Iterative Merge Sort in Python

Author: Deepali-Srivastava

sorting/merge-sort-iterative.py

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+# Copyright (C) Deepali Srivastava - All Rights Reserved
+# This code is part of DSA course available on CourseGalaxy.com  
+
+def merge_sort(a):
+    n=len(a)
+    temp = [None]*n
+    size = 1
+    while size <= n-1:
+        sort_pass(a, temp, size, n)
+        size = size * 2 
+             
+def sort_pass(a,temp,size,n):
+    low1 = 0
+    while low1+size <= n-1:
+        up1 = low1 + size - 1
+        low2 = low1 + size
+        up2 = low2 + size - 1
+
+        if up2 >= n: # if length of last sublist is less than size
+            up2 = n-1
+            
+        merge(a, temp, low1, up1, low2, up2)
+
+        low1 = up2 + 1  # Take next two sublists for merging
+        
+    for i in range(low1,n):
+        temp[i] = a[i]  # If any sublist is left alone
+        
+    copy(a, temp, n)  
+
+# a[low1]...a[up1] and a[low2]...a[up2] merged to temp[low1]...temp[up2] */
+def merge(a, temp, low1, up1, low2, up2):
+    i = low1 
+    j = low2 
+    k = low1 
+
+    while i <= up1  and j <= up2:
+            if a[i] <= a[j]:
+                temp[k] = a[i]
+                i+=1
+            else:
+                temp[k] = a[j]
+                j+=1
+            k+=1
+    
+    while i <= up1:
+                temp[k] = a[i]
+                i+=1
+                k+=1
+            
+    while j <= up2:
+                temp[k] = a[j]
+                j+=1
+                k+=1
+
+# copies temp[low]....temp[up] to a[low]...a[up]
+def copy(a, temp, n):
+    for i in range(n):
+        a[i] = temp[i] 
+
+
+#############################################################    
+             
+list1 = [6,3,1,5,9,8]
+merge_sort(list1)
+print(list1)
+
+list2 = [2,3,5,39,11,8,9,166,45,23]
+merge_sort(list2)
+print(list2)
+
+list3 = [1,2,3,4,5,6,7,8,9,10]
+merge_sort(list3)
+print(list3)
+
+list4 = [10,9,8,7,6,5,4,3,2,1]
+merge_sort(list4)
+print(list4)
+
+list5 = [4]
+merge_sort(list5)
+print(list5)
+
+#################################################################