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| author | Alex Beregszaszi <alex@rtfs.hu> | 2018-09-27 05:29:11 +0800 |
|---|---|---|
| committer | GitHub <noreply@github.com> | 2018-09-27 05:29:11 +0800 |
| commit | 9508406984c1e83e6ce571af4af593c33aed52e6 (patch) | |
| tree | 30dcdc60ebe64b80f73311c724191969c0d532f2 /docs | |
| parent | d51d4def9e53422cc8ed08bc3fb01c3a4ab82ab9 (diff) | |
| parent | 7d1c428838baf47540b87f2e2259012fe1321f23 (diff) | |
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Merge pull request #5087 from ethereum/semantics4
[DOCS] Semantics of negation.
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/types.rst | 15 |
1 files changed, 14 insertions, 1 deletions
diff --git a/docs/types.rst b/docs/types.rst index 43291af8..03896a96 100644 --- a/docs/types.rst +++ b/docs/types.rst @@ -82,10 +82,23 @@ Addition, Subtraction and Multiplication ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Addition, subtraction and multiplication have the usual semantics. -They wrap in two's complement notation, meaning that +They wrap in two's complement representation, meaning that for example ``uint256(0) - uint256(1) == 2**256 - 1``. You have to take these overflows into account when designing safe smart contracts. +The expression ``-x`` is equivalent to ``(T(0) - x)`` where +``T`` is the type of ``x``. This means that ``-x`` will not be negative +if the type of ``x`` is an unsigned integer type. Also, ``-x`` can be +positive if ``x`` is negative. There is another caveat also resulting +from two's complement representation:: + + int x = -2**255; + assert(-x == x); + +This means that even if a number is negative, you cannot assume that +its negation will be positive. + + Division ^^^^^^^^ |