Get Segmentation Fault Occasionally - c++

Question

I am trying to make a random number generator for a uni project.
I am trying to implement a function to get the period of this list of numbers generated (how long before the numbers start to repeat).
When I compile this and run it it returns 507 (which is the correct period).
Yet about 50% of the time it returns a Segmentation Fault.
Any idea what is happening:
#include<iostream>
#include<fstream>
#include<vector>
#include<math.h>
#include<ctime>
using namespace std;
class Random
{
public:
// Constructor
Random(int a, int b, int m)
{
this->setMagicNumbers(a,b,m);
}
// Function to set the magic numbers of the random number generator
void setMagicNumbers(int a, int b, int m)
{
this->A = a;
this->B = b;
this->M = m;
}
// Function that returns a list of uniformly distributed random numbers
vector<double> GetUniform(int N, double initialValue = time(0))
{
double currentNumber = initialValue;
vector<double> RandomNumbers;
for(int i = 0; i <= N; i++)
{
currentNumber = fmod((this->A*currentNumber) + this->B, this->M);
RandomNumbers.push_back(currentNumber / this->M); // The number is normalised here
}
return RandomNumbers;
}
private:
int A, B, M;
};
class NumberList
{
public:
// Function to add an element to the list
void push(double x)
{
this->v.push_back(x);
}
// Function to pop an element off the list
double pop()
{
if(v.size() > 0)
{
int popped = v.back();
v.pop_back();
return popped;
}
else
{
cout << "Cannot pop element off empty list." << endl;
return 0;
}
return 0;
}
// Function to get the value at a given location on the list
double getAt(int i)
{
return this->v[i];
}
// Function to set the value at a given location on the list
void setAt(int i, double value)
{
this->v[i] = value;
}
// Function to find the size of the list
unsigned int size()
{
return this->v.size();
}
// Function to get the vector itself
vector<double> getvector()
{
return this->v;
}
// Function to set the value of the vector itself
void setVector(vector<double> vec)
{
this->v = vec;
}
// Function to print the list of numbers as coordinates to a data file
void printCoordinates()
{
ofstream data("coordinates.dat");
for(unsigned int i = 0; i <= this->v.size(); i++)
{
data << this->v[i] << " " << this->v[i + 1] << "\n";
}
data.close();
}
// Function to print the list of numbers to the terminal
void print()
{
for(unsigned int i = 0; i <= this->v.size(); i++)
{
cout << this->v[i] << endl;
}
}
// Function to get the period of the list of numbers
int getPeriod()
{
int i = 2;
while(true)
{
if(isPeriod(i) == true)
{
return i;
}
else
{
i = i + 1;
}
}
}
private:
// Vector to hold the values for the list
vector<double> v;
// Function to test if 'i' is the period of the list
bool isPeriod(int i)
{
for(int j = 0; j != (i-1); j++)
{
if(this->getAt(j) != this->getAt(i + j))
{
return false;
}
}
return true;
}
};
int main()
{
Random RandNumGenerator(100,104001,714025); // Create a new random number generator with given magic numbers
NumberList numbers; // Create a blank list of numbers
numbers.setVector(RandNumGenerator.GetUniform(10000)); // Add 10000 random numbers to the list
numbers.printCoordinates(); // Print out the random numbers as coordinates to a data file
cout << numbers.getPeriod() << endl; // Print out the period of the random number list
return 0;
}
Thanks in advance.

