File Exchange Pick of the Week

September 11th, 2007

MATLAB Basics video: Sorting matrices

This video covers how to sort a matrix, either sorting each column independently or sorting such that the original rows stay together. Most new users are able to find the SORT command without much problem, but the very useful SORTROWS is often missed.

Find the files here.

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By Doug Hull

14:01 UTC | Posted in MATLAB Basics, Video | Permalink | 11 Comments »

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11 Responses to “MATLAB Basics video: Sorting matrices”

slaesche replied on October 17th, 2007 at 17:42 UTC :

Good tip! Just what I was looking for. Thanks.
mayank replied on July 2nd, 2008 at 21:51 UTC :

Hi,I want to sort the coordiantes.How to do this?
Eg- 1 0
3 2
21 0
4 4
2 3
5 4
so that the sorted coordinates come out to be:
1 0
21 0
3 2
2 3
4 4
5 4
Doug replied on July 3rd, 2008 at 08:00 UTC :

Mayank,

Please watch the video again. You will see Sortrows at the end. This is the function you are looking for.

Doug
Sanchay Subhedar replied on September 22nd, 2008 at 22:51 UTC :

I am getting this error on using simple sort function on the same magic square as your.

??? Index exceeds matrix dimensions.

How can I “sort” this problem ;-)
Doug replied on September 23rd, 2008 at 08:46 UTC :

Sanchay,

You need to show all the relevant code to get help.

Doug
Renato replied on April 21st, 2009 at 19:34 UTC :

Hi, is there any function that does the same as sortrows but for columns?

Thx for your time!
Doug replied on April 22nd, 2009 at 12:49 UTC :

Renato,

What you need to do is use the transpose operator (‘) to transpose the rows to columns. Then you can use sort rows and transpose right back.

Doug
Mark replied on July 15th, 2009 at 23:22 UTC :

Hello,

I need to sort the pixel intensity values of an image in descending order, but then also be able to keep track of each intensities original coordinate in the image. I assume I will need to create an Nx3 matrix where N is the total number of pixels and the 3 column values are: 1. the pixel intensity, 2. the original col, and 3. the row coordinate.

Could you help me to get started doing this?

Thanks,
Mark
Doug replied on July 16th, 2009 at 14:58 UTC :

@Mark

http://www.mathworks.com/access/helpdesk/help/techdoc/ref/sort.html

Use sort with the second output argument. That gives the absolute index, but you can use IND2SUB to get back to row and column.

-Doug
Fred replied on January 29th, 2010 at 05:18 UTC :

Doug,
I have a set of coordinates for the perimeter of an object in random order. I want to plot these using the statement
```
plot(X, Y, '-');
```
so that I get a continuous line plot of the object perimeter. This requires getting the x coordinates representing the top half of the object in increasing order, but the x coordinates for the bottom half of the object in decreasing order.

I have tried to get the coordinates sorted using dummy columns in a matrix using code such as below and using sortrows:
```
coords = [X Y X>mean(X) Y<mean(Y)];
coords = sortrows(coords, [4 1 3 2]);
plot(coords(:,1), coords(:,2), '-r');
```
This doesn’t quite work and I get a horizontal line across the middle of my object. I feel I am close using this approach, but I just can’t get it. Any suggestions?
Thanks.
Fred
Doug replied on February 1st, 2010 at 16:41 UTC :

From the convhull doc

http://www.mathworks.com/access/helpdesk/help/techdoc/ref/convhull.html
```
xx = -1:.05:1; yy = abs(sqrt(xx));
[x,y] = pol2cart(xx,yy);
k = convhull(x,y);
plot(x(k),y(k),'r-',x,y,'b+')
```
This should get you where you are going.