Standard Flowchart Design: Symbols and Logic Tracing
In Week 5, we learned that an algorithm is a step-by-step procedure for solving a problem, and we wrote algorithms using text-based pseudocode.
While pseudocode is powerful, humans are visual creatures! When a team of software engineers, systems analysts, and company managers sits down in a conference room to plan a billion-Naira banking platform, they do not read sixty pages of text. They look at a Flowchart!
A flowchart is a graphical blueprint that uses standardized geometric shapes and directional arrows to map out the flow of logic. In this lesson, we will master the standard ANSI flowchart symbols and learn how to trace logic visually.
Learning Outcomes
By the end of this lesson, you will be able to:
- Define a flowchart and state its advantages in systems planning.
- Draw and identify standard ANSI symbols: Oval, Parallelogram, Rectangle, Diamond, and Flowline.
- Trace logic paths through decision diamonds with alternative branches.
- Convert written pseudocode into an accurate graphical flowchart.
Key Vocabulary
| Symbol Name | Geometric Shape | Operational Meaning |
|---|---|---|
| Terminal | Rounded Oval | Marks the START or STOP boundary of the algorithm. |
| Input / Output | Parallelogram | Entering data (e.g., READ Age) or displaying output (e.g., PRINT Grade). |
| Process | Rectangle | An action, calculation, or internal data operation (e.g., Sum = A + B). |
| Decision | Diamond (Rhombus) | A conditional question evaluating to two exit paths (Yes/No or True/False). |
| Flowline | Arrow ($\rightarrow, \downarrow$) | Directs the path of execution sequence from one shape to the next. |
| Connector | Small Circle | Connects separate flowlines on a complex page to avoid messy overlapping lines. |
Standard ANSI Flowchart Symbols Guide
1. TERMINAL (Start / Stop) 2. INPUT / OUTPUT (Data)
+---------------+ /---------------+
( START ) / READ Mark /
+---------------+ /---------------+
3. PROCESS (Calculation/Action) 4. DECISION (Branching Question)
+---------------+ /\
| Total = A + B | / \
+---------------+ /Mark\ --- YES ---> [ Pass ]
\ >=50/
\ /
\/
| NO
v
[ Fail ]
Study Notes: Rules for Clean Flowchart Construction
- Direction of Flow: Flowcharts should naturally read from Top to Bottom or Left to Right.
- Every Symbol Has Flowlines:
- Terminals have only one exit arrow (for Start) or one entrance arrow (for Stop).
- Rectangles and Parallelograms have one entrance and one exit.
- Diamonds MUST have at least two labeled exit arrows (e.g., YES and NO, or TRUE and FALSE).
- Keep Text Inside Symbols Concise: Do not write whole paragraphs inside shapes; use brief expressions like
READ N,Count = Count + 1,X > 10?.
Worked Example: Student Grade Evaluation Flowchart
Let us convert a simple grading algorithm into a visual flowchart:
Algorithm:
- Start.
- Read Student Exam Score.
- Is Score $\ge 50$?
- If YES → Print "PASSED".
- If NO → Print "FAILED".
- Stop.
Flowchart Representation:
+-------------+
( START )
+-------------+
|
v
/-------------/
/ READ Score /
/-------------/
|
v
/ \
/ \ YES
/Score\ ------------> /-------------/
\ >=50/ / PRINT "PASS" /
\ / /-------------/
\ / |
| NO |
v |
/-------------/ |
/ PRINT "FAIL" / |
/-------------/ |
| |
+------------+------------+
|
v
+-------------+
( STOP )
+-------------+
Practical Class Activity: The Number Sign Checker
In your notebook, draw a neat flowchart with a ruler and pencil for the following problem:
Problem: A program asks a user for a number. If the number is greater than or equal to zero ($N \ge 0$), it displays "POSITIVE". Otherwise, it displays "NEGATIVE".
Check your drawing:
- [ ] Did you use an Oval for START and STOP?
- [ ] Did you use a Parallelogram for
READ N? - [ ] Did you use a Diamond for
N >= 0? - [ ] Did you clearly label the two exit arrows leaving the diamond: YES and NO?
Common Mistakes & Corrections
| Common Mistake | Why It Is Incorrect | Proper Correction |
|---|---|---|
| "Using rectangles for everything including Start, Inputs, and Decisions." | Each geometric shape has a standardized meaning recognized worldwide by engineers. | Use the correct shape: Ovals for Terminals, Parallelograms for I/O, Diamonds for Decisions. |
| "Drawing a decision diamond with only one exit arrow." | A decision is meaningless if there is only one path to take! | A decision diamond must always have at least two labeled exit paths (YES and NO). |
Notebook Summary
Copy Into Your Computing Notebook
- A Flowchart is a standardized graphical diagram representing an algorithm.
- Oval: Start / Stop (Terminal).
- Parallelogram: Input / Output operations.
- Rectangle: Process / Calculation / Assignment.
- Diamond: Decision question with Yes and No branching arrows.
- Flowlines: Arrows showing the exact sequence of program execution.