Learning Objectives
- Recognise and use standard flowchart symbols.
- Represent sequence, selection and repetition using flow lines.
- Trace a flowchart to determine its output.
- Identify unclear or incorrect flowchart logic.
Key Terms
- Flowchart
- A diagram that represents the order and control flow of an algorithm using standard symbols and arrows.
- Terminator
- The start or end symbol, usually drawn as an oval or rounded shape.
- Process symbol
- A rectangle representing an operation such as a calculation or assignment.
- Input/output symbol
- A parallelogram representing data entry or displayed output.
- Decision symbol
- A diamond containing a condition with labelled branches such as Yes and No.
- Flow line
- An arrow showing the direction in which control moves.
Purpose Of A Flowchart
A flowchart shows how an algorithm proceeds from one step to the next. Standard symbols allow sequence, decisions and loops to be understood without using a programming language.
A flowchart is a design method and can also be used to explain or check a given algorithm. The meaning comes from both the symbols and the arrows connecting them.
Core Symbols
| Symbol | Typical Shape | Use |
|---|---|---|
| Start/End | Oval or rounded terminator | Marks entry to or exit from the algorithm |
| Process | Rectangle | Calculation, assignment or other operation |
| Input/Output | Parallelogram | Read input or display/write output |
| Decision | Diamond | Test a condition and choose a branch |
| Flow line | Arrow | Show the direction of control flow |
Sequence
In sequence, steps occur one after another. A flowchart may start, input length and width, calculate area, output area and end. The arrows make the order unambiguous.
Each process box should contain a clear action. Multiple closely related assignments can be placed in one box, but overcrowding makes a diagram difficult to trace.
Selection
A decision symbol contains a condition such as Mark >= 50. Two outgoing arrows are labelled to show the result of the condition, commonly Yes/No or True/False. Each branch leads to the appropriate action.
Branches may rejoin after different actions. The labels must be placed beside the correct arrows; otherwise a reader cannot know which path represents the true condition.
Repetition
A loop is created when a flow line returns to an earlier decision or process. For input validation, the flowchart may input a value, test whether it is valid and return to the input symbol when the answer is No.
The flowchart must contain a possible exit from the loop. A missing update or incorrectly directed arrow can create an unintended infinite loop.
Reading And Checking Flowcharts
To trace a flowchart, begin at Start and follow only the arrow selected by each decision. Record changes to variables where necessary. Do not follow both decision branches at the same time.
Check that every decision branch leads somewhere, all arrows have a clear direction, the algorithm reaches End for valid cases and repeated paths eventually change the condition being tested.
Worked Examples
Positive Or Non-Positive
Question: Describe a flowchart that inputs a number and outputs whether it is positive.
- Start.
- Input Number.
- Decision: Number > 0?
- Yes branch outputs “Positive.”
- No branch outputs “Zero or negative.”
- Both branches lead to End.
Answer: The decision diamond creates two labelled paths based on the condition.
Validation Loop
Question: Design the control flow for entering a mark from 0 to 100.
- Input Mark.
- Decision: Mark < 0 OR Mark > 100?
- Yes branch outputs an error and returns to Input Mark.
- No branch continues to the next process.
Answer: The backward arrow creates repetition until the input is valid.
Examination Guidance
- Use the correct symbol for each type of operation.
- Label every decision branch clearly.
- Ensure arrows show one unambiguous direction.
- When drawing a loop, include an operation that can make the exit condition true.
- Trace from Start and record only the path selected by the data.
Common Mistakes
- Using a process rectangle for every item, including decisions.
- Leaving decision branches unlabelled.
- Drawing crossing arrows that make the direction unclear.
- Creating a loop with no possible exit.
- Using a structure diagram when asked for algorithm flow.
Knowledge Check
1. Which symbol represents a decision?
2. Which symbol represents input or output?
3. What do arrows represent?
4. How is repetition shown?
5. Why label decision branches?
