How can I convert the strings to integers?

[compscinerds]

Hello, everybody can y'all be so kind to help me with my program? I am trying to convert strings to integers. This is my task:
Open a file "numbers.txt"
read integer numbers as strings from the file (one number per line)
convert each string to a numeric representation (without using stoi() or similar functions from the string library)
store converted numbers in an array
close "numbers.txt"

So far I have read integer numbers as strings but cannot convert the string array to integers without using the string library. Can someone help? My code won't compile but I am trying to do this: 154: Sum = 4*1 + 5*10 + 1*100 = 154 so it can convert to an integer.
This is what I have so far:

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

int StringToInteger( string arraySize);

int main()
{
   ifstream infile;
   infile.open("numbers.txt");
   const int size = 1000;
   string myArray[size];
   int i = 0;
   while (infile >> myArray[i])
   {

      cout << "The string is \"" << myArray[i] << "\"" << endl;

      myArray[i].size();
      cout << i << endl;
      
      StringToInteger(i);
      ++i;

      
   }
}

int StringToInteger( string arraySize)
{
   int values = 0;
   for (int index = 0; index < arraySize.size; index++) {
      values *= 10;
      values += (arraySize[index] - '0');
   }
   return values;
}