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  • Intro
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      • Abstract
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      • Introduction
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        • Breaking the Tie
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        • Maintaining an Attack
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      • Combining and Splitting Value
        • Dynamically Sized Coins
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        • A Typical Example
        • Fan Out
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        • Stock Exchange Comparison
        • Key Re-Use
        • Privacy - Assessment 2
        • Linking Inputs
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      • Calculations
        • Attacking the Chain
        • Things the Attacker Cannot Achieve
        • The Only Thing an Attacker Can Achieve
        • The Binomial Random Walk
        • The Gambler's Ruin
        • Exponential Odds
        • Waiting For Confirmation
        • Attack Via Proof of Work
        • Vanishing Probabilities
      • Conclusion
        • Conclusion Explained
    • Introduction to Bitcoin Script
      • Chapter 1: About Bitcoin Script
        • 01 - Introduction
        • 02 - FORTH: A Precursor to Bitcoin Script
        • 03 - From FORTH to Bitcoin Script
        • 04 - Bitcoin's Transaction Protocol
        • 05 - Transaction Breakdown
        • 06 - nLockTime
        • 07 - The Script Evaluator
      • Chapter 2: Basic Script Syntax
        • 01 - Introduction
        • 02 - Rules Around Data and Scripting Grammar
        • 03 - The Stacks
      • Chapter 3: The Opcodes
        • 01 - Introduction
        • 02 - Constant Value and PUSHDATA Opcodes
        • 03 - IF Loops
        • 04 - OP_NOP, OP_VERIFY and its Derivatives
        • 05 - OP_RETURN
        • 06 - Stack Operations
        • 07 - Data transformation
        • 08 - Stack Data Queries
        • 09 - Bitwise transformations and Arithmetic
        • 10 - Cryptographic Functions
        • 11 - Disabled and Removed Opcodes
      • Chapter 4: Simple Scripts
        • 01 - Introduction
        • 01 - Pay to Public Key (P2PK)
        • 02 - Pay to Hash Puzzle
        • 03 - Pay to Public Key Hash (P2PKH)
        • 04 - Pay to MultiSig (P2MS)
        • 05 - Pay to MultiSignature Hash (P2MSH)
        • 06 - R-Puzzles
      • Chapter 5: OP_PUSH_TX
        • 01 - Turing Machines
        • 02 - Elliptic Curve Signatures in Bitcoin
        • 03 - OP_PUSH_TX
        • 04 - Signing and Checking the Pre-Image
        • 05 - nVersion
        • 06 - hashPrevouts
        • 07 - hashSequence
        • 08 - Outpoint
        • 09 - scriptLen and scriptPubKey
        • 10 - value
        • 11 - nSequence
        • 12 - hashOutputs
        • 13 - nLocktime
        • 14 - SIGHASH flags
      • Chapter 6: Conclusion
        • Conclusion
    • BSV Infrastructure
      • The Instructions
        • The Whitepaper
        • Steps to Run the Network
        • Step 1
        • Step 2
        • Step 3
        • Step 4
        • Step 5
        • Step 6
      • Rules and their Enforcement
        • Introduction
        • Consensus Rules
        • Block Consensus Rules
        • Transaction Consensus Rules
        • Script Language Rules
        • Standard Local Policies
      • Transactions, Payment Channels and Mempools
      • Block Assembly
      • The Small World Network
        • The Decentralisation of Power
        • Incentive Driven Behaviour
        • Lightspeed Propagation of Transactions
        • Ensuring Rapid Receipt and Propagation of New Blocks
        • Hardware Developments to Meet User Demand
        • Novel Service Delivery Methods
        • MinerID
      • Conclusion
  • Research and Development
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  • Support & Contribution
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  • IF / NOTIF statements
  • Using OP_ELSE
  • Multi-condition loops / Case statements

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  1. BSV Academy
  2. Introduction to Bitcoin Script
  3. Chapter 3: The Opcodes

03 - IF Loops

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Last updated 4 months ago

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Flow control opcodes are opcodes that allow a script to either execute or ignore particular sections of code, or to terminate the script process depending on the value of the topmost stack items.

There are a variety of OPCODES available to trigger entry into IF loops including:

OP_IF, OP_NOTIF, OP_ELSE

IF / NOTIF statements

IF statements are used to allow Bitcoin script to do comparative analysis on stack data items to control entry into IF loops. IF loops are the only means provided in Bitcoin script to perform different operations depending on previous processing.

The format of a simple IF loop is as follows:

<Expression>

OP_IF

<True action>

OP_ENDIF

In this example, if the operations performed in <expression> leave a non-zero item at the top of the stack, then the code in <True action> will be executed. Otherwise the script will jump to the opcode immediately after OP_ENDIF.

The alternative to OP_IF is OP_NOTIF.

<Expression>

OP_NOTIF

<False action>

OP_ENDIF

When using OP_NOTIF, if the operations performed in <expression> leave a zero-value item at the top of the stack, then the code in <False action> will be executed. Otherwise the script will jump to the opcode immediately after OP_ENDIF.

Bitcoin grammar rules require that every OP_IF / OP_NOTIF must have a corresponding OP_ENDIF. Transactions that try to create outputs that do not adhere to this scripting rule are considered invalid and will not be accepted by the network.

Using OP_ELSE

Any IF loop can contain an ELSE statement which will cause the script to branch into one of two paths depending on the outcome of the expression being evaluated.

<Expression>

OP_IF

<True action>

OP_ELSE

<False action>

OP_ENDIF

Multi-condition loops / Case statements

Using OP_ELSE, IF loops can be nested allowing for complex nested functions to be developed. This allows for similar functionality to a case statement to be implemented.

<Case 1 check>

OP_IF

<When 1 action>

OP_ELSE

<Case 2 check>

OP_IF

<When 2 action>

OP_ELSE

<Else action>

OP_ENDIF

OP_ENDIF

An alternative method to nested IF loops is repeating IF loops in the code, although care must be taken to ensure that any 'ELSE' case is captured in an IF loop separately.

<Case 1 check>

OP_IF

<When 1 action>

OP_ENDIF

<Case 2 check>

OP_IF

<When 2 action>

OP_ENDIF

<else check>

OP_IF

<Else action>

OP_ENDIF