Okay, I got it. Thanks, folks.
I had seen an example somewhere which, going by Mike's helpful bit of
info, must have been in column-major order - so I had things switched
around in my head. Actually, when I saw the example it didn't make
sense to me, but I just followed it along blindly! Anyway, this
straightens things out.
Much appreciated!
cheers,
J.
On 29-Mar-09, at 9:55 AM, Ken Thomases wrote:
On Mar 29, 2009, at 11:43 AM, James Maxwell wrote:
Sorry for the long post, but I'm trying to wrap my head around
NSData, NSMutableData and matrices.
So I made an example to test my (mis)conception of how this all
should work.
Here it is:
- (void) matrixTesting
{
int numRows = 5;
int numCols = 7;
NSMutableData *theMatrix = [NSMutableData dataWithLength:(numRows
* numCols * sizeof(float))];
[theMatrix retain];
float *matrix = [theMatrix mutableBytes];
// Set row 3, col 5 to 3.333
matrix[(3 * numRows) + 5] = 3.333;
You want to multiply by numCols, not numRows. You essentially want
to skip rows 0, 1, and 2. Each of those rows contains numCols
floats, not numRows floats.
// Set row 2, col 4 to 6.666
matrix[(2 * numRows) + 4] = 6.666;
Same here.
int i,j;
for(i=0;i < numRows;i++)
for(j=0;j < numCols;j++)
NSLog(@"Row %i, Col %i = %f", i, j, matrix[i * numRows
+ j]);
And here.
int getRow = 3;
NSRange rowRange = {
getRow * numRows, numCols * sizeof(float)
Here you get it half right, half wrong. The part you get right is
knowing that the size of one row is (numCols * sizeof(float)). Now
think: what's the size of the 3 preceding rows which you want to
jump past? It's (getRow * numCols * sizeof(float)), which is what
you should be using for the location of your range. That is, you
have two problems in the above line. You're multiplying getRow by
numRows instead of by numCols, and you're failing to multiply by
sizeof(float).
};
float theRow[numCols];
[theMatrix getBytes:theRow range:rowRange];
for(i=0;i < numCols;i++)
NSLog(@"\n Row test --------> Row 3, index %i = %f", i,
theRow[i]);
[theMatrix release];
}
Regards,
Ken
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