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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 for example ``uint256(0) - uint256(1) == 2**256 - 1``. You have to take these overflows
into account when designing safe smart contracts. 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 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. 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 Note that in contrast, division on :ref:`literals<rational_literals>` results in fractional values
of arbitrary precision. 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 Exponentiation
^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^