OP_ELSE ​

OP_ELSE is an essential part of conditional structures in Bitcoin scripting. It allows for alternative execution paths depending on conditions evaluated by preceding OP_IF or OP_NOTIF opcodes.

Operation ​

1. If the preceding OP_IF or OP_NOTIF condition was met (and its script executed), skip the opcodes following OP_ELSE until an OP_ENDIF is encountered.
2. If the preceding condition was not met, execute the opcodes after OP_ELSE until an OP_ENDIF is found.

The structure generally appears like so:

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

Or, in conjunction with OP_NOTIF:

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

Where <condition> is an operation or value that results in a 0 or non-0 value on top of the stack.

Notes ​

• OP_ELSE is not standalone and must be used within an OP_IF/OP_NOTIF and OP_ENDIF structure.
• A script will fail and not complete if there is no matching OP_ENDIF for an OP_ELSE.

Examples ​

Example 1 ​

The following script leaves 2 on the stack since the condition (1) is true in front of an OP_IF`.

# ASM script
OP_1 OP_IF OP_2 OP_ELSE OP_3 OP_ENDIF

# Raw script
516352675368

# Final stack
2

Example 2 ​

The following script leaves 3 on the stack since the condition (1) is true in front of an OP_NOTIF.

# ASM script
OP_1 OP_NOTIF OP_2 OP_ELSE OP_3 OP_ENDIF

# Raw script
516452675368

# Final stack
3