IGCSE Computer Science - Algorithms

Designing Algorithms

• Definition of an Algorithm:

  • An algorithm is a precise set of rules or instructions to solve a specific problem or task.

• Main Methods of Designing Algorithms:

  • Structure Diagrams
  • Flowcharts
  • Pseudocode

Structure Diagrams

• Definition:

  • Structure diagrams illustrate hierarchical top-down design visually.

• Functionality:

  • Problems are divided into sub-problems, and sub-problems can be further divided.
  • Each level of the diagram specifies tasks suitable for a single subroutine.

• Example:

  • A structure diagram for a mobile application would show components such as user interface, database, and processing tasks.

Flowcharts

• Definition:

  • Flowcharts are visual representations using shapes to describe an algorithm.

• Components:

  • Inputs and outputs
  • Process steps
  • Decisions and repetitions
  • Lines connecting shapes indicate flow of control.

Example Flowchart Scenario

• A casino application where the user inputs their first name to receive a greeting upon accessing the site.

Pseudocode

• Definition:

  • Pseudocode is a text-based tool that describes an algorithm using simple English statements.

• Advantages:

  • More structured than regular sentences, yet flexible for adjustments.

• Example Scenario:

  • A casino program asking for the user's age:
  • Initial Pseudocode:
    INPUT Age IF Age >= 18 THEN OUTPUT "Welcome to the site" ELSE OUTPUT "Sorry, this site is for users 18 and over" ENDIF
  • Refined Version with user’s first name input as well:
    INPUT FName INPUT Age IF Age >= 18 THEN OUTPUT "Welcome to the site, " + FName ELSE OUTPUT "Sorry, this site is for users 18 and over" ENDIF

Explaining Algorithms

• Purpose:

  • The goal of an algorithm is to solve a problem, enabling users unfamiliar with it to interpret its purpose.

• Interpretation Tools:

  • Flowcharts
  • Pseudocode
  • High-level programming languages (e.g., Python)

• Complex Algorithms:

  • Utilize comments in the code, consider context, and test the algorithm with various inputs to understand it better.

• Example to Explain:

  • Pseudocode Example:
    Count ← 1 Number ← 0 Total ← 0 REPEAT INPUT Number Total ← Total + Number Count ← Count + 1 UNTIL Count > 10 OUTPUT Total
  • What it does:
  • Adds ten user-entered numbers and outputs their total.
  • Key Processes:
    • Initialize variables (Count, Number, Total)
    • Inputting user number
    • Adding to Total and Count.

Worked Example in Pseudocode

• Purpose:

  • A teacher's algorithm for entering student marks and grading them.

  • Pseudocode:

  Count ← 0
  REPEAT
      INPUT Score[Count]
      IF Score[Count] >= 70 THEN
          Grade[Count] ← "A"
      ELSE  
          IF Score[Count] >= 60 THEN
              Grade[Count] ← "B"
          ELSE
              IF Score[Count] >= 50 THEN
                  Grade[Count] ← "C"
              ELSE
                  IF Score[Count] >= 40 THEN
                      Grade[Count] ← "D"
                  ELSE
                      IF Score[Count] >= 30 THEN
                          Grade[Count] ← "E"
                      ELSE
                          Grade[Count] ← "F"
                      ENDIF
                  ENDIF
              ENDIF
          ENDIF
      ENDIF
      Count ← Count + 1
  UNTIL Count = 30
  

• Explanation of the Algorithm:

  • Inputs for 30 student marks stored in Score[].
  • Each mark checked against specific boundaries for grading.
  • Matching grades assigned and stored in Grade[] at the same index as input marks.
  • The process stops after all marks are inputted successfully.