Lecture 1-3: Algorithm and Flowcharts

1. Core Concepts

Programming

• Definition: The process of writing instructions for a computer to execute. It involves problem-solving, logic, and creativity.
• Importance: Enhances logical thinking and problem-solving skills and is essential in fields like software development, data science, AI, and automation.

Problem Solving

• Core Principle: The central activity of computer science.
• Programmer's Task:
  1. Understand how a human solves the problem.
  2. Translate this human method into an "algorithm" a computer can perform.
  3. Write the algorithm using the specific syntax of a programming language.

2. The Problem-Solving Process

Stages of Problem Solving

1. Analyzing the problem: Identifying inputs, processes, and outputs.
2. Writing the algorithm steps: Using pseudocode or flowcharts.
3. Writing the program: Coding in a specific programming language.
4. Translating the program: Compiling the program into machine code.
5. Executing, testing, and debugging: Running the program and fixing errors.
6. Documenting the software: Creating documentation for future reference and maintenance.

Problem Analysis

This stage requires answering three fundamental questions:

• Q1: What are the desired results or outputs?
• Q2: What data is available, representing the program's inputs?
• Q3: What method or set of processes will be used to transform the inputs into the desired outputs?

3. Algorithms

Definition

• A well-defined set of rules and procedures for solving a problem in a finite number of steps.
• An ordered set of unambiguous, step-by-step instructions that describes a process.
• Can be implemented in more than one programming language.

Characteristics of a Good Algorithm

• Well-Defined Inputs and Outputs: Must have clear input values and produce a definite output.
• Definiteness (Unambiguity): Each step must be precise, with only one possible meaning.
• Finiteness: The algorithm must terminate after a finite number of steps.
• Effectiveness: Each step must be simple and executable in a reasonable time.
• Correctness: The algorithm must correctly solve the problem and produce accurate results.
• Generality: Should be applicable to a range of inputs for problems of the same type.
• Step-by-Step Execution: Follows a sequential or logical order.

Algorithm Design Methods

Algorithms are commonly designed using two main tools before coding:

• Pseudocode: Uses English-like phrases to outline the program's logic.
• Flowchart: Graphically depicts the logical steps and their relationships using standard symbols.

4. Pseudocode

Definition & Purpose

• A method for representing an algorithm using simplified, language-agnostic commands.
• Allows the programmer to focus on the problem-solving logic rather than programming language syntax.
• It is easily converted into actual code once complete.

Components

Pseudocode consists of Keywords, Sections, and Statements.

Syntax

Program start/end

BEGIN: Start of a program.
END: End of a program.

Conditionals

IF condition Then: Start of an if statement.
ELSE: else block.
ELSE IF condition Then: else-if block.
ENDIF: End of an if statement.

I/O

INPUT varname1, varname2: Take user input.
OUTPUT "Some text ", variable_name: Print output.

Variables

SET var = value: Initalize a variable.
var = new_value: Reassign/modify a variable.

Loops

WHILE condition DO: Start a while loop
ENDWHILE: End a while loop

Advantages and Disadvantages

Advantages	Disadvantages
Easy to understand.	Can become lengthy for very complex problems.
Does not require memorizing special symbols or formats.
Does not use any special syntax or strict rules.
Easy to convert into a program in any language.
Requires less space on paper compared to flowcharts.
Replaces symbolic representations with simple English phrases.

Statement Structures

1. Sequential Statements:
   • Definition: Instructions are executed in order, one after another, without any condition disrupting the sequence.
   • Example (Add two numbers):

BEGIN
  INPUT a, b
  SET sum = a + b
  OUTPUT "Sum is:", sum
END

2. Decision (Selection) Statements:
   • Definition: A choice is made between two or more alternatives based on a given condition (e.g., IF-ELSE).
   • Example (Divide two numbers with error checking):

BEGIN
  INPUT numerator, denominator
  IF denominator == 0 THEN
    OUTPUT "Error: Cannot divide by zero"
  ELSE
    SET result = numerator / denominator
    OUTPUT "Result is:", result
  ENDIF
END

3. Repetition (Looping) Statements:
   • Definition: An instruction or sequence of instructions is repeated several times. Also known as iteration.
   • While Loop: Repeats a set of statements as long as a specified condition is true.
   • Example (Print "Welcome" five times):

BEGIN
  SET count = 1
  WHILE count <= 5 DO
    OUTPUT "Welcome"
    SET count = count + 1
  ENDWHILE
END

5. Flowcharts

Definition & Purpose

• A graphical representation showing the sequence of steps in a process or program.
• Uses a set of globally agreed-upon geometric shapes where each shape represents a specific function (e.g., process, decision, input/output).
• They simplify understanding, aid in analysis, guide development, serve as documentation, and improve communication.

Flowchart Symbols

Symbol Name	Shape	Function
Terminator	Oval	Signifies the Start and End points of the process.
Process	Rectangle	Indicates a processing step, calculation, or operation.
Decision	Diamond	Represents a point where the flow can branch based on a yes/no condition.
Data (I/O)	Parallelogram	Indicates data input or output.
Connector	Circle	Connects different parts of the chart, often across pages or distant areas.
Preparation / Loop	Hexagon	Used to represent a for loop, defining initial conditions, increments, and termination.
Document	Wavy-bottomed Rectangle	Represents data that can be read by people, such as a printed output.
Flow Line	Arrow	Connects shapes and indicates the direction of flow.

Drawing Guidelines

• A flowchart must be neat, complete, easy to follow, and unambiguous.
• The natural flow direction is top-to-bottom and left-to-right.
• Start (Terminator) has only one outgoing line.
• End (Terminator) has only one incoming line.
• Process (Rectangle) has one incoming and one outgoing line.
• Decision (Diamond) has one incoming line and typically two outgoing lines (e.g., Yes/No).

Flowchart Examples

Example 1: Adding Two Numbers (Sequential)

Example 2: Checking if a Number is Positive (Decision)

Example 3: Printing Numbers 1 to 5 (Pre-test Loop)

Advantages and Disadvantages

Advantages	Disadvantages
Communication: Standard symbols make logic easy to explain.	Complexity: Flowcharts for complex logic can become very large and confusing.
Analysis: Helps in analyzing problems effectively.	Modification: Changes may require redrawing the entire chart.
Documentation: Serves as important program documentation.	Reproduction: Drawing symbols can be tedious.
Efficient Coding: Acts as a guide for writing code.	Excessive Detail: Too much detail can obscure the main solution path.
Error Detection: Logic can be traced to find errors early.

Repetition Structures in Flowcharts

Each loop must have a termination condition, which can be tested at two different points:

1. Pre-test Loop:
   • The termination condition is checked before the process inside the loop is executed.
   • If the condition is initially false, the loop body will not execute at all.
2. Post-test Loop:
   • The termination condition is checked after the process inside the loop is executed.
   • The loop body is guaranteed to execute at least once.