How to display and expand the elements in a 2D matirx presented in C++ vector form.

[nanchuangyeyu]

Hi,
I tried to present a 2D mathematic matrix with C++ vector and expand it by repeating the first and last row and column. An example for intuition, I initialized a 2D C++ vector vec_2d to be as following:
1, 2, 3, 4
5, 6, 7, 8
9,10,11,12
13,14,15,16,
and I want to expand it to be:
1, 1, 2, 3, 4, 4
1, 1, 2, 3, 4, 4
5, 5, 6, 7, 8, 8
9, 9,10,11,12,12
13,13,14,15,16,16
13,13,14,15,16,16
by first repeat the first and last rows and then the updated first and last columns.

The code is as following:

#include <iostream>
#include <vector>

using namespace std;

int main()
{
	vector<short>vec;
	vector< vector<short> >vec_2d;
	vector< vector<short> >vec_2d_padded;
	// Initialize a 2D C++ vector.
	for (int i=1; i<17; i++)
	{
		vec.push_back(i);
		if (i%4 == 0)
		{
			vec_2d.push_back(vec);
			vec.clear();
		}
	}
	// Expand by repeating the first and last row.

	vector< vector<short> >::iterator it;
	it = vec_2d.begin();
	vec_2d.insert(it,vec_2d.front());
	vec_2d.push_back(vec_2d.back());

	vector<short>::iterator it_1d;

	// Things seem to go bad here.
	for (it=vec_2d.begin(); it<vec_2d.end(); it++)
	{
		for (it_1d=(*it).begin(); it_1d=(*it).end(); it_1d++)
		{
			cout<<" "<<*it_1d;
		}
	}
	return 0;
}

I have written the code to expand the vector by repeating the first and last rows but I do not know how to repeat the first and last columns. Another problem is, I do not know how to check if I am right by printing all the elements in the vector, the code I have written for this purpose seems to work badly. Anyone can tell me why and help me out? Thank you in advance.

nanchuangyeyu

[Narue]

You had an assignment in your inner loop condition rather than a comparison. I also added the code to insert the new columns as it's an identical operation to inserting the new rows:

#include <iostream>
#include <vector>

using namespace std;

int main()
{
    vector<short>vec;
    vector< vector<short> >vec_2d;
    vector< vector<short> >vec_2d_padded;
    // Initialize a 2D C++ vector.
    for (int i=1; i<17; i++)
    {
        vec.push_back(i);
        if (i%4 == 0)
        {
            vec_2d.push_back(vec);
            vec.clear();
        }
    }

    // Expand by repeating the first and last row.
    vector< vector<short> >::iterator it;
    it = vec_2d.begin();
    vec_2d.insert(it,vec_2d.front());
    vec_2d.push_back(vec_2d.back());

    // Things seem to go bad here.
    for (it=vec_2d.begin(); it<vec_2d.end(); it++)
    {
        vector<short>::iterator it_1d;

        // Expand by repeating the first and last column
        it->insert ( it->begin(), it->front() );
        it->push_back ( it->back() );

        for (it_1d=(*it).begin(); it_1d != (*it).end(); it_1d++)
        {
            cout<<" "<<*it_1d;
        }

        cout<<'\n';
    }
    return 0;
}