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Merge pull request #5006 from ethereum/modSemantics

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Document modulo semantics
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chriseth 2018-09-26 17:33:51 +02:00 committed by GitHub
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docs/types.rst
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@ -86,8 +86,8 @@ They wrap in two's complement notation, meaning that
for example ``uint256(0) - uint256(1) == 2**256 - 1``. You have to take these overflows
into account when designing safe smart contracts.
Division and Modulus
^^^^^^^^^^^^^^^^^^^^
Division
^^^^^^^^
Since the type of the result of an operation is always the type of one of
the operands, division on integers always results in an integer.
@ -96,7 +96,23 @@ In Solidity, division rounds towards zero. This mean that ``int256(-5) / int256(
Note that in contrast, division on :ref:`literals<rational_literals>` results in fractional values
of arbitrary precision.
Division by zero and modulus with zero throws a runtime exception.
.. note::
Division by zero causes a failing assert.
Modulo
^^^^^^
The modulo operation ``a % n`` yields the remainder ``r`` after the division of the operand ``a``
by the operand ``n``, where ``q = int(a / n)`` and ``r = a - (n * q)``. This means that modulo
results in the same sign as its left operand (or zero) and ``a % n == -(abs(a) % n)`` holds for negative ``a``:
* ``int256(5) % int256(2) == int256(1)``
* ``int256(5) % int256(-2) == int256(1)``
* ``int256(-5) % int256(2) == int256(-1)``
* ``int256(-5) % int256(-2) == int256(-1)``
.. note::
Modulo with zero causes a failing assert.
Exponentiation
^^^^^^^^^^^^^^