Pseudocode and Algorithms
Definition
Pseudocode
A human-readable representation of an algorithm that combines natural language with programming constructs.
- Pseudocode bridges the gap between flowcharts and actual code, making it easier to understand, analyse and implement algorithms.
- It is used to outline the logic of a program without adhering to the syntax of a specific programming language.
Note
- Pseudocode is not meant to be executed by a computer.
- It's designed for humans to read and interpret.
Analogy
- Think of pseudocode as a blueprint.
- Just like architects use blueprints to plan buildings, programmers use pseudocode to plan algorithms.
It improves:
- Clarity: Pseudocode helps you focus on the logic of your algorithm without worrying about syntax errors.
- Communication: It's easy to share with others, even if they don't know a specific programming language.
- Planning: By writing pseudocode first, you can identify potential issues before coding.
Key Elements of Pseudocode
- Sequential Steps: List actions in the order they occur.
- Variables: Use simple names to store data.
- Control Structures: Include loops and conditionals to control the flow.
- Input/Output: Clearly state how data enters and exits the algorithm.
Hint
- Use capital letters for keywords like IF, THEN, ELSE, FOR, and WHILE.
- This makes your pseudocode easier to read.
Example
Think about an algorithm:
- Input: Enter a number.
- Process:
- If the number is even, divide it by 2.
- Otherwise, multiply it by 3 and add 1.
- Output: Display the result.
Pseudocode can look like:
input NUM
if NUM mod 2 = 0 then
RESULT = NUM div 2
else
RESULT = (NUM * 3) + 1
end if
output RESULTHow to Analyse Pseudocode
- Identify the Purpose
- Determine what the algorithm is trying to achieve.
- For example, is it sorting data, searching for an element, or performing calculations?
- Break Down the Steps
- Divide the pseudocode into smaller sections.
- Analyse each step to understand its role in the algorithm.
- Trace the Flow
- Follow the logic from start to finish.
- Evaluate Efficiency
- Consider the time and space complexity.
- Is there a more efficient way to achieve the same result?
Hint
- Use sample inputs to see how the algorithm processes data.
- When analysing pseudocode, try to visualise the process.
- Drawing a flowchart can help clarify the logic.
Common Mistake
Common Mistakes in Pseudocode Analysis:
- Overlooking Edge Cases: Ensure the algorithm handles all possible inputs.
- Ignoring Efficiency: A working algorithm isn't always the best solution.
- Misinterpreting Steps: Read carefully to avoid misunderstanding the logic.
Example
Let's examine situation when only pseudocode given:
LARGEST = LIST[0] // initialize with the first number in the list
loop INDEX from 1 to LENGTH-1
if LIST[INDEX] > LARGEST then
LARGEST = LIST[INDEX]
end if
end loop
output LARGEST
Then the analysis steps can look like:
- Purpose: The algorithm identifies the largest number in a list.
- Steps:
- Initialise a variable to store the first number in the list.
- Iterate through the list, updating the variable if a larger number is found.
- Flow: The algorithm starts with the first number and compares it to each subsequent number.
- Efficiency: This is a linear search, with a time complexity of O(n).
Self review
- Can you think of a way to improve this algorithm?
- What if the list is already sorted?
Common Mistake
- Don't assume the algorithm is correct.
- Always test it with different inputs to verify its accuracy.
How to Write Pseudocode
Hint
How to write effective pseudocode:
- Be Consistent: Use the same terms and structure throughout.
- Keep It Simple: Avoid unnecessary details.
- Focus on Logic: Don't worry about syntax or specific programming languages.
- Use Indentation: Indent loops and conditionals to show structure.
Let's start with a similar example as above: finding the smallest number in the array.
- Define the Problem
- We want to find the smallest number in a list of numbers.
- Outline the Steps
- Start with the first number as the smallest.
- Compare each number in the list to the current largest.
- If a number is smaller, update the largest.
- Repeat until all numbers are checked.
- Output the smallest number.
- Write the Pseudocode
SMALLEST = LIST[0] // initialise with the first number in the list
loop INDEX from 1 to LENGTH-1
if LIST[INDEX] < SMALLEST then
SMALLEST = LIST[INDEX]
end if
end loop
output SMALLEST
Note
- Notice how the pseudocode uses simple language and clear steps.
- It's easy to follow, even if you're new to programming.
Exam technique
- While there are different standards for writing pseudocode, IB has adapted its own.
- Part of the pseudocode documentation (rules) will be available during the examination.
- However, there is another official document that covers more than the handout and is essential to review.
Self review
- Try writing pseudocode for these problems:
- Calculate the sum of all numbers in a list.
- Check if a number is even or odd.
- Find the average of a list of numbers.
- Try to analyse your written pseudocode using the steps outlined above.
- How did you do?
- Compare your pseudocode to the examples in this article.
- Are your steps clear and logical?
