# How to rearrange the rows of a matrix so the column values loop

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Joshua on 19 Jun 2013
I have a matrix
rec =
[ 0 0
0 0.4
0.2 0
1.0 0.4
1.0 0.9 ]
I want it rearranged so that it appears like so
rec_new =
[ 0.2 0
0 0
0 0.4
1.0 0.9
1.0 0.4 ]
If you take any one of these columns and concatenate it onto itself, the values loop from a minimum to a maximum. How can I get a program to automatically do this for a number of matrices? I'm not sure how to start this, so any help would be appreciated.
Ultimately, I need the pairs of points rearranged so that when they are graphed they form a polygon that does not intersect itself.
Thanks
##### 2 CommentsShowHide 1 older comment
Joshua on 20 Jun 2013
Leah and Angus, I'd like to thank you both for your assistance. I appreciate it greatly.

Angus on 19 Jun 2013
Edited: Angus on 19 Jun 2013
Hey that was interesting, I think this is what you want (not sure under what conditions it might fail):
rec = [0 0;0 0.4;0.2 0;1 0.4;1 0.9]
K = convhull(rec);
rec(K,:)
ans =
0 0
0.2000 0
1.0000 0.4000
1.0000 0.9000
0 0.4000
0 0
Came across this via Gift wrapping algorithm and then found convhull from there if you are interested.
It basically draws a line around the outside of the points ... kinda or something :D
Hope this does it for you,
Cheers, Angus.
Joshua on 24 Jun 2013
I do want to rearrange the structure of the matrix. I need to be able to do some calculations with them for the area and perimeter of the polygon the points create

Leah on 19 Jun 2013
"If you only want them to be counter-clock wise, why not convert the [x,y] point to polar coordinate and sort them by angle?"
##### 2 CommentsShowHide 1 older comment
Leah on 20 Jun 2013
I don't really know that this makes sense in 3D. If you had a mesh making up a sphere what order would you want them to go in? Clockwise direction doesn't mean anything in 3D space. I think you need to rethink your desired outcome in 3D space.

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