GCSE · Computer Science · Edexcel · Spec 1CP2

Flowcharts

Could a friend play your guessing game from a diagram instead of code? With a flowchart they can, because the shapes do half the explaining.

Algorithms · Flowcharts

Follow the flow: a guessing game

Press Next and watch the highlight move. Each shape holds one step, and the shape tells you what kind of step it is.

Predict first: the secret number is 7, and the player guesses 4, then 9, then 7. How many times will the flow pass through the diamond?

YesNoStartGenerate secretInput guessguess = secret?Output "Correct!"End

Output

Step 1: Start here. The rounded terminal is the one way in. Flow runs down the page unless an arrow says otherwise.

1 / 10

Use Next to move one step at a time, and Back to rewind.

Step 1 of 10: Start here. The rounded terminal is the one way in. Flow runs down the page unless an arrow says otherwise..

Watch out: The No line runs back UP the page. Flow is normally read top to bottom, so a line going the other way needs an arrowhead, or nobody could tell which way to go.

Algorithms · Flowchart symbols

Which shape does each step need?

Pick a step, then pick the symbol that must hold it. Watch out for the two that look alike.

Still to sort

Terminal (oval) (0)

Marks the start or the end of the flowchart. It can also be drawn as a rectangle with curved ends.

Process (rectangle) (0)

An operation that changes the value, form or location of data.

Where the line is: Both a process and an input can end with a value stored in a variable. Ask where the value came from: made by the computer is a process; typed in by the user is an input.

Input / output (parallelogram) (0)

Data coming into the system, or data going out of it.

Where the line is: Input means taking data into the system, like storing what the user types. Generating a value inside the program is not an input.

Decision (diamond) (0)

A yes/no or true/false test that decides which one of two paths to take.

Subprogram (rectangle with two lines inside) (0)

A process with its own identifier that is defined somewhere else.

Where the line is: It looks like a process rectangle, but the two extra lines say the detail of this step is defined elsewhere, and it may have a flowchart of its own.

7 of 7 still to sort.

Algorithms · Building a flowchart

Turn the words into shapes

Convert this algorithm into a flowchart. "Ask the user for their test score out of 50. Double it to get a percentage. If the percentage is at least 50, display Pass. Otherwise, display Try again."

  1. Terminal: StartEvery flowchart begins with a start terminal.
  2. missing step
Which line is step 2?

Algorithms · Loops in flowcharts

How do you draw 'keep asking'?

A friend is drawing a flowchart for: "Keep asking the user for the PIN until they type the correct one."

How should the repeating part be drawn? Pick the idea closest to what you think right now.
How sure are you?

Algorithms · When to use a flowchart

Is a flowchart always worth drawing?

The claim

You should always draw a flowchart before writing code.

Place each piece of evidence to load the balance. Mark the strong ones — they count double.

  1. A flowchart gives visual clarity when the problem is complex.

    Evidence 1: does it support or challenge the claim?

  2. Laying the logic out as symbols makes it easier to understand.

    Evidence 2: does it support or challenge the claim?

  3. You can check the logic and the steps for errors before any code is written.

    Evidence 3: does it support or challenge the claim?

  4. A flowchart documents a process, so it can be followed later.

    Evidence 4: does it support or challenge the claim?

  5. For a simple program, the code itself may be clear enough.

    Evidence 5: does it support or challenge the claim?

  6. Drawing a flowchart that is not needed adds to development time.

    Evidence 6: does it support or challenge the claim?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

An algorithm you can see. Every shape tells you what kind of step it holds, and one diamond is all it takes to make a loop.

