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
– Oliver Charlesworth, Commented Jul 10, 2011 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
– Johnydep, Commented Jul 10, 2011 at 17:29
Then please update your question to explain what you're really trying to do... — Oliver Charlesworth
– Oliver Charlesworth, Commented Jul 10, 2011 at 17:31

Cornelius Dol · Accepted Answer · 2011-07-10 17:35:31Z

2

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

answered Jul 10, 2011 at 17:35

Cornelius Dol

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2 Comments

Oliver Charlesworth Over a year ago

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!

Johnydep Over a year ago

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.

zacheusz · Accepted Answer · 2011-07-10 17:33:25Z

0

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

answered Jul 10, 2011 at 17:33

zacheusz

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Comments

Community · Accepted Answer · 2017-05-23 10:34:20Z

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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, 2017 at 10:34

CommunityBot

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answered Jul 10, 2011 at 17:58

Ed Staub

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