C++ Vector of vectors

Question

I have a class header file called Grid.h that contains the following 2 private data object:

vector<int> column;
vector<vector<int>> row;

And a public method whose prototype in Grid.h is such:

int getElement (unsigned int& col, unsigned int& row);

The definition of above mentioned function is defined as such in Grid.cpp:

int getElement (unsigned int& col, unsigned int& row)
{
    return row[row][col] ;
}

When I run the program, I get this error:

error C2109: subscript requires array or pointer type

Whats going wrong?

Answer 1

In the line return row[row][col]; the first row is the int& , not the vector .

The variable declared in the inner scope is shadowing the variable in the outer scope, so the compiler is trying to index an int rather than a vector , which it obviously can't do.

You should fix your variable names so that they don't conflict.

EDIT: Also, while the error that you're getting indicates that the compiler is finding the wrong row variable, as A. Levy points out, you also have a problem with the declaration of your vector , so even if you fix the variable names, if you have indeed declared the vector as shown here, it won't compile. Nested templates need spaces between the > symbols, otherwise the compiler will read >> as a right-shift operator rather than part of a template declaration. It needs to be

std::vector<std::vector<int> > row;

or

std::vector< std::vector<int> > row;

In addition, as you're doing this in a header file, you're going to need to tack the std:: tag on the front of anything from the std namespace - such as vector . If it were in a cpp file, then you could use using namespace std; but that would be very bad to do in a header file (since it would pollute the global namespace). Without the std:: tag or the using statement, the compiler won't recognize vector .

Answer 2

This is probably not the index problem, but you also need a space between the nested angle brackets in your vector of vectors type declaration. C++ compilers have a hard time telling the difference between nested template types and the right bit shift operator.

Example:

vector<vector<int> >  vec2d;        // Good.

vector<vector<int>>   anotherVec2d; // Bad!

vector< vector<int> > yetAgain;     // Best IMHO. 
                                    // Keeps the white space balanced.

Answer 3

I think you want something like this... (although I cannot imagine why :-))

#include <vector>
#include <iostream>

using namespace std;

typedef vector<int> row;
typedef vector<row> matrix;

matrix mat(2,2);

int getElement (unsigned int ri, unsigned int ci)
{
    return mat[ri][ci] ;
}

int main() {

    mat[1][0] = 1234;
    cout << getElement(1,0) << endl;

    return 0;
}

Answer 4

这就是你需要的：

return Grid::row[row][col];

Answer 5

It appears to me (although you may need to verify this on your own, I don't feel like writing up a test application) that the problem is coming from the fact that your parameter contains a named row and your class has an inner variable row and there is a naming conflict.

You may need to qualify which row you're using. Consider:

return Grid::row[row][col];