Algorithms and Pseudocode

• Before writing a program, thoroughly understand the problem and plan a solution.

Algorithms

• Algorithms consist of a finite set of unambiguous rules/instructions.
• They specify a finite sequence of operations.
• They are a step-by-step procedure for solving a problem.
• Algorithms are a recipe or procedure.
• Translating an algorithm into computer code is an obstacle in computer problem-solving.
• Programs are written in languages such as C, VB, Java, or Python, which are then converted into machine code.
• Algorithms can be designed using pseudocode or flowcharts.

Pseudocode

• Pseudocode is a simpler version of programming code in plain English with short phrases.
• It defines an algorithm's procedural logic in a simple and easy-to-understand manner.
• Pseudocode describes an algorithm without specific programming language syntax.
• It's free of syntactical complications of Programming Language.
• Pseudocode models real programming code and exists in various forms.
• It borrows syntax from popular languages like C, Lisp, or FORTRAN.
• Natural language is used when details are unimportant or distracting.
• Variable names or placeholders include "sum," "num1," "num2," and "num3" and are changeable.

Algorithm Examples

• Adding three integer numbers involves steps like initializing sum, getting numbers, and adding them.
• Converting Fahrenheit to Celsius follows a formula: C = (5/9)*(F-32).
• Finding the greater number between two numbers involves reading inputs A and B and comparing them.

Calculating Even Numbers Between 0 and 10

• One approach involves initializing n to 0, writing n, incrementing n by 2, and repeating if n <= 10.
• Another way is to initiate n to 0, write n, increment n by 2 and repeating if n < 11, otherwise end.

Finding the Largest Value of Three Integers

• One solution inputs A, B, and C.
• Then it checks conditions to assign the maximum value to a variable Max.
• Finally, the maximum number is printed.

Flowcharts

• A flowchart is a graphical representation of operations in a data processing system.
• It shows steps in a process in sequential order for better understanding.

Flowchart Symbols and Their Functions

• Terminal: Indicates the start, stop, begin, or end of a program or process.
• Input/Output: Represents I/O operations where the computer obtains data or outputs results; Keywords: Input, Output, Read.
• Process: Represents arithmetic operations and data manipulations.
• Decision: Used to ask questions answerable by Yes/No or True/False.
• Flow Lines: Show the direction of flow.
• Predefined Process: Used to invoke a subroutine, function, procedure, or method.
• On-Page Connector: Allows flowcharts to be drawn without intersecting lines on the same page.
• Off-Page Connector: Connects to a different page.
• Preprocessor/Preparation: Used for initialization.

General Flowchart Rules

• All symbols in a flowchart are connected with arrows.
• A decision symbol has two exit points.
• Flowcharts generally flow from top to bottom.
• Subroutines have their own independent flowcharts.

Flowcharting Tips

• Chart the process as it is occurring.
• People modify processes to enable a more efficient process.

Advantages of Using Flowcharts

• Improved communication and effective analysis.
• They ensure proper documentation and efficient coding.
• Flowcharts are helpful for debugging and program maintenance.

Limitations of Using Flowcharts

• Complex logic can make flowcharts complex and clumsy.
• Alterations may require complete redrawing.

Examples

• A flowchart illustrates crossing the street safely by checking for traffic.
• Flowcharts and pseudocode can both represent adding three integer numbers.
• Flowcharts can visually represent finding the area of a circle or the sum, average, and product of three numbers.
• Flowcharts can visually depict finding the greater number between two numbers.

Control Structures

• Sequence: Display messages, read inputs, perform calculations, and display output in a linear order.
• Branching (Selection): Execute different paths based on conditions (e.g., finding the smallest of two numbers).
• Loop (Repetition): Repeat a block of code based on conditions.

Loop Types

• While loop: Repeats as long as a condition is true, used for 0 to Many repetitions.
• Do-while loop: Executes at least once and then repeats as long as a condition is true, used for 1 to Many rerpetitions.

Common Problems and Notes

• Incorrect or no initial value for variables can lead to wrong output or infinite loops.
• Always assign a value to a variable before computations.
• It is a semantic/logical error if you do not assign a value.
• The Decision symbol has two exit points that can be on the sides or the bottom and one side.
• Flowcharts generally flow from top to bottom.
• Subroutines have their own and independent flowcharts.

