Inserting into nested map in C++

Question

Assume I have a nested map of type pointer. Then is there a single line statement to insert into the nested map,

map<int, map<int, int> >* nestedMap;

Currently I am doing this in 2 steps. First creating innermap and then insert into outer map as below,

nestedMap->insert(make_pair(int, map<int, int>)(int, innermap));

If the map is not pointer type, then i can insert easily like this,

nestedMap[int][int] = int;

Is there any simple ways for inserting into nested map of type pointer ?

thanks Prabu

I think this is enough easy - just a single line on code. For better reading - I suggest you to make a class Map , then method `bool insert(T, T)` (something like a wrapper of this ugly line) — Don Angelo Annoni, Nov 19 '12 at 08:28

user541686 · Accepted Answer · 2020-11-18T09:17:55.303

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map::operator[] automatically creates the key/value pair if it doesn't exist.
(That's why it's not const!) So you don't need to create the inner map manually.

If you want to avoid creating the pair automatically, then use map::find() or map::at().

edited Nov 18 '20 at 09:17

answered Nov 19 '12 at 08:22

user541686

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score 3 · Answer 2 · answered Nov 19 '12 at 08:36

3

I believe the simplest one-liner is:

(*nestedMap)[int][int] = int;

answered Nov 19 '12 at 08:36

Angew is no longer proud of SO

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juanchopanza · Answer 3 · 2012-11-19T08:32:05.743

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access the operator[] via ->:

nestedMap->operator[](5)[6] = 7;

This is analogous to

nestedMap[5][6] = 7;

if nestedMap is not a pointer.

Note that in neither case do you have to explicitly insert a map.

edited Nov 19 '12 at 08:32

answered Nov 19 '12 at 08:25

juanchopanza

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score 1 · Answer 4 · answered Nov 19 '12 at 08:28

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If i understand your question properly, you can actually use reference instead of pointer. You are not having issue with nested map, instead your outter map.

See below code, is what you want?

map<int, map<int, int> >* nestedMap  = new map<int, map<int, int> >;   
map<int, map<int, int> > &nestedMapAlais = *nestedMap;
nestedMapAlais[1][2] = 3;

answered Nov 19 '12 at 08:28

billz

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So far I thought we can't insert via (*nestedMap)[][], so only i raised this question. Now it clears. Thanks for your information – Prabu Nov 19 '12 at 09:18

Inserting into nested map in C++

4 Answers4