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STL - Map keys difference

Today, I would like to show how to quickly find difference between two maps of STL library basing on keys only. We need to compare two maps of STL library and find third map which will contain only those elements of first map which keys are not being placed within second map.

Take a look on code: Output of this code is:

In point I we are defining struct type having overloaded of operator(). This structure will be used as function object to compare two elements of maps (std::pair) basing on keys only.

In point II we are defining two maps which will be compared. We are going to receive all std::pairs from firstMap which keys are not being placed within secondMap.

In point II we are declaring diffMap map which will contain result of our differentiation.

Point IV is key point of our function. In that point we are using std::set_difference algorithm. This algorithm returns difference of two ranges of STL containers. First 4 arguments are begins and ends of two ranges of containers which should be compared. 5th argument is inserter of different element. This inserter will fulfill our diffMap map which will contain difference which we are looking for. For most of STL containers those 5 arguments are enough for comparison and finding difference. However if want to compare keys of map only, we need to redefine our comparison function. In order to do that we are using overloaded operator() of struct comparatorStruct defined in point I.

That's it. In point V we are displaying our difference map using C++11 auto type.

As usual, if you would like to download that code and compile it you can find it (as other codes used in this blog) on our github account here: mapKeysDifference
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Comments

  1. developer5 December 2013 at 21:26

    Thanks for this example. It helped me today in part of my project.

    ReplyDelete

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