Segmentation fault in vector for loop-malloc.c: No such file or directory

I'm getting a segmentation fault in the following code for a greedy algorithm for the knapsack problem. I've never successfully solved a segmentation fault before, though I've seen them, so I'd appreciate some help.

The message I get when I run the debugger is that there is no "malloc.c". When I run valgrind, I get an "Invalid read of size 4". Between that and the nature of the bug, I'm guessing I'm trying to access a vector element that is nonexistant. But I've tried every way I could think of to try and make sure that the loop didn't overstep its bounds when iterating through the vector.

(I've done this without using a C++11 range-based for loop and still get the same error. It doesn't seem to matter what I choose for the parameters of the for loop, it still throws a segmentation error.)

Thanks in advance.

#include <iostream>
#include <vector>
#include <algorithm>
#include <stdlib.h>

using namespace std;

bool decreaseSort(int a, int b)
{
   return a > b;   //sort by decreasing value for better performance
}

double get_optimal_value(int capacity, vector<int> weights, vector<int> values) {

sort(weights.begin(),weights.end(), decreaseSort); 
sort(values.begin(),weights.end(), decreaseSort);

vector<int> ourKnapsack(values.size(), 0);

double totalValue = 0.0;
int ourInput = 0;

for (auto i : ourKnapsack){
  int ourValue = values.at(i);
  int ourWeight = weights.at(i);
  double unitValue = (double)ourValue/ourWeight;
  if (capacity == 0) 
      return totalValue;


  if (weights.at(i) < capacity){
      ourInput = weights.at(i);
  }
  else {
      ourInput = capacity;
  }


  totalValue = totalValue * (ourInput * unitValue); 
  weights.at(i)-=ourInput;
  ourKnapsack.at(i)+=ourInput;
  capacity-=ourInput;

 }
 return totalValue;
}

  int main() {
  int n = 3;
  int capacity = 50;

  vector<int> values(n);
  values = {60,100,120};
  vector<int> weights(n); 
  weights = {20,50,30};

  double optimal_value = get_optimal_value(capacity, weights, values);

  std::cout.precision(10);
  std::cout << optimal_value << std::endl;
  return 0;
}