Pseudocode and Flowcharts¶

Learning Objectives

By the end of this section, you will be able to:

Write clear, readable pseudocode using standard patterns
Create flowcharts
Convert between algorithms, pseudocode, and flowcharts
Use proper conventions for documenting algorithm logic
Represent complex control structures visually

Introduction¶

Once you’ve analyzed an algorithm’s inputs, outputs, and purpose, you need to express the logic clearly. Two main tools help communicate algorithms effectively:

Pseudocode: Structured English that describes steps without language-specific syntax
Flowcharts: Visual diagrams that show the flow of logic through shapes and arrows

Both methods bridge the gap between human thinking and computer code.

Pseudocode Patterns¶

Pseudocode uses everyday language with programming structure to describe algorithms clearly. It follows consistent patterns that anyone can read, regardless of their programming language experience.

Basic Structure¶

ALGORITHM AlgorithmName
BEGIN
    // Algorithm steps go here
END

Sequence Pattern¶

For steps that happen one after another:

ALGORITHM MakeTea
BEGIN
    Boil water
    Add tea bag to cup
    Pour hot water into cup
    Wait 3 minutes
    Remove tea bag
    Add milk if desired
END

Input/Output Patterns¶

INPUT variableName
OUTPUT message
OUTPUT variableName
OUTPUT "literal text"

Example:

ALGORITHM GreetUser
BEGIN
    OUTPUT "What is your name?"
    INPUT userName
    OUTPUT "Hello, " + userName + "!"
END

Variable Assignment Pattern¶

SET variableName = expression

Examples:

SET total = price + tax
SET average = (num1 + num2 + num3) / 3
SET isValid = (age >= 18) AND (hasLicense = true)

Selection Patterns¶

Simple IF

IF condition THEN
    statements
END IF

IF-ELSE

IF condition THEN
    statements for true
ELSE
    statements for false
END IF

IF-ELSE IF-ELSE

IF condition1 THEN
    statements for condition1
ELSE IF condition2 THEN
    statements for condition2
ELSE
    statements for all other cases
END IF

Example: Grade Calculator

ALGORITHM DetermineGrade
BEGIN
    INPUT score

    IF score >= 90 THEN
        SET grade = "A"
    ELSE IF score >= 80 THEN
        SET grade = "B"
    ELSE IF score >= 70 THEN
        SET grade = "C"
    ELSE IF score >= 60 THEN
        SET grade = "D"
    ELSE
        SET grade = "F"
    END IF

    OUTPUT "Your grade is: " + grade
END

Iteration Patterns¶

WHILE Loop

WHILE condition DO
    statements
END WHILE

FOR Loop

FOR variable = start TO end DO
    statements
END FOR

FOR EACH Loop

FOR EACH item IN collection DO
    statements
END FOR

Example: Find Maximum

ALGORITHM FindMaximum
BEGIN
    INPUT numbers (list)
    SET maximum = first number in list

    FOR EACH number IN numbers DO
        IF number > maximum THEN
            SET maximum = number
        END IF
    END FOR

    OUTPUT "The maximum is: " + maximum
END

Flowcharts¶

Flowcharts translate algorithm steps into diagrams that show the order of actions, decision points, and possible paths. They give teams a shared visual language before code is written.

Why Flowcharts Matter¶

Highlight the sequence of work, including where an algorithm branches or repeats
Provide a quick checkpoint for peers or stakeholders who prefer visuals over code
Reveal missing cases (for example, the “no” path on a decision) before coding begins
Support testing and debugging because each arrow represents a path that must be validated

Core Symbols in Context¶

Terminator (rounded rectangle) marks the start or end of a flow.
Parallelogram handles input or output.
Rectangle holds a processing step.
Diamond asks a question that splits the flow.
Connector reunites a branch or links to another page.

Following a Simple Flow¶

The example below shows the straight-line logic for logging into a system. Each arrow represents what happens next.

Handling Decisions¶

Decisions turn a single path into multiple outcomes. Use meaningful labels for each branch so the reader knows what the answer means.

