OP_IF

INFO

Opcode number: 99
Byte representation: 0x63
Short description: Pop and execute the next statements if a nonzero element was popped.

OP_IF is used to perform conditional execution. Scripts in Bitcoin can contain branches—parts of the script that are executed only if certain conditions are met. The OP_IF opcode allows the script to evaluate a value and conditionally execute the following script commands based on that value.

Operation

1. Pop the top stack item.
2. If the item is not 0, execute the following opcodes until an OP_ELSE or OP_ENDIF is encountered.
3. If the item is 0, skip the following opcodes until an OP_ELSE or OP_ENDIF is encountered.

You'll see it used like so:

<condition> OP_IF
    <script if condition is true>
OP_ELSE
    <script if condition is false>
OP_ENDIF

Where <condition> is some operation or value that will leave a 0 or non-0 value on top of the stack.

Notes

• Bitcoin script does not support loop structures, so all conditional logic is based on branches.
• The script will fail and not complete if there is no matching OP_ENDIF for an OP_IF.

Examples

Example 1

The following script would leave 2 on the stack as the condition (1) is true.

# ASM script
OP_1 OP_IF OP_2 OP_ENDIF

# Raw script
51635268

# Final stack
2

Example 2

The following script would leave 3 on the stack as the condition (0) is false.

# ASM script
OP_0 OP_IF OP_2 OP_ELSE OP_3 OP_ENDIF

# Raw script
006352675368

# Final stack
3