Program Design: Algorithms, Pseudocode and Flowcharts

Designing a Program

• Programs must be designed before they are written
• The program development cycle consists of; design, writing code, correcting syntax errors, testing the program, and correcting logic errors
• Design is the most important part of the program development cycle
• Understand the task of the program, working with the customer and creating software requirements

Algorithm

• Determine the steps to perform the task by breaking it down into a series of steps
• An algorithm is a set of well-defined logical steps to perform a task

Pseudocode

• Pseudocode is fake code written in an informal language with no syntax rules
• Informally models programs without compilation or execution, and is translated into actual code in any programming language
• Focus should be on program design without worrying about syntax errors

Flowcharts

• Flowcharts are diagrams that graphically depict the steps in a program
• Ovals are terminal symbols
• Parallelograms are input and output symbols
• Rectangles are processing symbols
• Symbols are connected by arrows represent the flow of the program

Input, Processing, and Output

• A computer typically performs the three step process of input, processing, and output
• Input is any data that the program receives while it is running, such as mathematical calculations
• Output is a result of the process of the input

Print Function

• A function is a piece of prewritten code that performs an operation
• A print function displays output on the screen
• An argument is data given to a function, used like examples for what is printed
• Statements in a program will execute in the same order that they appear

Strings and String Literals

• Strings are sequences of characters used as data
• String literals are strings represented in a program's code
• String literals must be enclosed in single (') or double (") quote marks
• String literals can be enclosed in triple quotes (''' or """)
• Enclosed strings can contain both single and double quotes and can have multiple lines

Comments

• Comments are notes of explanation within a program, which are ignored by Python and intended for a person reading the program's code
• Comments begin with a # character
• An end-line comment appears at the end of a line of code, typically explaining the purpose of that line

Variables

• Variables are names that represents a value stored in the computer memory, used to access and manipulate data stored in memory
• A variable references the value it represents
• An assignment statement is used to create a variable and make it reference data using a general format
• variable = expression, for example age = 29, where (=) is the assignment operator

Variable Assignments

• In an assignment statement, a variable receiving a value must be on the left side
• A variable can be passed as an argument to a function, with the variable name not enclosed in quote marks
• A variable can only be used if a value is assigned to it

Variable Naming Rules

• The rules for naming variables in Python are that; variable names cannot be Python key words, cannot contain spaces, the first character must be a letter or underscore, after the first character letters/digits/underscores are allowed, and variable names are case sensitive
• Variable names should reflect their use

Print Function Calls

• Python permits displaying multiple items with a single call to print
• Items are separated by commas when passed as arguments
• The arguments are displayed in the order they are passed to the function
• Items are automatically separated by a space when displayed on a screen

Variable Reassignment

• Variables can reference different values while the program is running
• Garbage collection removes used values that are no longer referenced by variables via the Python interpreter
• Variables can refer to items of any type, even if they have been assigned to a specific one

Data Types

• Data types categorize values in memory
• Int is for integer, float is for real numbers, and str is used for storing strings
• A numeric literal is a number written in a program
• Numbers with no decimal point are considered int, otherwise they are considered float
• Some operations behave differently depending on data type

Reassigning Types

• Variables in Python can refer to items of any type

Keyboard input

• Most programs require reading input from the user
• The built-in input function is used when reading input from a keyboard
• All input is returned as string data
• Variable = Input (Prompt) format
• Value is typically a string prompting the user to value

Input Function

• The input function always returns a string
• Built in functions convert between data types
• int(item) converts an item to an int
• float(item) converts an item to a float
• Nested function calls follow the Function1 (Function2 (Argument)) format
• Value returned by Function 2 is passed to Function 1
• Type conversion only works if the item is a valid numeric value, otherwise an exception is thrown

Calculations

• The Math expression performs calculations and gives a value
• Math operators is the tool used, operating on the surrounding variables
• The resulting value is typically assigned to a variable
• Regular division "/" is a floating point
• Integer division "//" truncates

Operator Precedence

• Python operator precedence is as follows
• Operations enclosed in parentheses
• Exponentiation (**)
• Multiplication (*), division (/ and //), and remainder (%)
• Addition (+) and subtraction (-)
• Higher precedence is performed first, with same level proceeding from left to right

Exponents and Remainders

• Exponent operator (**) raises a number to a power
• Remainder operator (%) performs division operations and returns the remainder
• Also known as a modulus operator
• They can convert times and distances, or to detect odd or even numbers

Long Statements

• Long statements cannot be viewed on screen without scrolling and cannot be printed without cutting them off
• Multiline continuation character () allows you to break a statement into multiple lines
• Any part of a statement enclosed in parentheses can be broken without the line continuation character

Data Output

• The print function displays lines of output end= ' delimiter' replaces new lines in output data
• sep='delimiter' replaces item separation in output data

String Operations

• Special characters appearing in a string literal are preceded by a backslash ()
• Examples are newline (\n), horizontal tab (\t)
• The + operator performs string concatenation

Number Formats

• You can format display of numbers on the screen using the built in format function
• It requires two items including; Numeric value to be formatted and a Format specifier
• Returns string containing formatted number
• Format specifier typically includes precision and data type
• Scientific notation, comma separators, and the minimum field used to display the value

Integers as Formats

• The % symbol can be used in the format function to format a number as a percentage
• To format an integer using the format function do not specify precision
• Use "d" as the type designator, and still use format function to set field width or comma separator

Magic Numbers

• A magic number is an unexplained numeric value that appears in a program's code
• It is difficult to determine the purpose of the number
• If this type of value needs to be changed it must be modified in every place it is used
• There is a risk of making a mistake each time you type in the value

Named Constants

• Named constants should be used instead of magic numbers
• Named constant is a name representing a value that does not change during the program's execution

Constant Advantages

• They make code self-explanatory
• Help to make code easier to maintain
• Help prevent typographical errors associated with manual entry