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2D array (vector) syntax

I am trying to get the value from a 2D array of vectors, that is a vector< vector<int> > type structure. I can assign values to the array locations using v[1][2] and v[x][y] just fine, but I cannot seem to get the value out of the location as easily as:

ofstreamObj << v[x][y];

When I step through it with the debugger, it shows the correct values of the array locations as it "passes over" the variables, but when I put v[0][0] in the Watch Box, it says "Error: Overloaded operation not found." It would make sense that if I can assign a value to a location using the notation above, I should be able to retrieve it doing the same. Perhaps an expect can shed some light on my subject. Thank you.

DarthNemesis

Not sure what's going wrong with your code; I just tried a test program and everything worked fine:

#include <vector.h>
#include <fstream.h>
int main()
{
// initialize 2D vector to 3 elements, each with
// initial value vector<int>(3)
vector< vector<int> > v(3,vector<int>(3));

ofstream OutFile("test.txt");
for (int j=0; j<3; j++) {
for (int i=0; i<3; i++) {
v[i][j] = (i+1)*(j+1);
OutFile << v[i][j];
}
OutFile << endl;
}
OutFile.close();
return 0;
}

If that program doesn't work for you, then your vector class must be outdated. It works on Borland C++ Builder 4.0.

klax33

ASKER

Thank you for your comment. Your code works correct when I tested it. I am thinking my assignment and printing code is wrong.

// matrix is global at the top
// row and col are declared to be 1 and 1

// set an index
matrix[row][col] = 1;

// this says 1 is at 1,1
cout << matrix[row][col] << " " << row << "," << col << endl;

// this shows the matrix is all 0's exectued right after
cout << matrix[0][0] << " ";
cout << matrix[1][0] << endl;
cout << matrix[1][0] << " ";
cout << matrix[1][1] << endl;

I am very confused as to why this is the case. Perhaps someone can help me. Thank you.

Mafalda

1)Better use the portable standart ANSI STL by changing to

#include <vector>
#include <fstream>
using namespace std;

2)

making changes in 1 and running your code shows the right result ! (I use VC++6 SP5 on Win 2000 Pro)

vector< vector<int> > matrix(3,vector<int>(3));
int col = 1;
int row = 1;
matrix[row][col] = 1;

// this says 1 is at 1,1
cout << matrix[row][col] << " " << row << "," << col << endl;

// this shows the matrix is 0,0,0,1
cout << matrix[0][0] << " ";
cout << matrix[1][0] << endl;
cout << matrix[1][0] << " ";
cout << matrix[1][1] << endl;

Mafalda

Do you initialize row and col to 1 ?
Are row and col defined as ints or what ?

klax33

ASKER

My array declaration code is as follows:

cout << "Attempting to declare a 2D array of size " << largest_index << "... ";

// declare a 2d array
char max_index = (char)largest_index -'0';

vector< vector<int> > matrix(max_index);

for (int h = 0; h < max_index; h++)
matrix[h].reserve(max_index);

// set all indices to 0
for (int i = 0; i < max_index; i++)
{
for (int j = 0; j < max_index; j++)
matrix[i][j] = 0;
}
cout << "done." << endl;

In my case, max_index, largest_index, row, and col are all chars because I'm reading them from a file (see previous post https://www.experts-exchange.com/questions/20544910/Converting-between-char-and-int-types.html). Thanks.

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Mafalda

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Mafalda

BTW there is a bug ... instead of

cout << matrix[0][0] << " ";
cout << matrix[1][0] << endl;
cout << matrix[1][0] << " ";
cout << matrix[1][1] << endl;

it should be

cout << matrix[0][0] << " ";
cout << matrix[0][1] << endl; // <<<---------- FIX
cout << matrix[1][0] << " ";
cout << matrix[1][1] << endl;

klax33

ASKER

I have it working now. Thanks for your help Mafalda.