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?

How does it happen that you and user Cute ask very similar questions at almost the same moments??? — sharptooth, Jul 01 '09 at 06:28
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. — sharptooth, Jul 01 '09 at 06:57
It looks like that's the right way to do it according to the documentation: [http://www.cplusplus.com/reference/stl/map/operator=/](http://www.cplusplus.com/reference/stl/map/operator=/) — Ray Hidayat, Jul 01 '09 at 05:41

score 65 · Answer 1 · answered Jul 14 '09 at 20:43

65

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!

answered Jul 14 '09 at 20:43

Nick Lewis

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Remember the space between the two `> >` :) – GManNickG Jul 14 '09 at 20:52
1

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. :) – Nick Lewis Jul 14 '09 at 20:55
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In C++1x space between >> is not necessary. – Kirill V. Lyadvinsky Jul 14 '09 at 20:58
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use `std::make_move_iterator` if map2 is not needed. – fishshrimp鱼虾爆栈 Sep 27 '18 at 08:26

score 8 · Answer 2 · answered Jul 01 '09 at 06:23

8

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());

answered Jul 01 '09 at 06:23

Naveen

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Its just an example..it means the key val : 1 maps to value 1 :-) – Naveen Jul 01 '09 at 07:02

score 4 · Answer 3 · answered Jul 01 '09 at 06:04

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

Use the copy constructor:

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

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

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;

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.

I'd like to add: std::copy(map1.begin(), map1.end(), insert_iterator(map2, map2.begin()) is another way — Ray Hidayat, Jul 01 '09 at 06:29

score 1 · Answer 4 · answered Mar 13 '20 at 12:02

Since C++17 std::map provides a merge() member function. It allows you to extract content from one map and insert it into another map. Example code based on your data could be written as follows:

using myMap = std::map<std::string, std::list<std::string>>;

myMap  map2 = { {"kiran", {"c:\\pf\\kiran.mdf", "c:\\pf\\kiran.ldf"}},
                {"test",  {"c:\\pf\\test.mdf",  "c:\\pf\\test.mdf"}} };

myMap  map1 = { {"temp",  {"c:\\pf\\test.mdff",    "c:\\pf\\test.ldf"}},
                {"model", {"c:\\model\\model.mdf", "c:\\pf\\model.ldf"}} };

map2.merge(map1);

for (auto const &kv : map2) {
    std::cout << kv.first << " ->";
    for (auto const &str : kv.second)
        std::cout << " " << str;
    std::cout << std::endl;
}

Output:

kiran -> c:\pf\kiran.mdf c:\pf\kiran.ldf
model -> c:\model\model.mdf c:\pf\model.ldf
temp -> c:\pf\test.mdff c:\pf\test.ldf
test -> c:\pf\test.mdf c:\pf\test.mdf

Notes:

A key can only exist once in a map (i.e. kiran, test, temp, model in your example). If both maps contain the same key, then the corresponding element will not be merged into map2 and remain in map1.
If all elements can be merged into map2, then map1 will become empty.
The merge() function is quite efficient, because element are neither copied nor moved. Instead, only the internal pointers of the map nodes are redirected.

Code on Coliru

score 0 · Answer 5 · answered Mar 17 '16 at 01:34

0

If you want to insert your map as you define it, this is nice:

payload.insert({
            { "key1", "one" },
            { "key2", 2 },
        });

answered Mar 17 '16 at 01:34

Pellet

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

5 Answers5