[Python-checkins] commit of r41564 - python/trunk/Doc/howto

[andrew.kuchling]

Author: andrew.kuchling
Date: Wed Nov 30 02:14:48 2005
New Revision: 41564

Removed:
   python/trunk/Doc/howto/sorting.tex
Log:
Remove sorting HOWTO, after converting it to a wiki page at http://wiki.python.org/moin/HowTo/Sorting

Deleted: /python/trunk/Doc/howto/sorting.tex
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--- /python/trunk/Doc/howto/sorting.tex	Wed Nov 30 02:14:48 2005
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@@ -1,266 +0,0 @@
-\documentclass{howto}
-
-\title{Sorting Mini-HOWTO}
-
-% Increment the release number whenever significant changes are made.
-% The author and/or editor can define 'significant' however they like.
-\release{0.01}
-
-\author{Andrew Dalke}
-\authoraddress{\email{dalke at bioreason.com}}
-
-\begin{document}
-\maketitle
-
-\begin{abstract}
-\noindent
-This document is a little tutorial
-showing a half dozen ways to sort a list with the built-in
-\method{sort()} method.  
-
-This document is available from the Python HOWTO page at
-\url{http://www.python.org/doc/howto}.
-\end{abstract}
-
-\tableofcontents
-
-Python lists have a built-in \method{sort()} method.  There are many
-ways to use it to sort a list and there doesn't appear to be a single,
-central place in the various manuals describing them, so I'll do so
-here.
-
-\section{Sorting basic data types}
-
-A simple ascending sort is easy; just call the \method{sort()} method of a list.
-
-\begin{verbatim}
->>> a = [5, 2, 3, 1, 4]
->>> a.sort()
->>> print a
-[1, 2, 3, 4, 5]
-\end{verbatim}
-
-Sort takes an optional function which can be called for doing the
-comparisons.  The default sort routine is equivalent to
-
-\begin{verbatim}
->>> a = [5, 2, 3, 1, 4]
->>> a.sort(cmp)
->>> print a
-[1, 2, 3, 4, 5]
-\end{verbatim}
-
-where \function{cmp} is the built-in function which compares two objects, \code{x} and
-\code{y}, and returns -1, 0 or 1 depending on whether $x<y$, $x==y$, or $x>y$.  During
-the course of the sort the relationships must stay the same for the
-final list to make sense.
-
-If you want, you can define your own function for the comparison.  For 
-integers (and numbers in general) we can do:
-
-\begin{verbatim}
->>> def numeric_compare(x, y):
->>>    return x-y
->>> 
->>> a = [5, 2, 3, 1, 4]
->>> a.sort(numeric_compare)
->>> print a
-[1, 2, 3, 4, 5]
-\end{verbatim}
-
-By the way, this function won't work if result of the subtraction
-is out of range, as in \code{sys.maxint - (-1)}.
-
-Or, if you don't want to define a new named function you can create an
-anonymous one using \keyword{lambda}, as in:
-
-\begin{verbatim}
->>> a = [5, 2, 3, 1, 4]
->>> a.sort(lambda x, y: x-y)
->>> print a
-[1, 2, 3, 4, 5]
-\end{verbatim}
-
-If you want the numbers sorted in reverse you can do
-
-\begin{verbatim}
->>> a = [5, 2, 3, 1, 4]
->>> def reverse_numeric(x, y):
->>>     return y-x
->>> 
->>> a.sort(reverse_numeric)
->>> print a
-[5, 4, 3, 2, 1]
-\end{verbatim}
-
-(a more general implementation could return \code{cmp(y,x)} or \code{-cmp(x,y)}).
-
-However, it's faster if Python doesn't have to call a function for
-every comparison, so if you want a reverse-sorted list of basic data
-types, do the forward sort first, then use the \method{reverse()} method.
-
-\begin{verbatim}
->>> a = [5, 2, 3, 1, 4]
->>> a.sort()
->>> a.reverse()
->>> print a
-[5, 4, 3, 2, 1]
-\end{verbatim}
-
-Here's a case-insensitive string comparison using a \keyword{lambda} function:
-
-\begin{verbatim}
->>> a = "This is a test string from Andrew".split()
->>> a.sort(lambda x, y: cmp(x.lower(), y.lower()))
->>> print a
-['a', 'Andrew', 'from', 'is', 'string', 'test', 'This']
-\end{verbatim}
-
-This goes through the overhead of converting a word to lower case
-every time it must be compared.  At times it may be faster to compute
-these once and use those values, and the following example shows how.
-
-\begin{verbatim}
->>> words = string.split("This is a test string from Andrew.")
->>> offsets = []
->>> for i in range(len(words)):
->>>     offsets.append( (string.lower(words[i]), i) )
->>> 
->>> offsets.sort()
->>> new_words = []
->>> for dontcare, i in offsets:
->>>      new_words.append(words[i])
->>> 
->>> print new_words
-\end{verbatim}
-
-The \code{offsets} list is initialized to a tuple of the lower-case string
-and its position in the \code{words} list.  It is then sorted.  Python's
