Unhandled Memory Exception - c++

Question

I'm working on a program that reads a set of data based on patient's ID numbers and their blood pressure readings. The program will then add all the readings together and come up with an average. It'll then display that average. This is my program so far:
#include <iostream>
#include <string>
#include <conio.h>
#include <fstream>
using namespace std;
int main()
{
//Initialize Required Variables For Program
int patientCount;
string id;
string rHowMany; //String To Read From File
int howMany;
int howManyCount;
int avg = 0;
int avg2;
string line;
int number_lines = 0;
ifstream reader ("data.txt"); //Open The Data File To Be Read From
patientCount = 0;
while (getline(reader, line))
{
number_lines += 1;
}
//getline(reader, id); //Get the patients ID
//getline(reader, rHowMany); //Get How Many BP Records The Patient Has
for (number_lines; number_lines > 0; number_lines--)
{
reader >> id;
reader >> rHowMany;
howMany = stoi(rHowMany);
howManyCount = howMany;
patientCount += 1;
cout << "Patient ID: " + id;
for (howManyCount; howManyCount > 0; howManyCount--)
{
reader >> avg2;
avg = avg + avg2;
}
}
cout << avg;
cout << "\n";
cout << howMany;
avg = avg / howMany;
cout << "\n";
cout << avg;
_getch();
return 0;
}
When I run the program I get this error:
Unhandled exception at at 0x756DB727 in Blood Pressure.exe: Microsoft C++ exception: std::invalid_argument at memory location 0x0042F794.
I'm not quite sure what that means, but it opens up a list of code I've never seen before. Like I said I'm not sure why it's throwing this error, or what the error means, but if someone could help me, I would appreciate it.

Answer 1

This line:
cout << "Patient ID: " + id;
Is flawed. You are trying to append id to "Patient ID: ", but that is not what is happening.
The string literal "Patient ID: " is actually a pointer to a sequence of characters (that is, an array). When you use the plus sign, you are performing pointer arithmetic between the memory address of the character sequence and id. If id is larger than the length of the string, this will probably lead to your program trying to access an invalid location.
To fix this, simply print id after the sequence by making two separate calls to the << operator:
cout << "Patient ID: " << id; // no +