This flow makes it clear that the system always produces exactly one message, even though different rules apply.

Representing Loops¶

Loops show repeated work and must always include an exit condition. The loop below keeps asking for guesses until the answer is correct or attempts run out.

Flowchart Tips¶

Make every decision branch lead to a follow-up step; never leave an arrow dangling.
Keep text short inside each shape—use verbs for actions and questions for decisions.
Check that every terminator is reachable; otherwise, you have a logical dead end.
Use connectors or sub-flowcharts when the diagram no longer fits comfortably on the page.
Align flowchart shapes with your pseudocode: each IF, loop, or assignment should appear in both representations.

Complete Example: Password Strength Checker¶

Let’s see the same algorithm expressed in both pseudocode and flowchart form:

Pseudocode Version¶

ALGORITHM CheckPasswordStrength
BEGIN
    OUTPUT "Enter your password:"
    INPUT password

    SET length = length of password
    SET hasUpper = false
    SET hasLower = false
    SET hasDigit = false
    SET hasSpecial = false

    FOR EACH character IN password DO
        IF character is uppercase letter THEN
            SET hasUpper = true
        END IF
        IF character is lowercase letter THEN
            SET hasLower = true
        END IF
        IF character is digit THEN
            SET hasDigit = true
        END IF
        IF character is special symbol THEN
            SET hasSpecial = true
        END IF
    END FOR

    SET strengthCount = 0

    IF length >= 8 THEN
        SET strengthCount = strengthCount + 1
    END IF

    IF hasUpper = true THEN
        SET strengthCount = strengthCount + 1
    END IF

    IF hasLower = true THEN
        SET strengthCount = strengthCount + 1
    END IF

    IF hasDigit = true THEN
        SET strengthCount = strengthCount + 1
    END IF

    IF hasSpecial = true THEN
        SET strengthCount = strengthCount + 1
    END IF

    IF strengthCount >= 4 THEN
        OUTPUT "Strong password"
    ELSE IF strengthCount >= 2 THEN
        OUTPUT "Medium password"
    ELSE
        OUTPUT "Weak password"
    END IF
END

Flowchart Version¶

Pseudocode Style Guidelines¶

1. Use Clear, Descriptive Names¶

Good:

SET totalPrice = itemPrice + tax + shipping
SET isEligibleForDiscount = (age >= 65) OR (isStudent = true)

Avoid:

SET x = a + b + c
SET flag = (n >= 65) OR (s = true)

2. Indent Consistently¶

Good:

IF weather = "sunny" THEN
    IF temperature > 20 THEN
        OUTPUT "Perfect day for a picnic!"
    ELSE
        OUTPUT "Sunny but too cold"
    END IF
ELSE
    OUTPUT "Not ideal weather"
END IF

Avoid:

IF weather = "sunny" THEN
IF temperature > 20 THEN
OUTPUT "Perfect day for a picnic!"
ELSE
OUTPUT "Sunny but too cold"
END IF
ELSE
OUTPUT "Not ideal weather"
END IF

3. One Action Per Line¶

Good:

INPUT firstName
INPUT lastName
SET fullName = firstName + " " + lastName
OUTPUT fullName

Avoid:

INPUT firstName, INPUT lastName, SET fullName = firstName + " " + lastName, OUTPUT fullName

4. Use Comments for Complex Logic¶

// Calculate compound interest: A = P(1 + r/n)^(nt)
SET amount = principal * (1 + rate/compoundFrequency)^(compoundFrequency * years)

Converting Between Representations¶

From Natural Language to Pseudocode¶

Natural Language: “Ask the user for three numbers. Find the largest number and display it.”

