How do I merge two vectors while keeping the duplicates

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I have two vectors A,B and I want to merge them to a vector C but I want to preserve any duplicates in each vector while discarding duplicates between the two.

For example if I have A{1,2,3,4,2,5} and B{7,8,2,2,2,3,3} I want C to be {1,2,2,2,3,3,4,5,7,8}. It doesn't need to be sorted. A and B can be sorted vectors if it's required.

What I have tried:

set_difference and set_symmetric_difference

Posted 11-Dec-21 4:16am

John Brg

Updated 11-Dec-21 10:00am

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Richard MacCutchan 11-Dec-21 10:35am

If you start by eliminating the entries in A that exist in B then you end with C=(1,2,2,2,3,3,4,5,7,8). However if you start by eliminating B versus A then C=(1,2,2,3,4,5,7,8). So you first need to decide which numbers to keep when you find duplicates.

John Brg 11-Dec-21 10:43am

I shouldn't have to decide which numbers to keep. If the duplicates are more in A keep those in A, if they are more in B keep those in B. Just don't add both.

Richard MacCutchan 11-Dec-21 11:12am

Start by sorting each list to make the comparisons easier. Then look for duplicates in the two lists. Once you find a duplicate vale then remove all of those values in the smaller subset from one list. In the example above you would remove the two number 2s, and the one 3, from list A. Once you have removed the duplicates the merged lists should provide a consistent result.

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John Brg · Answer 1 · 2021-12-11T10:00:00

Solution 1

Well it seems std::set_union does this. Nothing else is needed.

Posted 11-Dec-21 10:00am

John Brg

Comments

Shao Voon Wong 11-Dec-21 21:33pm

The two vectors need to be sorted before calling std::set_union().

https://www.codeproject.com/Articles/1223131/STL-Set-Algorithm-Operators

Stefan_Lang 13-Dec-21 3:52am

Very good article, must have missed that when it was published.

Shao Voon Wong 13-Dec-21 5:01am

Thank you.

How do I merge two vectors while keeping the duplicates

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How do I merge two vectors while keeping the duplicates

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