Comparing Arrays: What are tradeoffs b/w using a Dictionary vs a nested For-Each loops?

[joe31623]

I have two arrays, say Arr1 & Arr2. Arr1 will have unique elements (compared with other elements in Arr1 but not necessarily Arr2). Similarly for Arr2 (unique elements...). I want to compare the contents between Arr1 & Arr2 and if there are values in Arr2 which are not in Arr1, I want to add those to Arr1.

I want to know what the trade-offs are between using the following two methods for doing that, which I'll describe below. If there is a better way, please comment.

Method 1: Using a dictionary
Step 1: set all elements of Arr1 to the keys of a dictionary with arbitrary/empty items.
Step 2: loop through Arr2 with a For-Each loop and use the dictionary.Exists method to determine if there is an element in Arr2 which does not exist in Arr1.
Step 3: add keys in the dictionary to Arr1
Or...
Method 1:
Step 1: Loop over All elements in Arr1 & Arr2 with embedded For-Each loops
Step 2: Compare each element
Step 3: ReDim Preserve Arr1(UBound(Arr1) + 1) and Arr1(UBound(Arr1)) = New element in Arr2 which is not in Arr1

Are there substantially more memory resources used for a dictionary object?
Does the speed using the .Exists method overtake the nested For-Each loop? For small arrays? For large arrays?

I appreciate anyone's expertise they'd like to share.

[AlphaFrog]

I have two arrays, say Arr1 & Arr2. Arr1 will have unique elements (compared with other elements in Arr1 but not necessarily Arr2). Similarly for Arr2 (unique elements...). I want to compare the contents between Arr1 & Arr2 and if there are values in Arr2 which are not in Arr1, I want to add those to Arr1.

I want to know what the trade-offs are between using the following two methods for doing that, which I'll describe below. If there is a better way, please comment.

Method 1: Using a dictionary
Step 1: set all elements of Arr1 to the keys of a dictionary with arbitrary/empty items.
Step 2: loop through Arr2 with a For-Each loop and use the dictionary.Exists method to determine if there is an element in Arr2 which does not exist in Arr1.
Step 3: add keys in the dictionary to Arr1
Or...
Method 1:
Step 1: Loop over All elements in Arr1 & Arr2 with embedded For-Each loops
Step 2: Compare each element
Step 3: ReDim Preserve Arr1(UBound(Arr1) + 1) and Arr1(UBound(Arr1)) = New element in Arr2 which is not in Arr1

Are there substantially more memory resources used for a dictionary object?
Does the speed using the .Exists method overtake the nested For-Each loop? For small arrays? For large arrays?

I appreciate anyone's expertise they'd like to share.

The memory resources for Dictionaries are negligible. Not much more than having an array of results.

The dictionary method would be significantly faster for large arrays. Less noticeable for small arrays, but still faster.

If you use the Dictionary method, you wouldn't necessarily have to use .Exists to test. Just add both arrays to the dictionary. Unique items will be added and duplicates will be unchanged for the existing item (in other words, ignored).

Example:

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[joe31623]

That clears up some misc. questions I had about some of the more advanced users' solutions I see and will help me code better. Thank you for the detailed explanation!

[AlphaFrog]

You're welcome. Thanks for the feedback.