Efficient way to do std::map comparison based on their values

Asked Oct 01 '18 at 21:31

Active Oct 08 '18 at 18:57

Viewed 123 times

I have two maps which I want to compare based on values. The maps are of type:

std::map<std::string, CustomMap> compareMap;

where CustomMap is:

std::map<std::string, Struct> CustomMap;

and Struct is a struct of two strings. This is how the design is and I can't change it.

How do I efficiently compare two compareMaps based on their values? The only way I can think of now is to use 2 for loops. Is there some inbuilt way to compare the maps based on their values? Or some way to calculate the hash on the values and find out which entries differ and which entries are the same?

EDIT: Let me rephrase my question: Is there a better elegant way to compare the maps based on their values instead of using two for-loops? I know the maps are different and some keys have the same values in both the maps. I only want to know which keys have different values in the two maps.

P.S: I don't have C++11

edited Oct 01 '18 at 21:35

asked Oct 01 '18 at 21:31

Venkat Krishnan

Do you just want to know if `compareMap A` and `compareMap B` are equal? – NathanOliver Oct 01 '18 at 21:33
"_Is there some inbuilt way to compare the maps based on their values?_" Why would there be inbuilt way for this, if such usage isn't intended usage of `std::map`? – Algirdas Preidžius Oct 01 '18 at 21:35
@NathanOliver I know the maps are different and some keys have the same values in both the maps. I only want to know which keys have different values in the two maps. – Venkat Krishnan Oct 01 '18 at 21:38
@AlgirdasPreidžius yes I see that now, I have rephrased my question, apologies – Venkat Krishnan Oct 01 '18 at 21:38
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possibly `std::set_difference` with a custom comparator – NathanOliver Oct 01 '18 at 21:39
2

Possibly `std::set_difference` with a [flattening iterator](https://stackoverflow.com/questions/3623082/flattening-iterator)? – Mooing Duck Oct 01 '18 at 21:41
@NathanOliver and MooingDuck Thanks, I will look into it. – Venkat Krishnan Oct 01 '18 at 21:44
What if map A contains a key not in map B, or vice versa? Do you care about those or does that situation never arise? – Paul Sanders Oct 01 '18 at 21:52
I think you probably write the `for` loops making liberal use of comments (of course you thoroughly comment your code, right?) because anything "elegant" might may also be incomprehensible. – Richard Oct 01 '18 at 21:53
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@Richard And most likely no more efficient. – Paul Sanders Oct 01 '18 at 21:53
@PaulSanders: And perhaps less-so, since cobbling together a bunch of library functions may produce, e.g., intermediate data structures. – Richard Oct 01 '18 at 22:04
@PaulSanders that scenario is already handled, just doing a find on the key. This is the scenario I'm yet to implement. – Venkat Krishnan Oct 01 '18 at 22:06
Not having C++11 makes this somewhat harder (no `auto`, no ranged `for` loops). Is this a given? – Paul Sanders Oct 01 '18 at 22:48
@PaulSanders unfortunately yes – Venkat Krishnan Oct 01 '18 at 23:04

Efficient way to do std::map comparison based on their values

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