OP_LESSTHANOREQUAL - Less Than or Equal

14. OP_LESSTHANOREQUAL — Less Than or Equal

Opcode Reference

Opcode:     OP_LESSTHANOREQUAL
Hex:        0xA1
Decimal:    161
Input:      a b
Output:     1 if a <= b, else 0

Overview

OP_LESSTHANOREQUAL pushes 1 if the second-from-top element is less than or equal to the top element. This is the non-strict version of OP_LESSTHAN.

Stack Examples

Stack: [b=10, a=5] → Is 5 <= 10? Yes → [1] Stack: [b=5, a=5] → Is 5 <= 5? Yes → [1] (equal case also true) Stack: [b=3, a=10] → Is 10 <= 3? No → [0]

Bitcoin Core Implementation

case OP_LESSTHANOREQUAL:
{
    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_LESSTHANOREQUAL is critical for inclusive upper-bound checks. For example, verifying that a fee does not exceed a maximum:

<computed_fee> <fee_cap> OP_LESSTHANOREQUAL OP_VERIFY