finding max cost and printing string

[justStarting]

So I am having trouble finding the max price from a list of parts that is in a .txt file. I need to find what the highest cost is and cout the part number (string). The rest of my program works fine, just this part has got me stumped. The .txt file appears like:
partnumber (string), category (char), quantity(int), cost(double) which are all stored in a struct.

I know the code below is wrong/missing the part where I would display the string but like I mentioned... I'm a bit lost. Any direction would be appreciated.

string highest(const vector <Parts> &pVector)
{
	
	double max = 0;
	int size = pVector.size();
	for (int i = 0; i < size; i++)
	{
		if (pVector[i].cost > max)		
			max = pVector[i].cost;	
	}
	cout << max << endl;

	return 0;
}

[tpb]

string highest(const vector<Parts>& parts)
{
	if (parts.size() == 0)
	    return "";

	size_t max = 0;
	for (size_t i = 1; i < parts.size(); i++)
	{
		if (parts[i].cost > parts[max].cost)
			max = i;
	}

	return parts[max].partnumber;
}

[justStarting]

Thank you for your response, I knew I wasn't far off. I don't think I covered size_t yet, but after your input I did some reading on it(http://www.cplusplus.com/reference/cstring/size_t/). All worked. Thanks again.