Programming Forums > Script Programming > Python

Sane · Jul 26th, 2006, 9:34 AM

Yeah, Andro, that is quite silly. Matrix math should only be used if you're doing transformations of non 90 degree intervals, or reverse transformations. Since the former is impossible to have in a nested list, it's best to stick with inverse functions/linear transformations.

andro · Jul 26th, 2006, 1:03 PM

I may not have understood correctly, but..

Rotating about the X axis:

[1, 0, 0]
[0, cos x, -sin x]
[0, sin x, cos x]

Rotating about the Y axis:

[cos x, 0, sin x]
[0, 1, 0]
[-sin x, 0, cos x]

Rotating about Z:

[cos x, -sin x, 0]
[sin x, cos x, 0]
[0, 0, 1]

Although that may or may not do what you want, I haven't touched that stuff in a while now

EDIT: You know I think this applies to transforming actual objects in a 3D coordinate space... not physically rotating a 3x3 matrix in place like you are describing... never mind all of this.

Sane · Jul 26th, 2006, 2:22 PM

Your edit said it all.

hydroxide · Jul 27th, 2006, 10:03 AM

Quote:

Originally Posted by DaWei

What would be your Pythonic solution for rotating a 3x3 grid counterclockwise by 90 degrees?

If you will really be dealing with larger grids (ie: 3*3 is just to get sample code) then you might find NumPy, etc worthwhile - there's a rot90() function which does just that.

If you _are_ using 3*3, you feel numpy is overkill, you need to do this frequently, you need the speed, and you don't want to use one of the reallocating solutions you could just roll your own:

(Toggle Plain Text)

# Modifies a 3*3 list of lists inplace.  No, it's not incredibly pretty.
def rot90(grid):
    r1 = grid[0]
    r2 = grid[1]
    r3 = grid[2]

    (r1[0], r1[1], r1[2], 
     r2[0],        r2[2],
     r3[0], r3[1], r3[2]) =\

        (r3[0], r2[0], r1[0], 
         r3[1],        r1[1],
         r3[2], r2[2], r1[2])

-T.

stevengs · Jul 27th, 2006, 5:44 PM

Quote:

What would be your Pythonic solution for rotating a 3x3 grid counterclockwise by 90 degrees?...

...Why? Example?

hehe, I can't believe that noone has made any mention of "posting your code and showing that you made an effort"

-- I reckon I better duck ... incoming --

Sane · Jul 27th, 2006, 5:48 PM

I had assumed in DaWei's case it wasn't a question of inability, but a question of curiousity for the sake of seeing the alternatives. That, or he just wanted another tick on his little side counter there.

stevengs · Jul 27th, 2006, 5:57 PM

Naaa, it's probably a homework assignment, and we all know the rules about homework assignments..

Sane · Jul 27th, 2006, 6:04 PM

Hahahahaha. That's excellent. It must definitely be homework.

DaWei · Jul 28th, 2006, 8:58 AM

Yeah, it's homework. I got an F. Except from Grumpy, who gave me an upside down F, and Narue, who gave me two Fs. I got pissed and moved to Germany, next town over from Steven, Bad Thang.

stevengs · Jul 28th, 2006, 9:57 AM

Quote:

Originally Posted by DaWei

...an upside down F...

genial!

Quote:

Originally Posted by DaWei

... Bad Thang.

Well, the beer is good, but since you are a Glenlivet advocat, that wouldn't much make up for the 7$/Gallon at the pumps!

Jul 27th, 2006, 5:57 PM	#17
stevengs Professional Programmer Join Date: May 2005 Location: Bad Nauheim, Germany Posts: 436 Rep Power: 4	Naaa, it's probably a homework assignment, and we all know the rules about homework assignments.. __________________ -Steven "Is this a piece of your brain?" - Basil Fawlty

Jul 28th, 2006, 8:58 AM	#19
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Yeah, it's homework. I got an F. Except from Grumpy, who gave me an upside down F, and Narue, who gave me two Fs. I got pissed and moved to Germany, next town over from Steven, Bad Thang. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

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Jul 26th, 2006, 9:34 AM	#11
Sane Programming Guru Join Date: Apr 2005 Location: Waterloo, Ontario Posts: 2,028 Rep Power: 6	Yeah, Andro, that is quite silly. Matrix math should only be used if you're doing transformations of non 90 degree intervals, or reverse transformations. Since the former is impossible to have in a nested list, it's best to stick with inverse functions/linear transformations.

Jul 26th, 2006, 1:03 PM	#12
andro Professional Programmer Join Date: Oct 2005 Location: California Posts: 316 Rep Power: 4	I may not have understood correctly, but.. Rotating about the X axis: [1, 0, 0] [0, cos x, -sin x] [0, sin x, cos x] Rotating about the Y axis: [cos x, 0, sin x] [0, 1, 0] [-sin x, 0, cos x] Rotating about Z: [cos x, -sin x, 0] [sin x, cos x, 0] [0, 0, 1] Although that may or may not do what you want, I haven't touched that stuff in a while now EDIT: You know I think this applies to transforming actual objects in a 3D coordinate space... not physically rotating a 3x3 matrix in place like you are describing... never mind all of this.

Jul 26th, 2006, 2:22 PM	#13
Sane Programming Guru Join Date: Apr 2005 Location: Waterloo, Ontario Posts: 2,028 Rep Power: 6	Your edit said it all.

Jul 27th, 2006, 5:48 PM	#16
Sane Programming Guru Join Date: Apr 2005 Location: Waterloo, Ontario Posts: 2,028 Rep Power: 6	I had assumed in DaWei's case it wasn't a question of inability, but a question of curiousity for the sake of seeing the alternatives. That, or he just wanted another tick on his little side counter there.

Jul 27th, 2006, 6:04 PM	#18
Sane Programming Guru Join Date: Apr 2005 Location: Waterloo, Ontario Posts: 2,028 Rep Power: 6	Hahahahaha. That's excellent. It must definitely be homework.

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