Merge pull request #2871 from ethereum/fixedpointdocs

Explain fixed point types in docs

2 files changed, 24 insertions, 14 deletions

diff --git a/docs/frequently-asked-questions.rst b/docs/frequently-asked-questions.rst
index 9edfb4fb..f59d86e7 100644
--- a/docs/frequently-asked-questions.rst
+++ b/docs/frequently-asked-questions.rst
@@ -103,11 +103,6 @@ This is a limitation of the EVM and will be solved with the next protocol update
 
 Returning variably-sized data as part of an external transaction or call is fine.
 
-How do you represent ``double``/``float`` in Solidity?
-======================================================
-
-This is not yet possible.
-
 Is it possible to in-line initialize an array like so: ``string[] myarray = ["a", "b"];``
 =========================================================================================
 
diff --git a/docs/types.rst b/docs/types.rst
index d3ebfe14..3335655a 100644
--- a/docs/types.rst
+++ b/docs/types.rst
@@ -70,6 +70,30 @@ sign extends. Shifting by a negative amount throws a runtime exception.
     are going to be rounded towards zero (truncated). In other programming languages the shift right of negative values
     works like division with rounding down (towards negative infinity).
 
+.. index:: ! ufixed, ! fixed, ! fixed point number
+
+Fixed Point Numbers
+-------------------
+
+.. warning::
+    Fixed point numbers are not fully supported by Solidity yet. They can be declared, but
+    cannot be assigned to or from.
+
+``fixed`` / ``ufixed``: Signed and unsigned fixed point number of various sizes. Keywords ``ufixedMxN`` and ``fixedMxN``, where ``M`` represent the number of bits taken by 
+the type and ``N`` represent how many decimal points are available. ``M`` must be divisible by 8 and goes from 8 to 256 bits. ``N`` must be between 0 and 80, inclusive.
+``ufixed`` and ``fixed`` are aliases for ``ufixed128x19`` and ``fixed128x19``, respectively.
+
+Operators:
+
+* Comparisons: ``<=``, ``<``, ``==``, ``!=``, ``>=``, ``>`` (evaluate to ``bool``)
+* Arithmetic operators: ``+``, ``-``, unary ``-``, unary ``+``, ``*``, ``/``, ``%`` (remainder)
+
+.. note::
+    The main difference between floating point (``float`` and ``double`` in many languages, more precisely IEEE 754 numbers) and fixed point numbers is
+    that the number of bits used for the integer and the fractional part (the part after the decimal dot) is flexible in the former, while it is strictly
+    defined in the latter. Generally, in floating point almost the entire space is used to represent the number, while only a small number of bits define
+    where the decimal point is.
+
 .. index:: address, balance, send, call, callcode, delegatecall, transfer
 
 .. _address:
@@ -203,15 +227,6 @@ As a rule of thumb, use ``bytes`` for arbitrary-length raw byte data and ``strin
 for arbitrary-length string (UTF-8) data. If you can limit the length to a certain
 number of bytes, always use one of ``bytes1`` to ``bytes32`` because they are much cheaper.
 
-.. index:: ! ufixed, ! fixed, ! fixed point number
-
-Fixed Point Numbers
--------------------
-
-.. warning::
-    Fixed point numbers are not fully supported by Solidity yet. They can be declared, but
-    cannot be assigned to or from.
-
 .. index:: address, literal;address
 
 .. _address_literals: