How i can return a std::map containing the differences of two maps in c++?

问题

i have two maps and i need to find the differences and create a new map that only has the differences. Not sure how to do this. I tried using set_difference but don't really understand how it works. Any help would be appreciated. thanks

// header file
typedef std::map<std::string, int> MapCol;
typedef std::map<std::string, MapCol> MapRow;
MapRow m_mapRows;

//.cpp fle
CheckForDifferences( const Table& rhs )
{
    Table diffTable;
    vector<string> v;                           
    vector<string>::iterator it;
    it=set_difference (m_mapRows.begin(), m_mapRows.end(), diffTable.m_mapRows.begin(), diffTable.m_mapRows.end, v.begin());
}

edit:

std::set_difference( m_mapRows.begin(), m_mapRows.end(),
rhs.m_mapRows.begin(), rhs.m_mapRows.end(), diffTable.m_mapRows.begin());

ok this is what i tried, but i get errors, the first one being error C2678: binary '=' : no operator found which takes a left-hand operand of type 'const std::string' (or there is no acceptable conversion)

any ideas?

回答1:

If you have two sorted containers a and b and you want to copy the set of objects that aren't in either one into another container c, you'd do this:

std::set_symmetric_difference(
    a.begin(), a.end(),
    b.begin(), b.end(),
    std::back_inserter(c) );

If you just want the elements from a that aren't in b, use set_difference instead of set_symmetric_difference.

回答2:

In your code

std::set_difference( m_mapRows.begin(), m_mapRows.end(),
rhs.m_mapRows.begin(), rhs.m_mapRows.end(), diffTable.m_mapRows.begin());

you try to use regular iterator as output, that does not work. In your case you can use std::inserter

std::set_difference( m_mapRows.begin(), m_mapRows.end(),
rhs.m_mapRows.begin(), rhs.m_mapRows.end(), std::inserter( diffTable.m_mapRows, diffTable.m_mapRows.begin()));

回答3:

This is my take on it. Very basic. You start off from a copy of the left hand operand and remove all that is present in the right hand operand. It leaves you with the difference as defined by set theory.

template< class TMap >
auto    MapDiff( const TMap & lh, const TMap & rh )
{
  auto result = lh ;

  for( auto const & item : rh )
  {
    result.erase( item.first ) ;
  }

  return result ;
}

来源：https://stackoverflow.com/questions/13201463/how-i-can-return-a-stdmap-containing-the-differences-of-two-maps-in-c

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c++

map

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