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# 2d vector help

Posted on 2006-11-14
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I'm hoping someone could help me with 2-d vectors.

Could someone tell me how to:

1.  perform a push_back on a 2d vector
2.  perform a resize on a 2d vector
3.  make a copy of a 2d vector

Thanks!
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Question by:cfans
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LVL 12

Assisted Solution

rajeev_devin earned 100 total points
ID: 17945020
For 1.

vector<vector<int> > matrix;

vector<int> v;
v.push_back(10);
v.push_back(20);
v.push_back(30);

matrix.push_back(v);
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LVL 9

Accepted Solution

ID: 17945026
1)
vector< vector<int> > matrix(max_index); // to set the size of the array

for(int i = 0; i<matrix.size(); ++i)
matrix[i].reserve(max_index);  // to set the size of each column .. Now the matrix is (max_index X max_index)

// set all indecies to 0
for(int x = 0 ;x<matrix.size(); ++x)
for(int y = 0 ; y<matrix[0].size(); ++y)
matrix[i][j] = 0;

OR:

vector<int> v(max_index);
vector< vector<int> vv(max_index);
for(int i = 0; i<v.size(); ++i)
v.push_back(0);

vv.push_back(v);

2)

matrix.resize(max_index+1, vector<int>(max_index)); // max_index+1 is the new size of the matrix

3)
I didn't do that before, but try this

vector< vector<int> > copyMatrix = matrix;

0

LVL 12

Expert Comment

ID: 17945030
2.
//for 10x20
vector<vector<int> > matrix;

matrix.resize(10);
for (int i = 0; i < 10; ++i)
matrix[i].resize(20);

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LVL 12

Expert Comment

ID: 17945035
0

LVL 39

Expert Comment

ID: 17946091
You can set both dimensions when creating the 2d array, e. g.

vector<vector<int> > matrix(10, vector<int>(20, 0) );  // 10 x 20 all zeros

>>>> 1.  perform a push_back on a 2d vector

push_back is an operation defined for a 1d array. So if you look at the first index as the ROW index, you would add a new row by push_back.

For a 2d array push_back isn't well defined cause normally a 2d array has the same column size for all rows. A push_back for a row would add one cell to that row, what might be strange. If you want to increase the number of columns for all rows you better use the resize function for any row:

vector<vector<int> >::iterator i;
for (i = matrix.begin(); i != matrix.end(); ++i)
i->resize(i->size()+1, 0);  // increment and set to 0

Regards, Alex

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