← IGCSE Computer Science Paper 1

A record that cannot be altered

Cannot be changed or deleted once added. A blockchain record stays fixed, so mistakes are fixed by adding a new record and the history stays permanent.

Part of: IGCSE Computer Science 0478 — 5.2 Digital Currency — Module 16

Understanding A record that cannot be altered in IGCSE Computer Science Paper 1

What it does: Editing an existing record is not allowed on a blockchain, so the attempt to change block 5 is rejected and the history stays fixed. The old record is never changed. A new record is added to correct the mistake, and both records stay in the history. The history is permanent because records cannot be altered or deleted. A blockchain only grows by adding new records. Old records stay the same. The broken code tries to alter a record that is already on the blockchain, which is impossible because it is immutable.

Common mistakes: An old record on a blockchain cannot be edited. The mistake must be fixed by adding a new correction record with ADD, not by overwriting chain[2]. Old records cannot be deleted from a blockchain. The mistake is fixed by adding a new correction record, so line 3 should be ADD(chain, 'Fix: A pays B 4').

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This module's key terms are also in a printable crossword (Modules 510–519) and word search (Modules 510–546).

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Learning
Duo
A record has just been added to a blockchain. Can someone go back and edit it?
Yes, the owner can edit it at any timeNo, it cannot be changed once added
Matches
change_amount(old_record, 50)
↓
Records on a blockchain cannot be edited, so you cannot change the amount of an old record. Add a new correcting record instead.
Applying
Meanings
Term
immutable
Meaning
Cannot be changed or deleted once added. A blockchain record stays fixed, so mistakes are fixed by adding a new record and the history stays permanent.
Practice
// Old records are never edited or deleted
Testing
Gaps
blockchain.(record)
Check Answer
Spots
3ADD(chain, 'B pays C 2')
4chain[2] ← 'B pays C 3'

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Quick Questions

1. A record has just been added to a blockchain. Can someone go back and edit it?
A. Yes, the owner can edit it at any time
B. No, it cannot be changed once added ✓
Blockchain records are immutable. Once a record is added it cannot be edited, so the history stays fixed.
2. A user wants to remove an old transaction from the blockchain. What happens?
A. It cannot be deleted and stays in the history ✓
B. It is deleted after the user asks the network
Old records cannot be deleted from a blockchain. Every past record stays in the chain.
3. A payment was recorded with the wrong amount. How is the mistake fixed?
A. The old record is edited to the right amount
B. A new record is added to correct it ✓
The old record is never changed. A new record is added to correct the mistake, and both records stay in the history.
4. Which word describes a blockchain record that cannot be changed or deleted?
A. Immutable ✓
B. Editable
Immutable means cannot be changed. Blockchain records are immutable once added.
5. What happens to the history of transactions on a blockchain over time?
A. It stays permanent and fixed ✓
B. Old parts are erased to save space
The history is permanent because records cannot be altered or deleted.

Match the Pairs

immutable → A record that cannot be changed or deleted once it has been added to the blockchain
add_record(correction) → A mistake is fixed by adding a new record, not by editing the old one
Old records cannot be edited, so trying to change a past amount is refused → edit(block5, amount) -> REFUSED
Old records cannot be deleted, so the history stays permanent → delete(block5) -> REFUSED
change_amount(old_record, 50) → Records on a blockchain cannot be edited, so you cannot change the amount of an old record. Add a new correcting record instead.
remove(old_record) → Records on a blockchain cannot be deleted, so you cannot erase a mistake. Add a new record that corrects it instead.

Key Term

immutable
Cannot be changed or deleted once added. A blockchain record stays fixed, so mistakes are fixed by adding a new record and the history stays permanent.

Fill in the Blanks

1. blockchain.append(record)
Hint: Type the method name that adds a new record to the end of the chain instead of changing an old one.
2. correction ← NewRecord(-50) blockchain.append(correction)
Hint: Type the variable name that holds the new record used to fix the mistake.
3. immutable ← TRUE
Hint: Type the Boolean value that shows records cannot be changed once added.
4. IF record.immutable = TRUE THEN OUTPUT "Cannot edit"
Hint: Type the pseudocode keyword that displays a message to the user.
5. history ← history + newRecord
Hint: Type the operator that adds to the existing history without replacing it.

Spot the Error

Question 1
A chain ← []
B ADD(chain, 'A pays B 5')
C ADD(chain, 'B pays C 2')
D chain[2] ← 'B pays C 3'
E OUTPUT LENGTH(chain)
Line D — An old record on a blockchain cannot be edited. The mistake must be fixed by adding a new correction record with ADD, not by overwriting chain[2].
Question 2
A chain ← []
B ADD(chain, 'A pays B 5')
C DELETE(chain, 1)
D OUTPUT chain
Line C — Old records cannot be deleted from a blockchain. The mistake is fixed by adding a new correction record, so line 3 should be ADD(chain, 'Fix: A pays B 4').
Question 3
A record ← 'A pays B 5'
B locked ← TRUE
C IF locked = TRUE THEN
D record ← 'A pays B 50'
E ENDIF
Line D — When a record is locked it cannot be altered. The program should refuse the edit with a message, not change the record's value.
Question 4
A immutable ← FALSE
B IF immutable = TRUE THEN
C OUTPUT 'Record is fixed'
D ENDIF
Line A — Blockchain records are immutable, which means they cannot be changed. The variable should be set to TRUE, not FALSE.
Question 5
A chain ← ['A pays B 5']
B chain ← ['B pays C 2']
C OUTPUT LENGTH(chain)
D OUTPUT chain[1]
Line B — This replaces the whole chain and erases the first record. History must stay permanent, so the new record should be added with ADD(chain, 'B pays C 2').

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