Manipulating array of arrays...

I am trying to work with this a two dimensional array, that represents greyscale of an image... I want to pixelate the image by averaging 2x2 blocks and replacing each value by the average of the 2x2 grid... for example, if the input is the array

Code :

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  2,  4, 31, 31   
   3,  3, 21, 41
   1,  2, 10, 20
   3,  2, 20, 30

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then you start by looking at the first 2x2 sub-array (from the top-left)

Code :

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   2,  4
   3,  3

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and find the average of this sub-array (which is (2+4+3+3)/4 = 3). In the returned array, the first 2x2 sub-array will then be

Code :

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   3,  3
   3,  3

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We then look at the next 2x2 block

Code :

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   31,  31
   21,  41

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with average 31, The next 2x2 block in the output will be

Code :

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   31,  31
   31,  31

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I am wondering what approach would best solve this problem. Not too sure how to solve it... Thanks in advance

Try nested loops and walk through the dimensions by 2 vs by 1

but how do i make a loop to go through 2x2 portions of my array of array?

Quote:

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Originally Posted by choloboy

... what approach would best...

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I wouldn't worry about finding the "best" approach. Just think about an approach.

For example:

Code :

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Suppose you have a two-dimension array of ints named pixelArray.
 
Declare an int named numRows and set it equal to pixelArray.length;
Declare an int named numCols and set it equal to pixelArray[0].length;
 
First of all make sure that numRows and numCols are both even numbers.
(Use numRows%2 and numCols%2 to determine that.)

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Then make nested loops that go through the rows and columns of pixelArray two at a time:

Code :

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    for (int i = 0; i < numRows; /* What goes here to make it step through the rows two at a time? */) {
        for (int j = 0; j < numCols; /* What goes here to make it step through the columns two at a time? */) {
            //
            // Do the Good Stuff here
            //
        }
    }

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Now, what is the Good Stuff?

Code :

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Declare an int variable named sum, or some such thing.
 
In the innermost loop, calculate it by adding the values of the
elements of the 2x2 sub array:
   pixelArray[i][j] +
   pixelArray[i][j+1] +
   pixelArray[i+1][j] +
   pixelArray[i+1][j+1]
 
Declare an int variable named avg or some such thing
Set the value of avg equal to Math.round(sum/4.0f) to get the
int value closest to the average value of the four pixels.
 
Copy this average value back into the pixelArray elements
that were used to calculate the sum.

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Cheers!


Z