copying column of a 2D array as 1D array in java?

Question

Let's say I have a 2D array:

int[][] a = new int[4][3];

populated such that:

Is there any shortcut method in java to extract lets say column 1 as single array:

array1 = {1 4 7 2};

Currently what I am doing is traversing through the whole 2D matrix and with if condition (if j==0), I traverse over the rows and add values to 1D array.
Just wondering if there is any standard method offered in java for such tasks.

Why are you traversing the *whole* matrix? Why not simply: `for (i = 0; i < 4; i++) { b[i] = a[i][0]; }`? — Oliver Charlesworth, Jul 10 '11 at 17:27
that is because i need to extract all the columns as separate arrays. and problem is the matrix is so big, so i want to know if there is some simpler method available. — Johnydep, Jul 10 '11 at 17:29
Then please update your question to explain what you're *really* trying to do... — Oliver Charlesworth, Jul 10 '11 at 17:31

score 2 · Accepted Answer · answered Jul 10 '11 at 17:35

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No there is no shortcut to doing this. You have to loop over the arrays, switching the x & y indices.

answered Jul 10 '11 at 17:35

Lawrence Dol

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Of course, if it's a recurring pattern in your code, you should write a helper method to do it for you. Or consider whether the fact that you're continually transposing matrices means that your design is broken! – Oliver Charlesworth Jul 10 '11 at 17:36
actually i have to calculate cosineSimilarity which requires traversing over the transposed columns and it is recursive calculation over a huge matrix of the size [2500]x[3000]. The only idea i can think of is to integrated the calculation inside the matrix for now. – Johnydep Jul 10 '11 at 17:48

score 0 · Answer 2 · answered Jul 10 '11 at 17:33

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There is no such build-in method. You have to write a simple loop.

answered Jul 10 '11 at 17:33

zacheusz

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score 0 · Answer 3 · edited May 23 '17 at 10:34

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You might want to consider use of a matrix library. But this is pretty simple stuff - if this is all you need, you can probably write it quicker than you can get up to speed on a library.

edited May 23 '17 at 10:34

Community

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answered Jul 10 '11 at 17:58

Ed Staub

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copying column of a 2D array as 1D array in java?

3 Answers3