map

[mickey]

hi, I need to do this but don't compile...

std::map<int,Light> lightmap;
lightmap.insert(1,Light());

std::map<int,Light> lightmap;
lightmap.insert(1,Light());

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I'd like every element has an index, and insert and remove they; furthermore I need take it ordered for index (int): how this?
thanks.

[Michiel]

If it's a sequence of indices from 0 (or 1, I suppose), you might want to use a vector instead. It'll have faster inserts and lookups.

However, if you want to keep using the map, please post the entire code and the compiler error. I can't see anything wrong with the code you posted.

[mickey]

Yes I need a map because I need erase eg element 4 and 6 and compact map (is this automatically?). Then I need re-insert elemt 4 and 6 in their position...

(29): error C2664: 'std::_Tree<_Traits>::iterator std::_Tree<_Traits>::insert(std::_Tree<_Traits>::i terator,const std::_Tree<_Traits>::value_type &)' : cannot convert parameter 1 from 'int' to 'std::_Tree<_Traits>::iterator'
with
[
_Traits=std::_Tmap_traits<int,Light,std::less<int> ,std::allocator<std::pair<const int,Light>>,false>
]
and
[
_Traits=std::_Tmap_traits<int,Light,std::less<int> ,std::allocator<std::pair<const int,Light>>,false>
]

[jacek]

std::map::insert() wants an iterator as the first parameter.

Try this instead:

std::map<int,Light> lightmap;
lightmap[1] = Light();

std::map<int,Light> lightmap;
lightmap[1] = Light();

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[Michiel]

Of course. I always forget about that. (To be honest, I think it's a bit illogical.)

Here's a link that might come in handy: Click

[jacek]

To be honest, I think it's a bit illogical.

I agree, luckily QMap::insert() requires a key instead of iterator.

[Michiel]

Of course, if you already happen to have an iterator, the insert will be faster. But they could have overloaded the function to accept a key as well.

[mickey]

is it better use container Pair? thanks

[mickey]

Hi, I'm trying to use map, but I've got some problems; I'd like to insert in _map one element at begin. Is there a way more simple to do this below?

typedef  map<int,Light> MAP;
map<int,Light>::iterator ll;
MAP _map;
MAP::iterator _it;
_map.insert(MAP::value_type(1,Light()));

typedef  map<int,Light> MAP;
map<int,Light>::iterator ll;
MAP _map;
MAP::iterator _it;
_map.insert(MAP::value_type(1,Light()));

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Furhermore, After i inerted some elements in map I need add one other at the end and one at position eg 4; how to use insert to do this? thanks

[jacek]

Is there a way more simple to do this below?

Yes:

_map[1] = Light();

_map[1] = Light();

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[mickey]

Yes:

_map[1] = Light();

_map[1] = Light();

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sorry bu i don't understand: am I inserting an object Light at position 1 or an obj Light with key 1? I need insert element at the end;
With this:

_map[1] = Light()
_map[1] = Light()
_map[1] = Light()

_map[1] = Light()
_map[1] = Light()
_map[1] = Light()

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what am I doing? Have I inserted 3 element in map with key 1? thanks

[jacek]

am I inserting an object Light at position 1 or an obj Light with key 1?

The latter. There are no positions in a map.

I need insert element at the end;

There is no "end" in a map.

Have I inserted 3 element in map with key 1? thanks

No, you have inserted element with key 1 and then changed it twice.

[mickey]

sorry,

_map[0] = Light()
_map[1] = Light()  // == _map.insert(make_pair(1,Light())); //is it th same??
_map[2] = Light()

_map[0] = Light()
_map[1] = Light()  // == _map.insert(make_pair(1,Light())); //is it th same??
_map[2] = Light()

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I've 3 Light; now I need to delete the element with key '1';

_map.erase(_map.find(1));

_map.erase(_map.find(1));

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This??
thanks

[jacek]

http://www.cppreference.com/cppmap/erase.html

[mickey]

There are no positions in a map.
There is no "end" in a map.

A question: here I tried the use of end for a map? is this correct? isn't there 'end'? I don't understand
http://www.cppreference.com/cppmap/end.html

[jacek]

here I tried the use of end for a map? is this correct? isn't there 'end'? I don't understand

You can treat std::map as both a map (i.e. set of pairs) and a sorted list of pairs .