CODEWITHSUNDEEP
c++boostunordered-map
H'H
H'H

Reputation: 1678

modify value of key in boost unordered_map

I have a boost::unordered_map which I want to modify the value of a particular key. I have seen the question here.

what makes my question different is that in my map key is a simple int and my value is std::vector. I want to update this value by inserting a new PoitCoord at the second position of the vector.

One solution is like that:

auto it = map.find(key);
if(it != map.end()) 
{
    std::vector<PointCoord> pntcrds = it->second;
    pntcrds.insert((pntcrds.begin()+1), new_value);
    it->second = pntcrds;
}

I am wondering if there is less verbose solution.

Upvotes: 1

Views: 767

Answers (3)

Sam
Sam

Reputation: 20056

There is indeed a very simple solution, that covers all the scenarios you might think about. And it is

myMap[key].insert(pntcrds.begin()+1);

If the key does not exist, it will be inserted. otherwise, the value will be updated to the new one;

But you must make sure you have at least one element in your vector. Otherwise, it will crash.

A similar trick would be

myMap[key].push_back(new_value); // appends to the end of the vector

Upvotes: 1

Johannes S.
Johannes S.

Reputation: 4626

The map does not have anything to do with your insertion, if I understand your question correctly. You are only modifying a vector which happens to be stored in a map. You are not modifying the key of a map, but one of it's values.

So the short solution would be:

auto it = map.find(key);
if(it != map.end() && !it->second.empty() ) 
{
    it->second.insert( (pntcrds.begin()+1), new_value);
}

If you know that the key exists in your map, you can shorten this to:

std::vector<PointCords> & pntCords = map[key];
if( ! pntCords.empty() )
   pntCords.insert( pntCords.begin()+1, new_value );

NB: If you use the second method and the key does not yet exist, a default constructed (=empty) std::vector<PointCords> will be inserted into the map.

Upvotes: 2

user2090491
user2090491

Reputation: 600

You have to find the key iteration position then update directly the found key by

it->second.insert( (pntcrds.begin()+1), new_value);

but you have to be sure that you've found the iteration and as @Johannes said your vector is not empty.

Upvotes: 1

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