Problem Solving Example: Finding the Largest of Three Numbers

• A flowchart is constructed using test data to guide the process.
• Terminal symbols indicate start and stop, and I/O symbols ask the user to supply data.
• Decision symbols compare numbers to determine the largest.
• Processes update the state.
• PRINT MAX then STOP to output.

Controlling Repetition or Loops

• Counter-Controlled Repetition: A loop repeats until a counter reaches a certain value.
• Necessary elements include control variable name, initial value, condition for a final value, and increment/decrement.
• Sentinel-Controlled Repetition: A loop continues until a sentinel value is entered
• A sentinel value indicates the end of data entry.
• The sentinel is distinct from any other input value.
• FLAG-Controlled Repetition: Loops until the value of a flag changes, using a Boolean variable.
• End-of-File Controlled: The loop ends when the end of the file is detected.

Function Calls

• Function details are specified in another flowchart (function definition).
• A start terminal for a function differs from that of a main program.
• The end terminal within functions must be "Return".

Properties of Algorithms (Donald Ervin Knuth)

• Finiteness: Must terminate after a finite number of steps.
• Definiteness: Each step must be precisely defined.
• Input: Requires beginning values/quantities.
• Output: Expects a specific output/result.
• Effectiveness: Should use basic operations doable by a person with paper and pencil.

Data Flow Diagrams (DFD)

• Graphical representations of data processes, activities, and flows within an organization.
• They show the flow of data and logic within a system.
• They model sources, destinations, inputs, outputs, actions, and data maintained.

Objectives of Data Flow Diagrams

• Explain the purpose of data-flow diagrams.
• Describe the meaning of symbols.
• Identify diagramming mistakes.
• Construct simple and levelled data-flow diagrams.
• They document & organize information gathered during system analysis to represent how a business operates.

DFD Elements

• External Entity: A person, organization, or system external to the system but interacting with it.
• Process: An activity or function performed for a specific business reason.
• Data Store: A collection of data stored in some way.
• Data Flow: A single piece of data or a logical collection of data.

Context Diagram

• Shows the entire system in context with its environment.
• All process models have one context diagram.
• Data stores are usually not included.
• External entities receive information from or contribute information to the system.

Level 0 Diagram

• Shows all processes comprising the system, data stores, external entities, and data flows.

Level 1 Diagrams

• More explicit DFD that decomposes a Level 0 process, showing greater detail.
• Decomposition breaks down processes into a hierarchy of diagrams.
• All process models have as many level 1 diagrams as Level 0 processes, for example Say, processes number 2.1, 3.1 etc.
• Level 1 diagrams may not be needed for all Level 0 processes.

Level 2 Diagrams

• Show the next level of decomposition and may not be needed for all Level 1 processes.
• Numbering each process helps in understanding where it fits in the system.
• Balancing ensures that all information in a DFD at one level is accurately represented.

DFD Mistakes

• Black Hole: a process with inputs but no outputs.
• Miracle: a process with outputs but no inputs.
• Grey Hole: a process with insufficient input to produce the output.
• Illegal Data Flows: Data cannot move directly between sources/sinks without processing or between data stores without a process.

Developing Data Flow Diagrams (DFD) Using A Top-Down Approach

• List business activities to determine external entities, data flows, processes, and data stores.
• Create a context diagram without detailed processes or data stores.
• Draw Diagram 0, showing general processes & data stores.
• Create child diagrams and additional levels as necessary, and check for errors to ensure labels are meaningful.
• Develop physical DFD from logical DFD.
• Partition physical DFD for programming and implementation.

Logical vs. Physical DFDs

• A logical model shows how the business operates.
• A physical model shows how the system is implemented.
• The logical DFD illustrates processes involved without detailing the physical implementation.

Partitioning the Physical DFD

• Identifies distinct processes for different user groups.
• Separates processes performed at different times.
• Groups similar activities for efficiency.
• Combines activities or Separates them for consistency or security.

HIPO (Hierarchical plus Input-Process-Output)

• A tool for planning and documenting computer programs.
• HIPO offers diagrammatic and textual representations for systems, programs, and processes.
• Consists of a Visual Table of Contents (VTOC) and IPO charts.
• Designers can evaluate and refine design, correcting flaws prior to implementation.

Decision Tables

• Excellent in both testing and requirements management.
• Structured exercises used to formulate requirements for complex business rules.
• Represent conditions and actions with rules.
• Each table column has a rule describing the unique combination of circumstances and actions.
• Make possible to detect combination of conditions that would have otherwise not been detected