Using 2D vector as a member of class matrix

[dushyantkumar1]

Hi,

I am trying to declare a class matrix (containing 2D array) with following members:
int d1, d2; //dimensions
vector<vector<float>> p; // All values of matrix

I want to put all elements of matrix in double vector.
The code compiles fine. However, when I start setting the value, I get message that:

....
Expression: vector subscript out of range...

There is some error with function “set_value”.

// Matrix.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"  //must for MS visual studio
#include <iostream>  //For function cout<<
#include <fstream>   //For I/O from file
#include <string>    //For string op
#include <cstdlib>   //??
#include <vector>

using namespace std;


class matrix
{
	int d1, d2; //dimensions
	vector<vector<float>> p; // All values of matrix
public:
	//We are not using the default constructor
	matrix(int, int); //Constructor will only create the space here LLL
	~matrix();
	void set_value(int, int, float); // first two args are element position specifiers.
};


matrix::matrix(int d11, int d22)
{
	d1=d11;
	d2=d22;
	vector<vector<float>> p(d2, vector<float>(d1,0)); 
}

matrix::~matrix()
{

	//I believe that I do not need to write a destructor as there is no pointer involved.
}
void matrix::set_value(int i, int j, float value)
{
	p[j][i] = value;
}

int main()
{
	int m, n; //dimensions
    int i,j;
	float value;
//Enter size m>>n
//Enter element one by one
//Use the for loop

	cout<<"Enter the dimensions m, n : \n";
	cin>>m>>n;

	cout<<"Value of m and n \n";
	cout<<m<<", "<<n;
	matrix A(m,n), B(m,n);

//Reading A from key
	cout<<"Enter the values of elements one by one: ROWWISE\n";
	for (i = 0; i< m; i++)
		for (j=0; j < n; j++)
		{
			//cout<<"\n Please Enter (";cout<<i; cout<<","; cout<<j ;cout<<") th element \n";
			cout<<"\n Please Enter ("<<i<<","<<j<<") th element \n";
			cin>>value;
			A.set_value(i,j,value);
		}


	cout<<"\n";
	system("pause");
	//cin.get(); 
	return 0;
}

[shacktar]

This (within your constructor): vector<vector<float>> p(d2, vector<float>(d1,0)); actually defines a vector p which hides the p belonging to your matrix class!

Try this instead:

matrix::matrix(int d11, int d22)
{
         d1=d11;
         d2=d22;
         p.resize(d2, vector<float>(d1,0));
}

[johnnystarr]

You may want to include exception handling as well.

[dushyantkumar1]

Thanks shacktar and johnystarr.

shacktar,

your suggestion worksed. However, for some reason, the launching of console was taking 4-5 minutes. So, rather than using resizing, I have started using:

matrix::matrix(int d11, int d22)
{
	d1=d11;
	d2=d22;
	vector<vector<float>> p1(d2, vector<float>(d1,0));
	p = p1;
}

Anyway, shacktar, can you tell me how did you figure out the source of error?


johnnystarr,
Since I am new to C++, I only have limited idea about exception handling and I was under impression that it's used for memory allocation. Since vector is standard STL feature, I am assuming that it must be taking care of that.

Anyway, would you please show me an example in current context?

Regards,

[shacktar]

Anyway, shacktar, can you tell me how did you figure out the source of error?

Well, any time the compiler sees something like (type) (identifier) ... ; it treats it as a new definition. Combine that with C++'s scoping rules and you've got yourself a lost variable.

For instance:

int x;

void myFunction()
{
   int x;//now "myFunction()" sees this x, not the one declared above
}

This is precisely what was happening with your case above, where the vector p in the matrix class constructor hid the p in the scope of the whole matrix class. Seeing this comes with experience, I suppose.

Also, note that vector p was already constructed here (using the vector class default constructor): vector<vector<float>> p; // All values of matrix , so it doesn't make sense to try and construct it again.