-sort method sorts tuples by comparing terms; given \code{x} and \code{y}, compare
-\code{x[0]} to \code{y[0]}, then \code{x[1]} to \code{y[1]}, etc. until there is a difference.
-
-The result is that the \code{offsets} list is ordered by its first
-term, and the second term can be used to figure out where the original
-data was stored.  (The \code{for} loop assigns \code{dontcare} and
-\code{i} to the two fields of each term in the list, but we only need the
-index value.)
-
-Another way to implement this is to store the original data as the
-second term in the \code{offsets} list, as in:
-
-\begin{verbatim}
->>> words = string.split("This is a test string from Andrew.")
->>> offsets = []
->>> for word in words:
->>>     offsets.append( (string.lower(word), word) )
->>> 
->>> offsets.sort()
->>> new_words = []
->>> for word in offsets:
->>>     new_words.append(word[1])
->>> 
->>> print new_words
-\end{verbatim}
-
-This isn't always appropriate because the second terms in the list
-(the word, in this example) will be compared when the first terms are
-the same.  If this happens many times, then there will be the unneeded
-performance hit of comparing the two objects.  This can be a large
-cost if most terms are the same and the objects define their own
-\method{__cmp__} method, but there will still be some overhead to determine if
-\method{__cmp__} is defined.
-
-Still, for large lists, or for lists where the comparison information
-is expensive to calculate, the last two examples are likely to be the
-fastest way to sort a list.  It will not work on weakly sorted data,
-like complex numbers, but if you don't know what that means, you
-probably don't need to worry about it.
-
-\section{Comparing classes}
-
-The comparison for two basic data types, like ints to ints or string to
-string, is built into Python and makes sense.  There is a default way
-to compare class instances, but the default manner isn't usually very
-useful.  You can define your own comparison with the \method{__cmp__} method,
-as in:
-
-\begin{verbatim}
->>> class Spam:
->>>     def __init__(self, spam, eggs):
->>>         self.spam = spam
->>>         self.eggs = eggs
->>>     def __cmp__(self, other):
->>>         return cmp(self.spam+self.eggs, other.spam+other.eggs)
->>>     def __str__(self):
->>>         return str(self.spam + self.eggs)
->>> 
->>> a = [Spam(1, 4), Spam(9, 3), Spam(4,6)]
->>> a.sort()
->>> for spam in a:
->>>   print str(spam)
-5
-10
-12
-\end{verbatim}
-
-Sometimes you may want to sort by a specific attribute of a class.  If
-appropriate you should just define the \method{__cmp__} method to compare
-those values, but you cannot do this if you want to compare between
-different attributes at different times.  Instead, you'll need to go
-back to passing a comparison function to sort, as in:
-
-\begin{verbatim}
->>> a = [Spam(1, 4), Spam(9, 3), Spam(4,6)]
->>> a.sort(lambda x, y: cmp(x.eggs, y.eggs))
->>> for spam in a:
->>>   print spam.eggs, str(spam)
-3 12
-4 5
-6 10
-\end{verbatim}
-
-If you want to compare two arbitrary attributes (and aren't overly
-concerned about performance) you can even define your own comparison
-function object.  This uses the ability of a class instance to emulate
-an function by defining the \method{__call__} method, as in:
-
-\begin{verbatim}
->>> class CmpAttr:
->>>     def __init__(self, attr):
->>>         self.attr = attr
->>>     def __call__(self, x, y):
->>>         return cmp(getattr(x, self.attr), getattr(y, self.attr))
->>> 
->>> a = [Spam(1, 4), Spam(9, 3), Spam(4,6)]
->>> a.sort(CmpAttr("spam"))  # sort by the "spam" attribute
->>> for spam in a:
->>>    print spam.spam, spam.eggs, str(spam)
-1 4 5
-4 6 10
-9 3 12
-
->>> a.sort(CmpAttr("eggs"))   # re-sort by the "eggs" attribute
->>> for spam in a:
->>>    print spam.spam, spam.eggs, str(spam)
-9 3 12
-1 4 5
-4 6 10
-\end{verbatim}
-
-Of course, if you want a faster sort you can extract the attributes
-into an intermediate list and sort that list.
-
-
-So, there you have it; about a half-dozen different ways to define how
-to sort a list:
-\begin{itemize}
- \item sort using the default method
- \item sort using a comparison function
- \item reverse sort not using a comparison function
- \item sort on an intermediate list (two forms)
- \item sort using class defined __cmp__ method
- \item sort using a sort function object
-\end{itemize}
-
-\end{document}
-% LocalWords:  maxint