Week 4 Notes: Algorithms, Pseudocode, and Flowchart

Algorithms

  • Definition: A set of step-by-step instructions to accomplish a task.
  • Must have a start instruction and a stop instruction.
  • Each instruction must be:
    • precise
    • unambiguous
    • executable in finite time
  • Origin of the term:
    • The word algorithm derives from the phonetic pronunciation of the name Abu Ja'far Mohammed ibn Musa al-Khwarizmi, an Arabic mathematician who developed a set of rules for the four basic arithmetic operations.
  • A procedure is a finite sequence of well-defined instructions that can be mechanically carried out in a finite amount of time.
  • Relationship to problem solving:
    • Algorithms describe the method; pseudocode/flowcharts help design and communicate the method before coding.

Basic Algorithm Concept

  • Input List: A list of inputs required to solve the problem.
  • Output List: The results produced after processing the inputs.
  • A step-by-step method for solving a problem or doing a task.

Examples

  • Sum of two numbers

    • Steps:
    1. Start
    2. Declare variables num1, num2, sum
    3. Read values num1 and num2
    4. Compute sum = num1 + num2
    5. Display sum
    6. Stop
    • Input: two numbers (num1, num2)
    • Process: sum = num1 + num2
    • Output: display the sum (sum)
    • Formula: sum=num1+num2\text{sum} = \text{num1} + \text{num2}

  • Area of a rectangle

    • Steps:
    1. Start
    2. Declare variables length, width, areaR
    3. Read values length and width
    4. Compute areaR = length × width
    5. Display areaR
    6. Stop
    • Input: length and width
    • Process: areaR = length × width
    • Output: display the area (areaR)
    • Formula: areaR=length×width\text{areaR} = \text{length} \times \text{width}
    • Note: Some slides show areaR = length + width in the process line due to a typo; the correct operation is multiplication.

  • Largest of two numbers

    • Steps:
    1. Start
    2. Declare variables val1, val2, ans
    3. Read values val1 and val2
    4. If val1 > val2 then print that val1 is largest
    5. Else print that val2 is largest
    6. Stop
    • Process: if (val1 > val2) then print that val1 is largest; else print that val2 is largest
    • Output: display the answer (ans)

Flowchart

  • Flowchart definition: A tool to show the steps involved in a process.
  • It is a diagram made up of boxes, diamonds and other shapes, connected by arrows. Each shape represents a step and arrows show the order.
  • It depicts the flow or sequence of a program or process.

Flowcharting Basic Symbols

  • Start/End symbol: Terminator; marks the starting or ending point; usually contains Start or End.
  • Action or Process symbol: A box representing a single step or a sub-process.
  • Input/Output symbol: Represents data entering or leaving the system (input or output).
  • Decision symbol: A diamond indicating a branching point; multiple lines for different decisions.
  • Preparation symbol: Sets up to another step.
  • Flow Lines: Arrows showing direction of flow.

Additional Symbols (as listed on slides)

  • Terminal
  • Process
  • Condition
  • Data
  • Stored Data
  • Connector
  • Document
  • Delay
  • Display
  • Preparation
  • Manual Input
  • Multidocument
  • Database
  • Flow Line
  • Manual Operation
  • Predefined Process
  • Card
  • Paper Tape
  • Direct Data

General Rules for Flowcharting

  • All boxes are connected with arrows (flow lines).
  • Flowchart symbols have an entry point on the top of the symbol; exit point is on the bottom for most symbols; the Decision symbol has two exit points (sides or bottom).
  • Generally, flow proceeds from top to bottom; upward flow is allowed only up to 3 symbols.
  • Connectors are used to bridge breaks in the flowchart.
  • Subroutines and interrupt programs have their own independent flowcharts.
  • All flowcharts start with a Terminal or Predefined Process symbol (for interrupt programs or subroutines).
  • All flowcharts end with a Terminal symbol.

Algorithm/Flowchart to Find the Sum of Two Numbers

  • Steps (same as earlier): Start → Declare num1, num2, sum → Read num1, num2 → Compute sum → Print sum → Stop
  • Pseudocode (equivalent):
    • START
    • Declare num1, num2, sum
    • sum = num1 + num2
    • PRINT sum
    • STOP

Algorithm to Find the Largest of Two Numbers (Pseudocode/Flowchart)

  • Steps (same as earlier): Start → Declare val1, val2, ans → Read val1, val2 → If val1 > val2 then print val1 is largest else print val2 is largest → Stop
  • Pseudocode (equivalent):
    • START
    • DECLARE Val1, Val2, Ans
    • READ Val1, Val2
    • IF Val1 > Val2 THEN PRINT Val1 is largest
    • ELSE PRINT Val2 is largest
    • ENDIF
    • STOP

Pseudocode

  • Definition: An artificial and informal language that helps develop algorithms.
  • Purpose: Useful for developing algorithms that will later be translated into a programming language (e.g., C).
  • Characteristic: Similar to English statements; not executed by a computer.

