pseudocode using the three basic constructs of sequence, decision, and iteration

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A-Level Computer Science 9.2 Algorithms - Pseudo-code

A-Level Computer Science 9.2 Algorithms

Objective: Pseudo-code using Sequence, Decision, and Iteration

This section focuses on representing algorithms using pseudo-code, specifically utilizing the fundamental constructs of sequence, decision (if/else), and iteration (loops).

1. Sequence

Sequence refers to the order in which instructions are executed. Instructions are executed one after the other, in the order they appear.

  • Example: A simple algorithm to add two numbers.
    1. Start
    2. Input first number
    3. Input second number
    4. Calculate the sum (first number + second number)
    5. Output the sum
    6. End

2. Decision (If/Else)

Decision constructs allow an algorithm to make choices based on conditions. The most common construct is the 'if-else' statement.

If a condition is true, a block of code is executed. Else, if the condition is false, a different block of code is executed.

Pseudo-code Explanation 
                        IF condition THEN
                            // Code to execute if the condition is true
                        ELSE
                            // Code to execute if the condition is false
                        ENDIF
The algorithm checks a condition. If the condition is true, the code within the 'THEN' block is executed. Otherwise, the code within the 'ELSE' block is executed.

Example: Determining if a number is positive, negative, or zero.

  • Input a number
  • IF the number is greater than 0 THEN Output "Positive"
  • ELSE IF the number is less than 0 THEN Output "Negative"
  • ELSE Output "Zero"

3. Iteration (Loops)

Iteration allows a block of code to be executed repeatedly. Common types of loops include 'for' and 'while' loops.

a) For Loop

A 'for' loop is used when the number of iterations is known in advance.

Pseudo-code Explanation 
                        FOR each item IN sequence
                            // Code to execute for each item
                        ENDFOR
The code within the 'FOR' loop is executed once for each item in the specified sequence.

Example: Calculating the sum of numbers in a list.

  • Input a list of numbers
  • Set sum = 0
  • FOR each number IN the list:
  • sum = sum + number
  • ENDFOR
  • Output the sum

b) While Loop

A 'while' loop is used when the number of iterations is not known in advance, and the loop continues as long as a condition is true.

Pseudo-code Explanation 
                        WHILE condition IS TRUE
                            // Code to execute while the condition is true
                        ENDWHILE
The code within the 'WHILE' loop is executed repeatedly as long as the specified condition remains true.

Example: Repeatedly asking for input until a valid value is entered.

  • Input a value
  • WHILE the value is not valid:
  • Output "Invalid input. Please try again."
  • Input a value
  • ENDWHILE
  • Output the valid value

Combining Constructs

Algorithms often combine sequence, decision, and iteration constructs to solve complex problems. For example, an algorithm might use a 'while' loop to repeatedly ask for input until a certain condition is met, and then use an 'if-else' statement to process the input based on its value.

Figure: A diagram illustrating the combination of sequence, decision, and iteration in a simple algorithm (e.g., checking if a number is even or odd).

Suggested diagram: A flowchart showing the sequence of steps for determining if a number is even or odd, including a decision point and a loop (if applicable).