you want to display
• The lower-left corner of the first character will be positioned at the turtle's X and Y coordinates

Filling Shapes

• To fill a shape with color:
• Use the turtle.begin_fill() command before drawing the shape
• Use the turtle.end_fill() command after the shape is drawn
• When the turtle.end_fill() command executes, the shape will be filled with the current fill color

Getting Input With a Dialog Box

• Specify a default value, minimum value, and maximum value with turtle.numinput
• An error message will be displayed if the input value is less than minval or greater than maxval

Keeping the Graphics Window Open

• The graphics window closes immediately when the program finishes while running a turtle graphics program outside IDLE
• To prevent this, add the turtle.done() statement to the very end of your turtle graphics programs
• This will cause the graphics window to remain open after the program finishes