Thursday, April 11, 2013

Odd-Even sort visualized

The Odd/Even sort is a sorting algorithm which uses the concept of the Bubble Sort to move elements around. Unlike Bubble sort, the Odd/Even sort compares disjointed pairs by using alternating odd and even index values splitting the sorting in different phases. We have that in the odd phase all the element with an odd index i are compared with the adjacent even index i+1 and in the even phase all the element with an even index i are compared with the adjacent odd index i+1. These two phases are repeated until no exchanges are required. This is a Python snippet that make us able to visualize the behavior of the algorithm:
def oddeven_anim(a):
 imgidx = 0
 x = range(len(a))
 sort = False; 
 while not sort:
  sort = True;
  for i in range(1,len(a)-1,2): # odd phase
   if a[i] > a[i+1]:
    a[i+1], a[i] = a[i], a[i+1]
    sort = False;
  for i in range(0,len(a)-1,2): # even phase
   if a[i] > a[i+1]:
    a[i+1], a[i] = a[i], a[i+1]
    sort = False;
  pylab.plot(x,a,'k.',markersize=6)
  pylab.savefig("oddevensort/img" + '%04d' % imgidx + ".png")
  pylab.clf()
  imgidx =  imgidx + 1

# run the algorithm
a = range(300)
shuffle(a)
oddeven_anim(a)
As in the other examples of sorting visualization in this blog, we have an image for each step of the algorithm. The following video have been produced joining all the images (here is explained how):

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