Adding elements into a c++ map - c++

Question

I have a map in my program that stores the code of a Product p and the amount of it.
If a ask for a new request, and if the product already exists in the map, i just need to sum the second element of the pair(code, amount) to the element in the map.
How can I do this?
void Request :: addItem (Product p, double amount) {
if(this->isItemRequest(p)) {
//p already exists in the map.
}
this->rdata.insert(pair<int, double>((int)p.getCode(), amount));
}
Thanks a lot!

Answer 1

Assuming your map is declared within the Request class as std::map<int, double> rdata, the code can be:
void Request::addItem( Product p, double amount )
{
if ( this->isItemRequest(p) )
{
int Code = int(p.getCode);
this->rdata[ Code ] += amount;
}
this->rdata.insert( pair<int, double>(int(p.getCode), amount) );
}
However, if isItemRequest() is just a trivial check, your code can be simplified to just:
void Request::addItem( Product p, double amount )
{
int Code = int(p.getCode);
this->rdata[ Code ] += amount;
}
P.S. Maybe, it is a good idea (if you can change the interface) to pass Product by const reference.

Answer 2

If you call on:
this->rdata[key] = value;
you create a value using default constructor (int() initializes to 0), return reference to it, and call operator= on it. You can avoid that by checking if key exist first:
this->rdata.count(key) != 0
or simplier
this->rdata.count(key)
if it exists you can use operatror=, operator+= and so on on reference returned by operator[]:
if (this->rdata.count(key) == 0)
this->rdata.insert( pair<int, double>( key, value ) );
else
this->rdata[key] += value;
but in this simple case
this->rdata[key] += value;
should just do.

Answer 3

The simplest way to do that is
rdata[p.getCode()] += amount;
If it wasn't already in the map, then [] will insert an element with value zero, so the value will end up being amount. If it was, then amount is added to the existing value.

Answer 4

Easy: map.find :)
#include <iostream>
#include <map>
#include <string>
typedef std::map<std::string, int> StringIntMap;
int main() {
StringIntMap map;
map["coke"] = 10;
map["fries"] = 25;
map["pizza"] = 50;
std::cout << "Before increase" << std::endl;
StringIntMap::const_iterator it;
for (it = map.begin(); it != map.end(); it++) {
std::cout << it->first << ": " << it->second << std::endl;
}
std::cout << "Now, increase pizza +50" << std::endl;
StringIntMap::iterator item = map.find("pizza");
if (item != map.end()) {
// pizza exists increase 50
item->second += 50;
} else {
std::cout << "Sorry, no pizza here" << std::endl;
}
std::cout << "after increase" << std::endl;
for (it = map.begin(); it != map.end(); it++) {
std::cout << it->first << ": " << it->second << std::endl;
}
return 0;
}