Map comparison between key value pairs

[Evyn]

Hi,

How do I compare 2 maps for identical keys, and if they have identical
keys, check if they have identical values? In either case I want to
copy ONLY that key value pair to one of two different maps.

I know how to copy the entire map to another:
// Copy fset1 to fset3
std::copy(fset1.begin(),fset1.end(),std::inserter(fset3,
fset3.begin()));

And I can for example iterate through the entire map, but the key pair
comparison is beyond me at this point.

// Print test
for (std::map<double,double>::const_iterator iter = fset3.begin();
iter != fset3.end(); ++iter )
{
std::cout << iter->first << " , " << iter->second << "\n";
}


Thank you for your time
Evyn

 

[Kai-Uwe Bux]

Hi,

How do I compare 2 maps for identical keys, and if they have identical
keys, check if they have identical values? In either case I want to
copy ONLY that key value pair to one of two different maps.

I know how to copy the entire map to another:
// Copy fset1 to fset3
std::copy(fset1.begin(),fset1.end(),std::inserter(fset3,
fset3.begin()));

And I can for example iterate through the entire map, but the key pair
comparison is beyond me at this point.

// Print test
for (std::map<double,double>::const_iterator iter = fset3.begin();
iter != fset3.end(); ++iter )
{
std::cout << iter->first << " , " << iter->second << "\n";
}

Ponder:

#include <map>
#include <algorithm>
#include <iterator>
#include <iostream>

int main ( void ) {
std::map< int, int > a;
std::map< int, int > b;
b[2] = 1;
b[4] = 2;
b[3] = 1;
a[3] = 0;
a[5] = 2;
a[4] = 5;
a[2] = 3;

typedef std::map<int,int>::const_iterator const_map_iter;

const_map_iter a_iter = a.begin();
const_map_iter b_iter = b.begin();
while ( a_iter != a.end() && b_iter != b.end() ) {
if ( a_iter->first < b_iter->first ) {
++ a_iter;
continue;
}
if ( b_iter->first < a_iter->first ) {
++ b_iter;
continue;
}
// if we get here, keys match:
std::cout << "key : "
<< a_iter->first
<< " matches "
<< b_iter->first
<< ". value in a : "
<< a_iter->second
<< ". value in b : "
<< b_iter->second
<< '\n';
// do not forget to increment in this case:
++ a_iter;
++ b_iter;
}
}


Best

Kai-Uwe Bux

 

[=?iso-8859-1?q?Erik_Wikstr=F6m?=]

Hi,

How do I compare 2 maps for identical keys, and if they have identical
keys, check if they have identical values? In either case I want to
copy ONLY that key value pair to one of two different maps.

I know how to copy the entire map to another:
// Copy fset1 to fset3
std::copy(fset1.begin(),fset1.end(),std::inserter(fset3,
fset3.begin()));

And I can for example iterate through the entire map, but the key pair
comparison is beyond me at this point.

You need to understand that an iterator to a map points to a std:air,
so what's stored in the map is acctually std:air-objects which
contains both the key and the value. The nice thing about this is that
you can compare std:airs to each other, if you have two pairs, x and
y then
x == y
is the same thing as
x.first == y.first && x.second == y.second
which means that comparing the pairs in the tree compares both the keys
and the values.

I don't quite understand what you mean whit copying only to one of the
maps. Do you mean that you want to extract the key/value pairs that are
unique in the two maps? You might want to look at the
set_symmetric_difference or set_difference algorithms.

 

[Daniel T.]

How do I compare 2 maps for identical keys, and if they have identical
keys, check if they have identical values? In either case I want to
copy ONLY that key value pair to one of two different maps.

So after the operation, the map should be identical, or do you want the
first map to have all the key/values it had plus any key/values that are
in the second map that the first didn't have?

I know how to copy the entire map to another:
// Copy fset1 to fset3
std::copy(fset1.begin(),fset1.end(),std::inserter(fset3,
fset3.begin()));

Is the end product of this operation what you want?

 

[Evyn]

Essentially, I want to compare the key value pair in 2 maps and copy
all unique key value pairs to a third map. In the case where the keys
are not unique, I still want to copy the key value pair to the third
map, but where the value is higher...

Thanks for your time again

 

[Ondra Holub]

Evyn napsal:

Hi,

How do I compare 2 maps for identical keys, and if they have identical
keys, check if they have identical values? In either case I want to
copy ONLY that key value pair to one of two different maps.

I know how to copy the entire map to another:
// Copy fset1 to fset3
std::copy(fset1.begin(),fset1.end(),std::inserter(fset3,
fset3.begin()));

And I can for example iterate through the entire map, but the key pair
comparison is beyond me at this point.

// Print test
for (std::map<double,double>::const_iterator iter = fset3.begin();
iter != fset3.end(); ++iter )
{
std::cout << iter->first << " , " << iter->second << "\n";
}


Thank you for your time
Evyn

See function Join:

#include <map>
#include <string>
#include <iostream>

typedef
std::map<std::string, int> M;

void Dump(const std::string& prompt, const M& m)
{
std::cout << prompt << ":\n";
for (M::const_iterator it = m.begin(); it != m.end(); ++it)
{
std::cout << '[' << it->first << "] = " << it->second << '\n';
}
std::cout << '\n';
}

template<typename KEY, typename DATA>
void Join(
std::map<KEY, DATA>& result,
const std::map<KEY, DATA>& m1,
const std::map<KEY, DATA>& m2
)
{
result.insert(m1.begin(), m1.end());
for (typename std::map<KEY, DATA>::const_iterator it = m2.begin();
it != m2.end();
++it
)
{
typename std::map<KEY, DATA>::iterator fi =
result.find(it->first);
if (fi == result.end())
result.insert(*it);
else
if (fi->second < it->second)
fi->second = it->second;
}
}

int main()
{
M m1;
M m2;

m1["a"] = 1;
m1["b"] = 2;
m1["c"] = 33;

m2["b"] = 22;
m2["c"] = 3;
m2["d"] = 44;

Dump("m1", m1);
Dump("m2", m2);

M result;
Join(result, m1, m2);

Dump("result", result);
};

 

[=?iso-8859-1?q?Erik_Wikstr=F6m?=]

Essentially, I want to compare the key value pair in 2 maps and copy
all unique key value pairs to a third map. In the case where the keys
are not unique, I still want to copy the key value pair to the third
map, but where the value is higher...

std::set_union is what you want, together with an insert_iterator.

#include <algorithm>
#include <iostream>
#include <iterator>
#include <map>

bool cmp(
const std:air<int, int>& p1,
const std:air<int, int>& p2
)
{
if (p1.first < p2.first)
return true;
if (p1.first == p2.first)
return p1.second > p2.second;
return false;
}

int main()
{
std::map<int, int> m1;
std::map<int, int> m2;
std::map<int, int> mR;

// Set values
m1[1] = 1; m1[2] = 2; m1[3] = 3;
m1[4] = 4; m1[5] = 5; m1[6] = 6;

m2[1] = 3; m2[2] = 2; m2[4] = 5;
m2[5] = 3; m2[8] = 1; m2[9] = 4;

// Get insert iterator and run set_union
std::insert_iterator<std::map<int, int> > ins(mR, mR.begin());
std::set_union(m1.begin(), m1.end(), m2.begin(), m2.end(), ins, cmp);

// Print
for (std::map<int, int>::iterator i = mR.begin(); i != mR.end(); ++i)
std::cout << i->first << " " << i->second << "\n";

return 0;
}

Hope the code survives the line-breaks.