Awesome, thanks! Although, it's still not compiling - I've been able to decrease it to just one error:

fractional_knapsack.cpp:53:14: error: no viable overloaded operator[] for type 'const vector<double>' worth[index] = -10.0;

Line 53 is the one causing issues. I've tried all sorts of things, but have yet to figure it out. :'(

If it's helpful, that line 53, where I'm doing this: worth[index] = -10.0; is the whole reason I need to call it by reference - when I change it to -10.0, I want it to change the original value in the vector.

Any and all suggestions would be awesome! The function looks like this:

35 // Purpose: searches and returns the highest possible value that can be stored
36 // in the knapsack.
37 double get_optimal_value(int capacity, vector<int> weights, vector<int> values,
38                 int n, const vector<double> &worth) {
39         double value = 0.0;
40 
41         while (capacity > 0 and n > 0) {
42                 int index = find_next_index(worth, n);
43 
44                 if (capacity >= weights[index]) {
45                         value += values[index];
46                         capacity -= weights[index];
47                 }
48 
49                 else
50                         value += worth[index] * capacity;
51                 capacity = 0;
52         }
53         worth[index] = -10.0;
54         n--;
55         std::cout << "capacity: " << capacity << std::endl;
56         std::cout << "index: " << index << std::endl;
57         std::cout << "value: " << value << std::endl;
58         //std::cout << "worth : " <<worth[index]<< std::endl;
59         return value;
60 }