add Week 3 PA

Author: SSQ

Programming Assignment 3/QuickSort.py

@@ -0,0 +1,73 @@
+
+with open('QuickSort.txt') as f:
+ a = [int(x) for x in f]
+
+def FindMedian(A):
+ minvalue = min(A)
+ maxvalue = max(A)
+ for i in range(3):
+ if A[i] != minvalue and A[i] != maxvalue:
+ return A[i]
+
+def ChoosePivot(A,flag):
+ n = len(A)
+ first = A[0]
+ final = A[n-1]
+ if n/2*2==n:
+ k = n/2 - 1
+ middle = A[k]
+ elif n/2*2<n:
+ k = n/2
+ middle = A[k]
+ else:
+ print 'error in ChoosePivot to choose middle element of A'
+
+ B = [first,middle,final]
+ med = FindMedian(B)
+ if med==B[0]:
+ position = 0
+ elif med==B[1]:
+ position = k
+ else:
+ position = n-1
+ 
+ if flag==1:
+ return 0
+ if flag==2:
+ return n-1
+ if flag==3:
+ return position
+ else:
+ print 'wrong flag'
+
+def Swap(A,first,second):
+ second_value = A[second]
+ first_value = A[first]
+ A[first] = second_value
+ A[second] = first_value
+ return A
+
+def Partition(A):
+ pivot = A[0]
+ r = len(A)
+ i = 1
+ for j in range(1,r):
+ if A[j]<pivot:
+ A = Swap(A,i,j)
+ i +=1
+ A = Swap(A,0,i-1)
+ return A,i-1
+ 
+def QuickSort(A,flag):
+ n = len(A)
+ 
+ if n>1:
+ p = ChoosePivot(A,flag)
+ A = Swap(A,0,p)
+ A,pivot_position = Partition(A)
+ A[:pivot_position],left = QuickSort(A[:pivot_position],flag)
+ A[pivot_position+1:],right = QuickSort(A[pivot_position+1:],flag)
+ 
+ return A,left+right+n-1
+ else:
+ return A,0