Flowcharts

A flowchart is a diagrammatic representation of an algorithm. It uses standardised symbols connected by arrows to show the sequence of steps, decisions, and flow of control in a process. Flowcharts are an important part of GCSE Computer Science — you must be able to read, interpret, complete, and create flowcharts.

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Why Use Flowcharts?

Flowcharts are useful because:

• Visual representation — they make the logic of an algorithm easy to see at a glance
• Easy to follow — the flow of the algorithm is shown with arrows
• Good for showing decisions — branching logic (IF statements) is clearly represented
• Language-independent — like pseudocode, flowcharts do not depend on any programming language
• Help with debugging — you can trace through a flowchart to find logical errors

Exam Tip: Flowchart questions are common in GCSE Computer Science exams. You may be asked to trace through a flowchart, complete a missing part, identify the output, or draw a flowchart from a description. Always follow the arrows carefully and use a trace table to track variable values.

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Standard Flowchart Symbols

You must know and use these standard symbols:

Symbol	Shape	Purpose	Example
Terminal	Rounded rectangle (oval)	Marks the START or END of the algorithm	START, STOP
Process	Rectangle	An instruction or calculation	score ← score + 1
Input/Output	Parallelogram	Input from user or output to screen	INPUT name, OUTPUT total
Decision	Diamond	A question with two outcomes (YES/NO or TRUE/FALSE)	Is score > 50?
Flow line	Arrow	Shows the direction of flow between steps	→
Subroutine	Rectangle with double vertical lines	Calls a separate procedure or function	CALL calculateGrade()

Important Rules

• Every flowchart must have exactly one START terminal and at least one END terminal
• Arrows must show the direction of flow — do not leave gaps
• Decision diamonds must have exactly two exits — typically labelled YES and NO (or TRUE and FALSE)
• All symbols must be connected — there should be no isolated shapes

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Example 1: A Simple Flowchart

This flowchart represents an algorithm that checks if a number is positive or negative:

[START]
    ↓
[INPUT number]          ← Parallelogram (Input)
    ↓
<Is number >= 0?>       ← Diamond (Decision)
   / \
 YES   NO
  ↓     ↓
[OUTPUT  [OUTPUT
"Positive"] "Negative"]  ← Parallelograms (Output)
  ↓     ↓
  ↓     ↓
[STOP]  [STOP]          ← Rounded rectangles (Terminal)

Pseudocode equivalent:

INPUT number
IF number >= 0 THEN
    OUTPUT "Positive"
ELSE
    OUTPUT "Negative"
END IF

The same algorithm rendered with proper flowchart shapes (terminator, input/output, decision, process):

flowchart TD
    Start(["START"]) --> In[/"INPUT number"/]
    In --> Dec{"number >= 0?"}
    Dec -- YES --> P1["OUTPUT ’Positive’"]
    Dec -- NO --> P2["OUTPUT ’Negative’"]
    P1 --> Stop(["STOP"])
    P2 --> Stop

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Example 2: Flowchart with a Loop

This flowchart represents a counting loop that outputs the numbers 1 to 5:

[START]
    ↓
[count ← 1]              ← Process
    ↓
<Is count <= 5?>          ← Decision
   / \
 YES   NO
  ↓     ↓
[OUTPUT  [STOP]
 count]
  ↓
[count ← count + 1]      ← Process
  ↓
  ↑ (loops back to the decision)

Pseudocode equivalent:

count ← 1
WHILE count <= 5
    OUTPUT count
    count ← count + 1
END WHILE

Trace table:

Step	count	count <= 5?	Output
1	1	YES	1
2	2	YES	2
3	3	YES	3
4	4	YES	4
5	5	YES	5
6	6	NO	—

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Example 3: Input Validation Flowchart

This flowchart validates that a user enters a number between 1 and 100:

[START]
    ↓
[OUTPUT "Enter a number between 1 and 100:"]
    ↓
[INPUT number]
    ↓
<Is number >= 1 AND number <= 100?>
   / \
 YES   NO
  ↓     ↓
[OUTPUT  [OUTPUT "Invalid input.
"Valid!"]  Try again."]
  ↓     ↓
[STOP]   ↑ (loops back to OUTPUT prompt)

Pseudocode equivalent:

REPEAT
    OUTPUT "Enter a number between 1 and 100:"
    INPUT number
UNTIL number >= 1 AND number <= 100
OUTPUT "Valid!"

Exam Tip: When drawing flowcharts, make sure loops are clearly shown with arrows going back to a previous step. Label each decision exit with YES and NO. Keep the flowchart neat and easy to follow.

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Converting Between Pseudocode and Flowcharts

You must be able to convert in both directions:

Pseudocode to Flowchart

1. Start with a START terminal
2. Convert each line of pseudocode into the appropriate symbol
3. Convert IF statements into Decision diamonds with YES/NO branches
4. Convert loops into Decision diamonds with an arrow looping back
5. End with a STOP terminal

Flowchart to Pseudocode

1. Follow the arrows from START
2. Convert each symbol into the appropriate pseudocode line
3. Decisions become IF...THEN...ELSE or WHILE loops
4. Loop-back arrows indicate iteration

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Common Mistakes

• Missing START or STOP — every flowchart needs these
• Unlabelled decision exits — always label YES and NO
• Arrows going the wrong way — always check the direction of flow
• Missing arrows — every symbol must be connected
• Using the wrong symbol — make sure you use the correct shape for each type of operation
• Crossing arrows — try to keep the flowchart tidy and avoid crossing lines

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Flowcharts vs Pseudocode

Feature	Flowcharts	Pseudocode
Format	Visual / diagrammatic	Text-based
Best for	Showing flow and decisions	Detailed step-by-step logic
Ease of editing	Harder to modify once drawn	Easier to edit
Space	Can take up a lot of room	More compact
Exam use	Commonly tested	Commonly tested

Both are valid ways to represent algorithms. In exams, you should be comfortable using either method.

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Summary

Flowcharts use standardised symbols (terminals, processes, inputs/outputs, decisions, and flow lines) to represent algorithms visually. You must know all the standard symbols and be able to read, trace, complete, and draw flowcharts. Practice converting between flowcharts and pseudocode, and always use trace tables to verify your answers.

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Extended Study: Flowcharts as a Design and Communication Tool