Using Map to insert an object and a string C++

Question

I am reading in a file and I am trying to make it to where if I find an object already in the map, it will add that value of the new object to the object already found in the map together. What I don't know is how to do this with the correct syntax with maps. Here's what I have:

struct ap_pair {
    ap_pair(float tp, float tm) : total_price(tp), total_amount(tm) {};
    ap_pair & operator+=(const ap_pair &);
    float total_price;
    float total_amount;
};


void APC :: compute_total ()
{

    string name;
    map<string, ap_pair> :: iterator my_it;
    float num1, num2, num3;

    while (!fs.eof() )
    {
        fs >> name >> num1 >> num2; //read in file

        ap_pair myobj(num1, num2); //send the weight/count and per unit price ap_pair 

        my_it = mymap.find(name); //returns iterator



       if (my_it != mymap.end()) 
    {                          
    //  myobj+=   //ERROR here. how can I add the new object to the object already in the map?


    }
    else
        mymap.insert(pair<string, ap_pair>(name, myobj));



        if (fs.eof()) break; //makes it so the last line is not repeated

        num3= num1*num2;
        total_amount+=num1;
        total_price+= num3;

    }




}

I am going through an iterator with the if condition. it should find a match with the same names, but how can I add the values of the object found with the object in the map already?

bstamour · Accepted Answer · 2014-01-17 20:18:47Z

4

A std::map iterator is a pair. The first element is the key and the second is the value. If you want to add the new object to the found one, you can do so like this:

my_it->second += myobj;

The ->second will give you a reference to the object at that place in the map, and then you just call your defined += operator on it.

Furthermore, if you create a default constructor for your pair type (perhaps it zeros out the two fields), then your code can be simplified to

while (!fs.eof() )
{
    fs >> name >> num1 >> num2; //read in file
    mymap[name] += ap_pair(num1, num2);

    // ... Rest of the loop...
}

If the operator [] doesn't find a value associated with name, it will default-construct one and then perform the addition.

answered Jan 17, 2014 at 20:18

bstamour

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2 Comments

Mdjon26 Over a year ago

Fantastic! It looks like it works. If I want to output the values of the container in the map, how can I do that? I can output the names with the for loop: for (my_it =mymap.begin(); my_it != mymap.end(); my_it++) cout << my_it->first<<endl;

bstamour Over a year ago

You would either need to overload operator << for your ap_pair type, or access the members individually through my_it->second. e.g. cout << my_it->second.total_price. Also, if this answer was satisfactory for you, feel free to click the check mark and "accept" it.

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