CODEWITHSUNDEEP
c++stldictionaryiteratorcopy
Raj
Raj

Reputation: 795

How i can copy reverse of a map to another map?

For example map1 is gaving values 1 to 10 with some address(begin to end). i want to have values 10 to 1 with corresponding address in map2(begin to end) 

map<long , int* > v;
map<long , int* > rv;

int i,a[10];
for(i=0; i<10; i++)
{
a[i] = i+1;
v.insert(pair<long, int *>(i+1,&a[i]));
}
itr = v.begin();
while(itr != v.end())
{
 cout << itr->first << " "<<itr->second;
 cout << endl;
 itr++;
}
rv.insert(v.rbegin(),v.rend());
cout << "copied array: "<<endl;
itr = rv.begin();
while(itr != rv.end())
{
cout << itr->first << " "<<itr->second;
cout << endl;
itr++;
 }

i tried above one but am getting values 1 to 10 only..my expected values 10 to 1. please help me to find out....

Upvotes: 2

Views: 397

Answers (3)

ks1322
ks1322

Reputation: 35716

This is impossible because std::map is ordered associative container. If you want to preserve order of insertion use other containers such as std::list or std::vector.

Upvotes: 1

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726599

STL map is an ordered container. The order of items that you get during the iteration is independent of the order in which you insert the items into the container.

The order of iteration is determined by two things:

  • The value of the key, and
  • The Compare class passed as the template parameter to the map

You can iterate the map in reverse order (your code snippet shows that you already know how it is done). The performance penalty for reverse-iterating a map, if any, is negligible. You can also provide a non-default Compare (std::greater<long> instead of the default std::less<long>) to have the default order of iteration altered.

Upvotes: 8

JTeagle
JTeagle

Reputation: 2196

Maps sort by increasing value (as dictated by operator< ), so no matter how you insert the elements, they will be returned in sorted order. You are certainly doing the insertion in reverse, but every element placed is being duly sorted into proper ascending order.

Upvotes: 0

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