Adding to a vector of pair

问题:

I have a vector of pair like such:

vector<pair<string,double>> revenue;

I want to add a string and a double from a map like this:

revenue[i].first = "string";
revenue[i].second = map[i].second;

But since revenue isn't initialized, it comes up with an out of bounds error. So I tried using vector::push_back like this:

revenue.push_back("string",map[i].second);

But that says cannot take two arguments. So how can I add to this vector of pair?

回答1:

Use std::make_pair:

revenue.push_back(std::make_pair("string",map[i].second));

回答2:

IMHO, a very nice solution is to use c++11 emplace_back function:

revenue.emplace_back("string", map[i].second);

It just creates a new element in place.

回答3:

revenue.pushback("string",map[i].second);

But that says cannot take two arguments. So how can I add to this vector pair?

You're on the right path, but think about it; what does your vector hold? It certainly doesn't hold a string and an int in one position, it holds a Pair. So...

revenue.push_back( std::make_pair( "string", map[i].second ) );     

回答4:

Read the following documentation:

http://cplusplus.com/reference/std/utility/make_pair/

or

http://en.cppreference.com/w/cpp/utility/pair/make_pair

I think that will help. Those sites are good resources for C++, though the latter seems to be the preferred reference these days.

回答5:

Or you can use initialize list:

revenue.push_back({"string", map[i].second});

回答6:

revenue.push_back(pair<string,double> ("String",map[i].second));

this will work.

回答7:

Try using another temporary pair:

pair<string,double> temp;
vector<pair<string,double>> revenue;

// Inside the loop
temp.first = "string";
temp.second = map[i].second;
revenue[i].push_back(temp);