How can I append the content of one map to another map?

Question

I have the following two maps:

map< string, list < string > > map1;
map< string, list < string > > map2;

I populated map1 with the following content:

1. kiran; c:\pf\kiran.mdf, c:\pf\kiran.ldf
2. test;  c:\pf\test.mdf, c:\pf\test.mdf

Then I copied the content of map1 into map2 as follows:

map2 = map1;

Then I filled map1 again with the following new content:

1. temp; c:\pf\test.mdf, c:\pf\test.ldf
2. model; c:\model\model.mdf, c:\pf\model.ldf

Now I have to append this content to map2. I cannot use map2 = map1;, because this will overwrite the existing content in map2. So, how can I do this?

Solution 1

map<int,int> map1;
map<int,int> map2;
map1.insert(map2.begin(), map2.end());

This will insert into map1 the elements from the beginning to the end of map2. This method is standard to all STL data structure, so you could even do something like

map<int,int> map1;
vector<pair<int,int>> vector1;
vector1.insert(map1.begin(), map1.end());

Furthermore, pointers can also function as iterators!

char str1[] = "Hello world";
string str2;
str2.insert(str1, str1+strlen(str1));

Highly recommend studying the magic of the STL and iterators!

Solution 2

You can use use insert method of the map. For example:

   std::map<int, int> map1;
    std::map<int, int> map2;

    map1[1] = 1;

    map2.insert(map1.begin(), map1.end());
    map1.clear();

    map1[2] =2;
    map2.insert(map1.begin(), map1.end());

Solution 3

You can do this several ways depending on what you want to do:

1. Use the copy constructor:

   map< string, list < string > > map1;
   // fill in map1
   
   map< string, list < string > > map2(map1);
   

2. Use the assignment operator as you indicate in the question:

   map< string, list < string > > map1;
   map< string, list < string > > map2;
   
   // fill in map1
   
   map2 = map1;
   

3. Do it all yourself manually:

   map< string, list < string > > map1;
   map< string, list < string > > map2;
   
   // fill in map1
   
   for (map< string, list < string > >::iterator i = map1.begin();
        i <= map1.end(); ++i) {
     map2[i.first()] = i.second();
   }
   

It sounds like (1) is what you want.

Comments

[sharptooth]

How does it happen that you and user Cute ask very similar questions at almost the same moments???

[sharptooth]

Okay, but what's the point in asking the question twice? One of you could ask the question and then both of you could see the answers. Question duplication just dilutes the community effort and you get less good answers.

[Ray Hidayat]

I'd like to add: std::copy(map1.begin(), map1.end(), insert_iterator(map2, map2.begin()) is another way

[Kiran Kumar]

Its just an example..it means the key val : 1 maps to value 1 :-)

[GManNickG]

Remember the space between the two > > :)

[Nick Lewis]

Oh yes, certain compilers will complain about nested template parameters (cough GCC), as the >> looks like a stream extraction operator. So you may need a space in the middle. I come from a Windows world, though, and the compiler in Visual Studio does the right thing. :)

[Kirill V. Lyadvinsky]

In C++1x space between >> is not necessary.

[小文件]

use std::make_move_iterator if map2 is not needed.