What you need to know

  • A flowchart is a diagram that uses symbols to show each step of an algorithm. Each symbol holds a single step.
  • Flowchart symbols are standardised, so they are used consistently and a flowchart drawn by one person can be read by someone else. The shape of a symbol shows the type of step inside it.
  • Terminal (an oval, or a rectangle with curved ends): marks the start and the end. A well-designed flowchart has one entry point and one exit point.
  • Process (rectangle): an operation that changes the value, form or location of data, such as multiplying a score by 10 or storing a randomly generated value in a variable.
  • Input/output (parallelogram): taking data into the system, such as storing what the user types, or sending data out, such as displaying a message.
  • Decision (diamond): a yes/no or true/false test that decides which one of two paths the algorithm takes. Each path leaving it must be labelled with its result.
  • Subprogram (rectangle with two lines inside): a process with its own identifier that is defined elsewhere, such as a function. It may have its own separate flowchart.
  • Flowlines (arrows) show the direction of flow. Flow is assumed to run top to bottom and left to right; arrowheads are needed where the flow goes in a non-standard direction.
  • Repetition: a decision whose result sends the flowline back to an earlier step makes those steps repeat, until the condition leads along the other path out of the loop.
  • Use only the symbols the logic needs (no decision symbol if there is no decision) and keep the flowchart as simple and clear as possible.
  • Flowcharts give visual clarity to complex problems, make logic easier to understand, let logic be checked for errors before coding, and document a process. For a simple program one may not be needed and could add to development time.

The big picture

A flowchart shows each step of an algorithm, one step per symbol. The symbols are standardised, so the shape tells anyone what kind of step it is: a terminal (oval) for start and end, a rectangle for a process, a parallelogram for input or output, a diamond for a decision and a rectangle with two lines inside for a subprogram. Flowlines show the direction of flow, normally top to bottom and left to right. A decision sends the flow down one of two labelled paths, and when one leads back to an earlier step, the steps repeat. That is a loop.

Key points

1One symbol, one step, and the shape tells you the type of step.
2Storing what the user types is an input; storing a value the computer generates is a process.
3A diamond has two ways out, and both must be labelled.
4A loop is a decision whose labelled path points back to an earlier step. No special symbol is needed.
5A line that goes back up the page needs an arrowhead.
6One start, one end, and no symbols the logic doesn't need.

Worked example

Problem

A student draws a flowchart for: "Input a number. If it is over 100, output Big; otherwise output Small." Their flowchart is: an oval Start; a rectangle Input number; a diamond number > 100? with two unlabelled arrows leaving it; a parallelogram Output "Big" and a parallelogram Output "Small", each leading to its own oval End. Find and fix the three problems.

⚠ Watch out

Mixing up input and process. Storing what the user types is an input (parallelogram), but storing a number the computer generates, such as a random number, is a process (rectangle). Both put a value in a variable, so ask where the value came from.

🧠

Memory hook

Ovals open and close. Rectangles do. Parallelograms talk (in and out). Diamonds ask. And a loop? Just a diamond pointing back.

✓

Check yourself

Cover the page. Which symbol holds each step: add 1 to lives; show the score; ask if lives equals 0; run a function defined elsewhere? Then say how a flowchart repeats steps.

Flashcards

(15)
What is a flowchart?
A diagram that uses symbols to show each step of an algorithm, with one step in each symbol.
Why are flowchart symbols standardised?
So they are used consistently, and a flowchart drawn by one person can be read and understood by someone else.
What does the terminal symbol look like, and what is it for?
An oval (or a rectangle with curved ends). It marks the start and the end of the flowchart.
How many entry and exit points should a well-designed flowchart have?
One entry point and one exit point.
What does a rectangle show?
A process: an operation that changes the value, form or location of data, e.g. multiplying a score by 10.
What does a parallelogram show?
An input (data taken into the system) or an output (data sent out of the system).
Storing a randomly generated number in a variable: which symbol?
A rectangle (process). The computer makes the value; nothing comes in from the user.
What does a diamond show?
A decision: a yes/no or true/false test that decides which one of two paths to take.
What must every path leaving a decision have?
A label showing its result, such as Yes/No or True/False.
What does a rectangle with two lines inside show?
A subprogram: a process with its own identifier, defined elsewhere, which may have its own flowchart.
Which way is flow assumed to run, and when do you need an arrowhead?
Top to bottom and left to right. Arrowheads are needed where the flow goes in a non-standard direction.
How does a flowchart show repetition?
A decision's labelled path sends the flow back to an earlier step, so the steps repeat until the condition leads out along the other path.
Should you add a decision symbol just in case?
No. Use only the symbols needed to show the logic, and keep the flowchart simple and clear.
Give two benefits of drawing a flowchart.
Any two: visual clarity for complex problems; logic easier to understand; logic can be checked for errors before coding; documents a process.
When might a flowchart not be worth drawing?
For a simple program whose code is clear enough on its own, where the flowchart would only add development time.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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