Pseudocode Constructs

  • SEQUENCE: linear tasks performed one after another.
  • WHILE: a loop with a condition at its beginning.
  • REPEAT-UNTIL: a loop with a condition at the bottom.
  • FOR: another looping construct.
  • IF-THEN-ELSE: conditional branching.
  • CASE: generalization of IF-THEN-ELSE (switch-like).
  • These constructs are keywords used to describe the control flow of the algorithm.

Pseudocode Constructs: Keywords and Mappings

  • SEQUENCE: Input: READ, OBTAIN, GET; Output: PRINT, DISPLAY, SHOW; Compute: COMPUTE, CALCULATE, DETERMINE; Initialize: SET, INIT; Add: INCREMENT, BUMP; Sub: DECREMENT
  • FOR: for iteration bounds
  • WHILE: while condition, sequence, ENDWHILE
  • CASE: CASE expression OF … ENDCASE
  • REPEAT-UNTIL: REPEAT, UNTIL condition
  • IF-THEN-ELSE: IF condition THEN … ELSE … ENDIF

Example: Area of a Rectangle (Pseudocode variant)

  • Steps: Start → Declare length, width, areaR → Read length, width → Compute areaR = length × width → Print areaR → Stop
  • Alternative starting code (as shown on slides):
    • Start
    • Set length = 0, width = 0, areaR = 0
    • Read length, width
    • Compute areaR = length × width
    • Print areaR
    • Stop

Example: Largest of Two Numbers (Pseudocode variant)

  • Steps: Start → Declare val1, val2, ans → Read val1, val2 → If val1 > val2 then print val1 is largest else print val2 is largest → Stop
  • Alternative starting code (as shown on slides):
    • Start
    • INIT val1 = 0, val2 = 0, ans = 0
    • GET val1, val2
    • IF Val1 > Val2 THEN PRINT val1 is largest
    • ELSE PRINT val2 is largest
    • ENDIF
    • Stop

Basic Structure of a C Program

  • Example shown:
    • #include
    • int main() { // your code
    • printf("Hello World!");
    • return 0;
    • }
  • Notes:
    • #include is a header file that lets us work with input and output functions, such as printf().
    • Main function is the entry point of the program; code inside {} is executed.
    • Every C statement ends with a semicolon ;
    • printf() is a function used to output text to the screen.
    • return 0; ends the main function.
    • Header files add functionality to C programs.

Practical Connections

  • Flowcharts and pseudocode are tools to design, communicate, and debug algorithms before coding.
  • They help ensure clarity, reduce implementation errors, and facilitate collaboration between team members.
  • In coursework, these concepts bridge from problem analysis to actual programming (C, etc.).

Real-World Relevance and Implications

  • Correctness and unambiguity in algorithms are crucial for predictable software behavior.
  • Flowcharts aid in documentation and maintenance, especially for complex business processes or legacy systems.
  • Pseudocode acts as a transitional artifact between design and implementation across programming languages.
  • Ethical/practical implications: clear communication of logic, avoiding ambiguous symbols, and ensuring accessibility for teams with varying expertise.

Quick Reference: Key Formulas and Rules

  • Sum of two numbers: sum=num1+num2\text{sum} = \text{num1} + \text{num2}
  • Area of a rectangle: area=length×width\text{area} = \text{length} \times \text{width}
  • Flow direction: generally top-to-bottom; upward flows allowed up to 3 symbols.
  • Common pseudocode keywords:
    • Input: READ, OBTAIN, GET
    • Output: PRINT, DISPLAY, SHOW
    • Compute: COMPUTE, CALCULATE, DETERMINE
    • Initialize: SET, INIT
    • Looping: FOR, WHILE, REPEAT-UNTIL
    • Conditional: IF-THEN-ELSE, CASE
  • Example program structure for C:
    • Preprocessor directive: #include
    • Entry point: int main() { … }
    • Output: printf("…");
    • Termination: return 0;

Summary

  • Algorithms define the method; flowcharts and pseudocode help design and visualize the method.
  • Simple problems (sum, area, max) illustrate how to translate a task into steps, a flowchart, and pseudocode.
  • C program basics connect the algorithmic design to actual implementation.