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Fixup intro paragraphs for the itertools docs. Add some tables for quick reference.

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Raymond Hettinger 2009-02-17 20:00:59 +00:00
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Doc/library/itertools.rst
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@ -13,39 +13,61 @@
from itertools import *
This module implements a number of :term:`iterator` building blocks inspired by
constructs from the Haskell and SML programming languages. Each has been recast
in a form suitable for Python.
This module implements a number of :term:`iterator` building blocks inspired
by constructs from APL, Haskell, and SML. Each has been recast in a form
suitable for Python.
The module standardizes a core set of fast, memory efficient tools that are
useful by themselves or in combination. Standardization helps avoid the
readability and reliability problems which arise when many different individuals
create their own slightly varying implementations, each with their own quirks
and naming conventions.
The tools are designed to combine readily with one another. This makes it easy
to construct more specialized tools succinctly and efficiently in pure Python.
useful by themselves or in combination. Together, they form an "iterator
algebra" making it possible to construct specialized tools succinctly and
efficiently in pure Python.
For instance, SML provides a tabulation tool: ``tabulate(f)`` which produces a
sequence ``f(0), f(1), ...``. But, this effect can be achieved in Python
by combining :func:`map` and :func:`count` to form ``map(f, count())``.
Likewise, the functional tools are designed to work well with the high-speed
functions provided by the :mod:`operator` module.
Whether cast in pure python form or compiled code, tools that use iterators are
more memory efficient (and often faster) than their list based counterparts. Adopting
the principles of just-in-time manufacturing, they create data when and where
needed instead of consuming memory with the computer equivalent of "inventory".
The tools also work well with the high-speed functions in the :mod:`operator`
module. For example, the plus-operator can be mapped across two vectors to
form an efficient dot-product: ``sum(map(operator.add, vector1, vector2))``.
.. seealso::
**Infinite Iterators:**
The Standard ML Basis Library, `The Standard ML Basis Library
<http://www.standardml.org/Basis/>`_.
================== ================= =================================================
Iterator Arguments Results
================== ================= =================================================
:func:`count` start, [step] start, start+step, start+2*step, ...
:func:`cycle` p p0, p1, ... plast, p0, p1, ...
:func:`repeat` elem [,n] elem, elem, elem, ... endlessly or up to n times
================== ================= =================================================
Haskell, A Purely Functional Language, `Definition of Haskell and the Standard
Libraries <http://www.haskell.org/definition/>`_.
**Iterators terminating on the shortest input sequence:**
==================== ============================ =================================================
Iterator Arguments Results
==================== ============================ =================================================
:func:`chain` p, q, ... p0, p1, ... plast, q0, q1, ...
:func:`compress` data, selectors (d[0] if s[0]), (d[1] if s[1]), ...
:func:`dropwhile` pred, seq seq[n], seq[n+1], starting when pred fails
:func:`filterfalse` pred, seq elements of seq where pred(elem) is False
:func:`groupby` iterable[, keyfunc] sub-iterators grouped by value of keyfunc(v)
:func:`islice` seq, [start,] stop [, step] elements from seq[start:stop:step]
:func:`starmap` func, seq func(\*seq[0]), fun(\*seq[1]), ...
:func:`tee` it, n it1, it2 , ... itn splits one iterator into n
:func:`takewhile` pred, seq seq[0], seq[1], until pred fails
:func:`zip_longest` p, q, ... (p[0], q[0]), (p[1], q[1]), ...
==================== ============================ =================================================
**Combinatoric generators:**
===================================== ==================== =================================================
Iterator Arguments Results
===================================== ==================== =================================================
:func:`product` p, q, ... [repeat=1] cartesian product
:func:`permutations` p[, r] r-length permutations (without repeated elements)
:func:`combinations` p[, r] r-length combinations (sorted and no repeats)
:func:`combinations_with_replacement` p[, r] r-length combinations (sorted but with repeats)
===================================== ==================== =================================================
.. _itertools-functions: