# OP\_IF - Executing Code When True

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**OP\_IF** is the fundamental conditional opcode in Bitcoin Script. It examines the top item on the stack and executes the enclosed code block only if that item represents a **true value** (non-zero).

#### Basic OP\_IF structure:

```
<Expression>
OP_IF
  <True action>
OP_ENDIF
```

#### How OP\_IF evaluates stack values:

* **Non-zero values** → Treated as TRUE, code block executes
* **Zero or empty array** → Treated as FALSE, code block is skipped
* The top stack item is **consumed** by OP\_IF (removed from stack)

#### Execution flow:

If the expression before OP\_IF leaves a non-zero value on the stack, the script executes all opcodes between OP\_IF and OP\_ENDIF. If the value is zero, the script jumps directly to the opcode after OP\_ENDIF, completely ignoring the code inside the block.

#### Practical example:

```
<signature> <public_key> OP_CHECKSIG
OP_IF
  <recipient_pubkey_hash> OP_EQUALVERIFY
OP_ENDIF
```

In this example, **OP\_CHECKSIG** validates the signature and pushes TRUE (1) or FALSE (0) onto the stack. **OP\_IF** examines this result. If the signature is valid (TRUE), an additional check on the recipient address executes. If invalid (FALSE), the script skips the check and fails validation.

**Critical grammar rule:** Every OP\_IF must have a corresponding OP\_ENDIF. Scripts violating this rule are considered invalid and will be rejected by the network.


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