Pseudocode:

ALGORITHM FindLargestOfThree
BEGIN
    OUTPUT "Enter three numbers:"
    INPUT num1
    INPUT num2
    INPUT num3

    SET largest = num1

    IF num2 > largest THEN
        SET largest = num2
    END IF

    IF num3 > largest THEN
        SET largest = num3
    END IF

    OUTPUT "The largest number is: " + largest
END

From Pseudocode to Flowchart¶

Take the pseudocode above and convert it:

Practice Activity: Library Book System¶

Let’s create both pseudocode and flowchart for a library book borrowing system that builds on the control structures from Section 2.1:

Requirements¶

Ask for the user’s library card number
Ask for the book ID they want to borrow
Check if the user has any overdue books
Check if the book is available
If both checks pass, allow borrowing and set due date
If either check fails, explain why borrowing is denied

Your Task: Complete the Pseudocode¶

ALGORITHM LibraryBookBorrowing
BEGIN
    OUTPUT "Welcome to the Library System"

    // Get user information
    OUTPUT "Enter your library card number:"
    INPUT cardNumber

    OUTPUT "Enter the book ID you want to borrow:"
    INPUT bookID

    // Check user status
    // TODO: Add pseudocode to check for overdue books
    // (Hint: use a boolean variable hasOverdueBooks)

    // Check book availability
    // TODO: Add pseudocode to check if book is available
    // (Hint: use a boolean variable isBookAvailable)

    // Make borrowing decision
    // TODO: Add selection structure to handle four cases:
    // 1. Both checks pass - allow borrowing
    // 2. Has overdue books - deny with message
    // 3. Book not available - deny with message
    // 4. Both problems - deny with both messages

END

Solution¶

ALGORITHM LibraryBookBorrowing
BEGIN
    OUTPUT "Welcome to the Library System"

    OUTPUT "Enter your library card number:"
    INPUT cardNumber

    OUTPUT "Enter the book ID you want to borrow:"
    INPUT bookID

    // Check user status (simplified - in real system would query database)
    OUTPUT "Do you have any overdue books? (true/false):"
    INPUT hasOverdueBooks

    // Check book availability (simplified - in real system would query catalog)
    OUTPUT "Is the book available? (true/false):"
    INPUT isBookAvailable

    // Make borrowing decision
    IF hasOverdueBooks = false AND isBookAvailable = true THEN
        OUTPUT "Book borrowed successfully!"
        OUTPUT "Due date: 2 weeks from today"
    ELSE IF hasOverdueBooks = true AND isBookAvailable = false THEN
        OUTPUT "Cannot borrow: You have overdue books AND the book is not available"
    ELSE IF hasOverdueBooks = true THEN
        OUTPUT "Cannot borrow: Please return your overdue books first"
    ELSE IF isBookAvailable = false THEN
        OUTPUT "Cannot borrow: Book is currently checked out"
    END IF
END

Flowchart Version¶

Common Pseudocode Patterns Summary¶

Pattern	Template	Example
Sequence	Step1, Step2, Step3	Boil water, Add tea, Pour water
Input	INPUT variableName	INPUT userName
Output	OUTPUT message	OUTPUT “Hello ” + userName
Assignment	SET var = expression	SET total = price + tax
Simple IF	IF condition THEN … END IF	IF age >= 18 THEN …
IF-ELSE	IF condition THEN … ELSE … END IF	IF score >= 60 THEN pass ELSE fail
WHILE	WHILE condition DO … END WHILE	WHILE count < 10 DO …
FOR	FOR var = start TO end DO … END FOR	FOR i = 1 TO 5 DO …

When to Use Each Representation¶

Use Pseudocode When¶

Planning algorithm logic before coding
Communicating with non-programmers
Documenting algorithm steps clearly
Working through complex logic problems
Teaching algorithmic thinking

Use Flowcharts When¶

Visualizing decision paths
Showing complex branching logic
Training or presenting to visual learners
Debugging logic flow problems
Documenting system processes

Use Both When¶

Complex algorithms need multiple perspectives
Teaching algorithm design
Formal documentation requirements
Peer review and collaboration

Summary¶

Pseudocode provides structured English for describing algorithms:

Key Patterns:

Sequence: steps in order
Selection: IF-THEN-ELSE structures
Iteration: WHILE and FOR loops
Clear variable names and consistent indentation

Flowcharts visualize algorithm flow using standard shapes:

Both methods help bridge the gap between human thinking and computer code, making algorithms easier to understand, verify, and implement.