How does my code work when num_voltages is out of domain in the if statements?

Question

I am calculating the largest value in the second column of all my files and then finding their proportional effect. My code works properly and gives me the correct values. However, num_voltages is not defined in the domain and so I'm confused about why this code works. I'm a complete beginner, I wrote this code with trial and error. When I replace 'num_voltages' with 'i', I get the wrong values. Anytime I try changing what I have currently written, it gives me the wrong values. There are 6 files with 2 columns. Does anyone have any suggestions on how to improve my code and solve the domain problem?

#include <fstream>
#include <sstream>
#include <iostream>
#include <cstring>
using namespace std;

int
main ()
{
  ifstream infile;
  ofstream outfile;
  int num_voltages;
  int num_concs;
  float peak_current;
  string filename;
  int conc;
  string conc_string;
  int i;

  i = 0;

  num_concs = 6;
  num_voltages = 16;
  peak_current = -10;
  float current[num_voltages];
  float voltage[num_voltages];

  outfile.open ("Conc_vs_peak_current.txt");

  for (conc = 0; conc <= 10; conc += 2)

    {
      filename = "IK_IV_" + to_string (conc) + ".txt";

      infile.open (filename.c_str ());

      cout << "The System is Reading " << filename << endl;

      if (!infile)
    {
      cout << "File Not Found. Exiting" << endl;
      exit (1);
    }

      for (i = 0; i < 16; i++)
    {
      infile >> voltage[num_voltages] >> current[num_voltages];
    }

      infile.close ();

      cout << "File Read" << endl;

      for (i = 0; i < 16; i++)
    {
      if (current[i] > peak_current)
        peak_current = current[num_voltages];
    }

      cout << "Peak Value = " << peak_current << ", " << "Voltage = " <<
    voltage[num_voltages] << endl;

      outfile << conc << " " << peak_current << " " << voltage[i] << endl;

      cout << endl << endl;

    }

  outfile.close ();


  ///////////////////////////////

  float current0[15];
  float voltage0[15];
  float current10[15];
  float voltage10[15];

  infile.open ("IK_IV_0.txt");

  for (i = 0; i < 16; i++)
    {
      infile >> voltage[num_voltages] >> current[num_voltages];
    }

  infile.close ();

  for (i = 0; i < 16; i++)
    {
      if (current[i] > peak_current)
    peak_current = current[num_voltages];
      current0[15] = peak_current;
    }


  infile.open ("IK_IV_10.txt");

  for (i = 0; i < 16; i++)
    {
      infile >> voltage[num_voltages] >> current[num_voltages];
    }

  infile.close ();

  for (i = 0; i < 16; i++)
    {
      if (current[i] > peak_current)
    peak_current = current[num_voltages];
      current10[15] = peak_current;
    }


  cout << endl << endl;

  /////////////////////////////


  outfile.open ("Proportional_effect.txt");

  for (conc = 0; conc <= 10; conc += 2)

    {
      filename = "IK_IV_" + to_string (conc) + ".txt";

      infile.open (filename.c_str ());

      cout << "The System is Reading " << filename << endl;

      if (!infile)
    {
      cout << "File Not Found. Exiting" << endl;
      exit (1);
    }

      for (i = 0; i < 16; i++)
    {
      infile >> voltage[num_voltages] >> current[num_voltages];
    }

      infile.close ();

      cout << "File Read" << endl;

      for (i = 0; i < 16; i++)
    {
      if (current[i] > peak_current)
        peak_current = current[num_voltages];
    }

      float numerator = peak_current - current0[15];
      float denominator = current10[15] - current0[15];
      float value = numerator / denominator;
      float absolute_value = abs (value);

      cout << "The Proportional Effect is " << absolute_value << endl;

      outfile << conc << " " << absolute_value << endl;

      cout << endl << endl;

    }

  outfile.close ();

}