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<p>我使用来自<em>SciPy</em>的数组重写了来自<em>SciPy</em>的Python原始基数排序算法,以获得性能并减少代码长度,这是我成功完成的。然后,我从<em>文学编程</em>中选取了<em>经典的</em>(内存中,基于枢轴的)快速排序算法,并比较了它们的性能。在</p>
<p>我预期基数排序会超过某个阈值,而快速排序则没有。此外,我发现<a href="http://erik.gorset.no/2011/04/radix-sort-is-faster-than-quicksort.html" rel="nofollow">Erik Gorset's Blog's</a>提出了一个问题“<strong>对于整数数组,基数排序是否比快速排序快?</strong>。答案是这样的</p>
<blockquote>
<p>.. the benchmark shows the MSB in-place radix sort to be consistently over 3 times faster than quicksort for large arrays.</p>
</blockquote>
<p>不幸的是,我无法重现结果;不同之处在于:(a)Erik选择了Java而不是Python;(b)他使用<em>MSB代替基数排序,而我只是在Python字典中填充<em>bucket</em>。在</p>
<p>根据理论,与快速排序相比,基数排序应该更快(线性);但显然它很大程度上取决于实现。我的错误在哪里?在</p>
<p>下面是比较两种算法的代码:</p>
<pre><code>from sys import argv
from time import clock
from pylab import array, vectorize
from pylab import absolute, log10, randint
from pylab import semilogy, grid, legend, title, show
###############################################################################
# radix sort
###############################################################################
def splitmerge0 (ls, digit): ## python (pure!)
seq = map (lambda n: ((n // 10 ** digit) % 10, n), ls)
buf = {0:[], 1:[], 2:[], 3:[], 4:[], 5:[], 6:[], 7:[], 8:[], 9:[]}
return reduce (lambda acc, key: acc.extend(buf[key]) or acc,
reduce (lambda _, (d,n): buf[d].<a href="https://www.cnpython.com/list/append" class="inner-link">append</a> (n) or buf, seq, buf), [])
def splitmergeX (ls, digit): ## python & numpy
seq = array (vectorize (lambda n: ((n // 10 ** digit) % 10, n)) (ls)).T
buf = {0:[], 1:[], 2:[], 3:[], 4:[], 5:[], 6:[], 7:[], 8:[], 9:[]}
return array (reduce (lambda acc, key: acc.extend(buf[key]) or acc,
reduce (lambda _, (d,n): buf[d].append (n) or buf, seq, buf), []))
def radixsort (ls, fn = splitmergeX):
return reduce (fn, xrange (int (log10 (absolute (ls).max ()) + 1)), ls)
###############################################################################
# quick sort
###############################################################################
def partition (ls, start, end, pivot_index):
lower = start
upper = end - 1
pivot = ls[pivot_index]
ls[pivot_index] = ls[end]
while True:
while lower <= upper and ls[lower] < pivot: lower += 1
while lower <= upper and ls[upper] >= pivot: upper -= 1
if lower > upper: break
ls[lower], ls[upper] = ls[upper], ls[lower]
ls[end] = ls[lower]
ls[lower] = pivot
return lower
def qsort_range (ls, start, end):
if end - start + 1 < 32:
insertion_sort(ls, start, end)
else:
pivot_index = partition (ls, start, end, randint (start, end))
qsort_range (ls, start, pivot_index - 1)
qsort_range (ls, pivot_index + 1, end)
return ls
def insertion_sort (ls, start, end):
for idx in xrange (start, end + 1):
el = ls[idx]
for jdx in reversed (xrange(0, idx)):
if ls[jdx] <= el:
ls[jdx + 1] = el
break
ls[jdx + 1] = ls[jdx]
else:
ls[0] = el
return ls
def quicksort (ls):
return qsort_range (ls, 0, len (ls) - 1)
###############################################################################
if __name__ == "__main__":
###############################################################################
lower = int (argv [1]) ## requires: >= 2
upper = int (argv [2]) ## requires: >= 2
color = dict (enumerate (3*['r','g','b','c','m','k']))
rslbl = "radix sort"
qslbl = "quick sort"
for value in xrange (lower, upper):
#######################################################################
ls = randint (1, value, size=value)
t0 = clock ()
rs = radixsort (ls)
dt = clock () - t0
print "%06d -- t0:%0.6e, dt:%0.6e" % (value, t0, dt)
semilogy (value, dt, '%s.' % color[int (log10 (value))], label=rslbl)
#######################################################################
ls = randint (1, value, size=value)
t0 = clock ()
rs = quicksort (ls)
dt = clock () - t0
print "%06d -- t0:%0.6e, dt:%0.6e" % (value, t0, dt)
semilogy (value, dt, '%sx' % color[int (log10 (value))], label=qslbl)
grid ()
legend ((rslbl,qslbl), numpoints=3, shadow=True, prop={'size':'small'})
title ('radix & quick sort: #(integer) vs duration [s]')
show ()
###############################################################################
###############################################################################
</code></pre>
<hr/>
<p>下面是比较大小在2到1250(横轴)范围内的整数数组的排序持续时间(对数垂直轴)的结果;下曲线属于快速排序:</p>
<ul>
<li><a href="http://db.tt/wfczeCOX" rel="nofollow">Radix vs Quick Sort Comparison</a></li>
</ul>
<p>快速排序在幂次变化时是平滑的(例如,在10、100或1000),但是基数排序只是跳跃一点,但在其他方面遵循与快速排序相同的路径,只是慢得多!在</p>