DeterministicSelection.py finally approved

Author: armanjscript

DataStructures/Section11/DeterministicSelection.py

@@ -0,0 +1,68 @@
+def partition(unsorted_array, first_index, last_index): 
+    if first_index == last_index: 
+        return first_index 
+    else: 
+        nearest_median = median_of_medians(unsorted_array[first_index:last_index]) 
+    index_of_nearest_median = get_index_of_nearest_median(unsorted_array, first_index, last_index, nearest_median) 
+    swap(unsorted_array, first_index, index_of_nearest_median) 
+ 
+    pivot = unsorted_array[first_index] 
+    pivot_index = first_index
+
+    index_of_last_element = last_index 
+    less_than_pivot_index = index_of_last_element 
+    greater_than_pivot_index = first_index + 1 
+ 
+    ## This while loop is used to correctly place pivot element at its correct position 
+    while 1:
+        while unsorted_array[greater_than_pivot_index] < pivot and greater_than_pivot_index < last_index:
+            greater_than_pivot_index += 1
+        while unsorted_array[less_than_pivot_index] > pivot and less_than_pivot_index >= first_index:
+            less_than_pivot_index -= 1
+ 
+        if greater_than_pivot_index < less_than_pivot_index:
+            temp = unsorted_array[greater_than_pivot_index]
+            unsorted_array[greater_than_pivot_index] = unsorted_array[less_than_pivot_index]
+            unsorted_array[less_than_pivot_index] = temp
+        else:
+            break
+ 
+    unsorted_array[pivot_index]=unsorted_array[less_than_pivot_index]
+    unsorted_array[less_than_pivot_index]=pivot
+    return less_than_pivot_index
+
+def median_of_medians(elems):  
+    sublists = [elems[j:j+5] for j in range(0, len(elems), 5)] 
+    medians = [] 
+    for sublist in sublists: 
+        medians.append(sorted(sublist)[int(len(sublist)/2)]) 
+    if len(medians) <= 5: 
+        return sorted(medians)[int(len(medians)/2)]
+    else:
+        return median_of_medians(medians)
+
+def get_index_of_nearest_median(array_list, first, last, median): 
+    if first == last: 
+        return first 
+    else: 
+        return array_list.index(median)
+
+def swap(array_list, first, index_of_nearest_median):
+    temp = array_list[first]
+    array_list[first] = array_list[index_of_nearest_median]
+    array_list[index_of_nearest_median] = temp
+
+
+def deterministic_selection(array_list, start, end, k): 
+    split = partition(array_list, start, end) 
+    if split == k: 
+        return array_list[split] 
+    elif split < k:
+        return deterministic_selection(array_list, split + 1, end, k) 
+    else: 
+        return deterministic_selection(array_list, start, split-1, k)
+
+
+#Output of deterministic selection
+list1= [2, 3, 5, 4, 1, 12, 11, 13, 16, 7, 8, 6, 10, 9, 17, 15, 19, 20, 18, 23, 21, 22, 25, 24, 14]
+print("The 6th smallest element is", deterministic_selection(list1, 0, len(list1)-1, 5))