Interleave different length lists, elimating duplicates, and preserve order

Question

I have two lists, let\'s say:

keys1 = [\'A\', \'B\', \'C\', \'D\', \'E\',           \'H\', \'I\']
keys2 = [\'A\', \'B\',           \'E\', \'F\', \'G\', \'H\',            


        

Answer 1

By using only lists, you can achieve this with few simple for loops and .copy():

def mergeLists(list1, list2):
    # Exit if list2 is empty
    if not len(list2):
        return list1
    # Copy the content of list2 into merged list
    merged = list2.copy()

    # Create a list for storing temporary elements
    elements = []
    # Create a variable for storing previous element found in both lists
    previous = None

    # Loop through the elements of list1
    for e in list1:
        # Append the element to "elements" list if it's not in list2
        if e not in merged:
            elements.append(e)

        # If it is in list2 (is a common element)
        else:

            # Loop through the stored elements
            for x in elements:
                # Insert all the stored elements after the previous common element
                merged.insert(previous and merged.index(previous) + 1 or 0, x)
            # Save new common element to previous
            previous = e
            # Empty temporary elements
            del elements[:]

    # If no more common elements were found but there are elements still stored
    if len(elements)
        # Insert them after the previous match
        for e in elements:
            merged.insert(previous and merged.index(previous) + 1 or 0, e)
    # Return the merged list
    return merged

In [1]: keys1 = ["A", "B",      "D",      "F", "G", "H"]
In [2]: keys2 = ["A",      "C", "D", "E", "F",      "H"]
In [3]: mergeLists(keys1, keys2)
Out[3]: ["A", "B", "C", "D", "E", "F", "G", "H"]

English is not my first language, and this one is pretty hard to explain, but if you care about the explanation, here's what it does:

• There's a local list called elements which can store temporary elements.
• There's a local variable called previous which stores the previous element that was in both lists.
• When ever it finds an element that is NOT in list2 but is in list1, it will append that element to elements list and continue the loop.
• Once it hits an element that is in both lists, it loops through the elements list, appending all elements after previous element to list2.
• The new match is then stored into previous and elements is reset to [] and the loop continues.
• Beginning of the lists and end of the lists are counted as a common element, if first or last element is not a common element in both lists.

This way it will always follow this format:

1. Previous common element
2. Elements from list1, between two common elements
3. Elements in list2, between two common elements
4. New common element

So for example:

l1 = ["A", "B", "C",      "E"]
l2 = ["A",           "D", "E"]

1. The revious common element A will be first in the merged list.
2. Elements from l1 between the previous common element A and the new common element E will be inserted right after A.
3. Elements from l2 between the previous common elmeent A and the new common elmeent E will be inserted right after the elements from l1.
4. The new common element E will be last element.

5. Back to step 1 if more common elements found.

   ["A", "B", "C", "D", "E"]