Answer 1

This line could cause a problem
for(unsigned int i = 0; i <= this->v.size(); i++)
With it you are using size as the last index, which is overflow, try changing it for this:
EDIT: change for this actually
for(unsigned int i = 0; i < this->v.size()-1; i++)
On the loop you are accessing the i+1 th element of the vector.
EDIT:
This probably wouldn't cause a crash but instead of creating N elements you are creating N+1
for(int i = 0; i <= N; i++)
Change if for this:
for(int i = 0; i < N; i++)
EDIT:
With fixes
#include<iostream>
#include<fstream>
#include<vector>
#include<math.h>
#include<ctime>
using namespace std;
class Random
{
public:
// Constructor
Random(long a, long b, long m)
{
this->setMagicNumbers(a,b,m);
}
// Function to set the magic numbers of the random number generator
void setMagicNumbers(long a, long b, long m)
{
this->A = a;
this->B = b;
this->M = m;
}
// Function that returns a list of uniformly distributed random numbers
vector<double> GetUniform(int N, double initialValue = time(0))
{
double currentNumber = initialValue;
vector<double> RandomNumbers;
for(int i = 0; i < N; i++)
{
currentNumber = fmod((this->A*currentNumber) + this->B, this->M);
RandomNumbers.push_back(currentNumber / this->M); // The number is normalised here
}
return RandomNumbers;
}
private:
long A, B, M;
};
class NumberList
{
public:
// Function to add an element to the list
void push(double x)
{
this->v.push_back(x);
}
// Function to pop an element off the list
double pop()
{
if(v.size() > 0)
{
int popped = v.back();
v.pop_back();
return popped;
}
else
{
cout << "Cannot pop element off empty list." << endl;
return 0;
}
return 0;
}
// Function to get the value at a given location on the list
double getAt(int i)
{
return this->v[i];
}
// Function to set the value at a given location on the list
void setAt(int i, double value)
{
this->v[i] = value;
}
// Function to find the size of the list
unsigned int size()
{
return this->v.size();
}
// Function to get the vector itself
vector<double> getvector()
{
return this->v;
}
// Function to set the value of the vector itself
void setVector(vector<double> vec)
{
this->v = vec;
}
// Function to print the list of numbers as coordinates to a data file
void printCoordinates()
{
ofstream data("coordinates.dat");
for(unsigned int i = 0; i < this->v.size()-1; i++)
{
data << this->v[i] << " " << this->v[i + 1] << "\n";
}
data.close();
}
// Function to print the list of numbers to the terminal
void print()
{
for(unsigned int i = 0; i < this->v.size(); i++)
{
cout << this->v[i] << endl;
}
}
// Function to get the period of the list of numbers
int getPeriod()
{
int i = 2;
while(true)
{
if(isPeriod(i) == true)
{
return i;
}
else
{
i = i + 1;
}
}
}
private:
// Vector to hold the values for the list
vector<double> v;
// Function to test if 'i' is the period of the list
bool isPeriod(int i)
{
std::cout << "trying period " << i << endl;
if (i >= v.size()) return true;
for(int j = 0; j < v.size()-1; j++)
{
if(this->getAt(j) == this->getAt(i + j))
{
std::cout << "value at j " << this->getAt(j) << "value at i+j " << this->getAt(i+j) <<endl;
return true;
}
}
return false;
}
};
int main()
{
Random RandNumGenerator(100,104001,714025); // Create a new random number generator with given magic numbers
NumberList numbers; // Create a blank list of numbers
numbers.setVector(RandNumGenerator.GetUniform(10000)); // Add 10000 random numbers to the list
numbers.printCoordinates(); // Print out the random numbers as coordinates to a data file
cout << numbers.getPeriod() << endl; // Print out the period of the random number list
return 0;
}

Answer 2

Played around with this a bit and I initially thought that your vector of doubles was somehow getting corrupted. I found that if you size it to the exact value that you want - which is a known value - the seg faults stop:
vector<double> RandomNumbers(N); // In GetUniform()
Running it through gdb was inconclusive as it looked as if the stack was getting corrupted. I also found that when I changed the push_back() just to take 0:
RandomNumbers.push_back(0.0);
instead of the normalised number, even without the initial size for the vector (above) it worked fine, suggesting an issue with one of the variables being used.
I also found that if I set currentNumber to 1, say, instead of calling fmod() - even with everything as it originally was, I got no seg faults:
currentNumber = 1;
This kind of suggests that it might be something to do with the use of fmod()
Interestingly, I then took out the call to push_back completely and found that it still had segfaults. Sorry I can't be much more help than that, but it certainly looks as if some kind of corruption is happening somewhere in this area.

Answer 3

In C++ index of array and vector will be from 0..size - 1, and you misused it all over your code(inside GetUniform, inside printCoordinates, ...) and beside that in printCoordinates you written this->v[i + 1] at last it will be converted to this->v[this->v.size() + 1] that is 2 index after last valid index.
But I think source of your error is in getPeriod and isPeriod, look at it you start from 0 to i - 1 and check if that index match item at index i + j, so if i become size() - 1 and size be 500, then in worse case you are accessing v[499 + 498] that is really large than last valid index and you certainly get a segmentation fault. Now to solve it check your index and never go beyond end of the vector size, that mean you should never use index > v.size() - 1