How to get rows and columns if arrays is only 1D using for loops

Question

I was used to Matlab's feature where you can make a matrix and get A[i][j] and things like that. Now I am using Java and we can only use one dimensional array. I am suppose to modify the entries (i:for rows and j:for columns) using a nested for loop but I am not sure how to access them if they are stored in an 1D array. Can someone please help me out? How difficult is it?

Yes only double[] array; and such no double[][] array unfortunately. — Amani Lama, Mar 30 '13 at 19:03
Is this a homework problem, or is there some other reason why you cannot use a 2D array? — Ben Hocking, Mar 30 '13 at 19:04
Yes it is hmw problem, the problem is to crop an image manually with java but I do not even know how to access i and j from 1 D array... — Amani Lama, Mar 30 '13 at 19:05

score 7 · Accepted Answer · answered Mar 30 '13 at 19:06

7

 int rows = 3;
 int cols = 4;
 int[] array = new int[rows*cols];
 int[] currentRow = new int[cols];
 for (int i = 0; i < rows; ++i) {
     for (int j = 0; j < cols; ++j) {
         currentRow[j] = array[i*cols + j];
     }
 }

answered Mar 30 '13 at 19:06

Rob G

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It worked great, thank you and everyone else. Could someone please tell me what my question got a -1? I would like to avoid such things, so I would like to know what I did wrong.... – Amani Lama Apr 01 '13 at 14:34
this logic made my day 'array[i*cols + j]'. Previously, I need to use a variable to store the 'columnCount++' and a 'if-else' to check columnCount is less than the input array length. Thanks @RobG – sky91 Dec 20 '15 at 14:19

score 5 · Answer 2 · answered Mar 30 '13 at 19:15

5

private int getElem(int[] arr, int i, int j){
  return arr[i*colNum+j];
}

answered Mar 30 '13 at 19:15

gkiko

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score 1 · Answer 3 · edited May 23 '17 at 12:00

1

Note that in Java, you can also use a 2D array like so:

int[][] my2DArr = new int[4][3]; //creates a 2D array with 4 rows and 3 columns
int value = my2DArr[2][1]; //gets the value at row 2 and column 1

Now, if you have to use a 1D array which is representing a 2D array, you can do some simple math to find out the position of a given row, column pair if you know the number of columns and number of rows. See here: Convert a 2D array into a 1D array

edited May 23 '17 at 12:00

Community

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answered Mar 30 '13 at 19:06

CodeGuy

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It is worth noting that this is less efficient than using a 1D array. – timelmer Apr 30 '18 at 23:28

score 1 · Answer 4 · answered Mar 30 '13 at 19:10

I hope I'm understanding this correctly.

Say you have a 10x10 2D array and you want it to be one dimensional.

You can make array[0] to array[9] the first row of the 2D array. Then array[10] to array[19] is the second row of the 2D array.

There is probably a more efficient way of doing this.

score 1 · Answer 5 · answered Mar 30 '13 at 19:38

All arrays the computer keeps track of are 1D because of sequential memory locations in which they are stored (C Programming: A Modern Approach - K.N.King). So if you want to use a 1D array like a 2D array, simply do this: (example using String array)

String str[] = {"a", "b", "c", "d", "e", "f"};

which is like:

String str[][] = {{"a", "b", "c"}, {"d", "e", "f"}};

So you can specify how many rows and columns there are manually so in the above str[][] there are 2 rows and 3 columns or any other permutation of the numbers (1 and 6, 3 and 2, 6 and 1). Simply write your code like this:

int rows = 2;
int cols = (str.length / rows);
for (int i = 0; i < rows*cols; i+=cols)
    for (int j = 0; j < cols; j++)
        System.out.println(str[(i + j)]);

and replace the println() with whatever code you want and i+j will always be the row and column location as long as your rows variable is how many rows you are working with.

How to get rows and columns if arrays is only 1D using for loops

5 Answers5