I've recently been working on a number of small test programs for the different sections which make up a game I'm planning on writing. One of these test systems involved a series of polyominoes which I needed to rotate. Internally, the data for these shapes are stored as a simple boolean array, which I access as though it were two dimensions.
One of the requirements was that the player needs to be able to rotate these shapes at 90° intervals, and so there were two ways I could have solved this
- Define pre-rotated versions of all shapes
- Rotate the shapes on the fly
Clearly, I went with option two otherwise there would be no need for this article! I choose not to go with the pre-rotated approach, as firstly I'm using a lot of shapes and creating up to 4 versions of each of these is not really worthwhile, and secondly I don't want to store them either, or have to care which orientation is currently in use.
This article describes how to rotate a 2D array in fixed 90° intervals, and also how to rotate 1D arrays that masquerade as 2D arrays.
Note: The code in this article will only work with rectangle arrays. I don't usually use jagged arrays, so this code has no special provisions to work with them.
Creating a simple sample
First up, we need an array to rotate. For the purposes of our demo, we'll use the following array - note that the width and the height of the array don't match.
We can visualize the contents of the array but dumping it in a friendly fashion to the console
All of which provides the following stunning output
Rotating the array clockwise
This function will rotate an array 90° clockwise
How does it work? First we get the width and height of the array
GetUpperBound method of the
Array class. As arrays
are zero based, we add
1 to each of these results, otherwise
the new array will be too small to hold the data.
Next, we create a new array - with the width and height ready previously swapped, allowing us to correctly handle non-square arrays.
Finally, we loop through each row and each column. For each entry, we calculate the new row and column, then assign the value from the source array to the transposed location in the destination array
- To calculate the new row, we simply set the row to the existing column value
- To calculate the new column, we take the current row, add one to it, then subtract that value from the original array's height
If we now call
RotateArrayClockwise using our source array,
we'll get the following output
Rotating the array anti-clockwise
Rotating the array anti-clockwise (or counter clockwise depending on your terminology) uses most of the same code as previous, but the calculation for the new row and column is slightly different
- To calculate the new row we take the current column, add one to it, then subtract that value from the original array's width
- The new column is the current row
Using our trusty source array, this is what we get
Rotating 1D arrays
Rotating a 1D array follows the same principles outlined above, with the following differences
- As the array has only a single dimension, you cannot get the width and the height automatically - you must know these in advance
- When calculating the new index position using row-major
order remember that as the width and the height have been
swapped, the calculation will be something similar to
newIndex = newRow * height + newCol
The following functions show how I rotate a 1D boolean array.
- 2015-12-24 - First published
- 2020-11-21 - Updated formatting
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