Algorithm design (cs)

• Program Development Life Cycle (PDLC)


  1. Analysis

  2. Design

  3. Coding

  4. Testing

  5. Launch
• Analysis

  Understanding the requirement specification - what the program aims to achieve
• Analysis

  • Decomposition - breaking down a complex problem into smaller parts
  • Abstraction - keeping the essential elements and discarding unnecessary details
• Design

  Developers continue the decomposition and start visualizing the end product using methods like structure diagrams, flowcharts and pseudocode
• Coding

  Programmers work on the design modules and write code to make each part of the program functional
• Testing

  The system is divided into smaller sections to be tested for input, output and validation, using test data
• PDLC

  It is a cyclical process, not linear - after launch, programmers go back to analysis to identify new features to add
• Computer systems are made up of software, data, hardware, communication and people
• Subsystems

  Computer systems can be divided into subsystems, which can be further divided into smaller subsystems
• Structure diagram

  Shows a top-down design in diagrammatic form, breaking down a problem into subsystems
• Alarm clock app(Example)

  • Input: set time, turn alarm on/off
  • Process: check if time has reached, sound alarm
  • Output: sound alarm, send notification
• A security system on a door allows a user to type a code into a key to enter the room and if an incorrect code is input and every message is shown and a short alarm sound is played
• Security system

  • Input: Keypad to allow user to enter code
  • Process: Check if code entered is correct
  • Output: Sound alarm if incorrect code
• Algorithm

  A set of instructions for solving a problem or accomplishing a task
• Flowcharts and pseudocode are effective in communicating how the algorithm that makes up a system or subsystem work because it shows the algorithm in a diagrammatic manner
• Flowchart components

  • Terminal symbol (rounded rectangle): Start/end
  • Process (rectangle): Instruction/step
  • Input/Output (parallelogram): User input/output
  • Subroutine (special symbol): Smaller procedure within program
  • Flow line (arrow): Direction of flow
• Flowchart for computer program

  1. Input two numbers

  2. Multiply numbers

  3. Check if product is greater than 100

  4. Output "Greater than 100" if true

  5. Ask user if they want to continue

  6. Loop back to input if yes, end if no
• Flowchart for concert ticket sales

  1. Input number of tickets

  2. Check if number of tickets is greater than 25

  3. If true, ask user to re-enter

  4. Calculate total cost based on number of tickets and discounts

  5. Output total cost
• Decision variable

  1. Check if number of tickets is greater than 25

  2. Check if number of tickets is less than 0
• If the condition is met, we'll ask the user to input again. If the condition is not met, the user can continue
• If the user has keyed in a valid value between 1 to 25, we need to check if they are eligible for a discount
• Discount calculation

  1. Check if number of tickets is greater than 10

  2. Check if number of tickets is greater than 20

  3. Calculate discount percentage

  4. Calculate total price after discount
• The final step is to output the total price for the user
• Pseudocode

  A way to write down the algorithm of a program using English keywords, without the strict syntax rules of programming languages
• Pseudocode

  • It is universally understandable, not bounded by programming language syntax
  • It can be easily translated into any programming language
• Assignment statement

  Assigning a value to a variable
• Array

  A container that can store multiple items under one variable name
• Declaring an array

  1. Declare array_name array 1 to 10

  2. Access element at index 12: array_name[12]
• Input

  Asking the user for input
• Output

  Displaying information on the screen
• Conditional

  Allowing the algorithm to do different things based on different conditions
• Conditional structure

  1. If condition then

  2. Else

  3. End if
• if

  Key word used to start a conditional statement
• then

  Key word used to indicate the code to be executed if the condition is true
• else

  Key word used to indicate the code to be executed if the condition is false
• Condition

  The expression that evaluates to true or false
• The code encapsulated by the then keyword will be executed if the condition is true, the code encapsulated by the else keyword will be executed if the condition is false
• The end if statement signifies the end of the conditional logic
• Comparison operators like greater than, less than, equal to are used to create the condition in an if statement
• Pseudocode for program that asks user height and weight, outputs message if height>170 and weight<85

  1. Input height

  2. Input weight

  3. If height is greater than 170 AND weight is lighter than 85

  4. Output "You meet the requirement"

  5. Else

  6. Output "You need to be taller and lighter"

  7. End if