OP_NUMNOTEQUAL - Numeric Inequality Test

11. OP_NUMNOTEQUAL — Numeric Inequality Test

Opcode Reference

Opcode:     OP_NUMNOTEQUAL
Hex:        0x9E
Decimal:    158
Input:      a b
Output:     1 if a != b, else 0

Overview

OP_NUMNOTEQUAL is the inverse of OP_NUMEQUAL. It pushes 1 if the two top stack elements represent different integers, and 0 if they are equal.

Stack Examples

Input: [5, 5]   → OP_NUMNOTEQUAL → [0]
Input: [5, 6]   → OP_NUMNOTEQUAL → [1]
Input: [-3, 3]  → OP_NUMNOTEQUAL → [1]
Input: [0, 0]   → OP_NUMNOTEQUAL → [0]

Bitcoin Core Implementation

case OP_NUMNOTEQUAL:
{
    if (stack.size() < 2)
        return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
    CScriptNum bn1(stacktop(-2), fRequireMinimal);
    CScriptNum bn2(stacktop(-1), fRequireMinimal);
    popstack(stack);
    popstack(stack);
    stack.push_back((bn1 != bn2) ? vchTrue : vchFalse);
}
break;

Use Cases

OP_NUMNOTEQUAL is used when you want to enforce that two values must differ. For example, in a script enforcing that a submitted nonce is different from a previously used value (if that prior value is embedded in the locking script):

Locking script:   <used_nonce> OP_NUMNOTEQUAL OP_VERIFY
Unlocking script: <new_nonce>

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