Comments on: Reordering Arrays http://blogs.mathworks.com/loren/2007/02/16/reordering-arrays/ Loren Shure works on design of the MATLAB language at MathWorks. She writes here about once a week on MATLAB programming and related topics. Thu, 09 Feb 2012 04:19:21 +0000 hourly 1 http://wordpress.org/?v=3.2.1 By: Navaneethan Santhanam http://blogs.mathworks.com/loren/2007/02/16/reordering-arrays/#comment-29606 Navaneethan Santhanam Mon, 21 Jul 2008 15:35:58 +0000 http://blogs.mathworks.com/loren/?p=77#comment-29606 Thanks a lot, Loren! That worked perfectly. Thanks a lot, Loren! That worked perfectly.

]]> By: Loren http://blogs.mathworks.com/loren/2007/02/16/reordering-arrays/#comment-29595 Loren Wed, 16 Jul 2008 20:16:48 +0000 http://blogs.mathworks.com/loren/?p=77#comment-29595 Use the values as indices into A. <pre><code> A = [1.2 3.8 5.7 90 63.7 88.65] n = randperm(length(A)) ind = n(1:3) B=A(ind) </code></pre> --Loren Use the values as indices into A.


A = [1.2 3.8 5.7 90 63.7 88.65]
n = randperm(length(A))
ind = n(1:3)
B=A(ind)

–Loren

]]> By: Navaneethan Santhanam http://blogs.mathworks.com/loren/2007/02/16/reordering-arrays/#comment-29594 Navaneethan Santhanam Wed, 16 Jul 2008 20:14:07 +0000 http://blogs.mathworks.com/loren/?p=77#comment-29594 Loren, Thank you for the prompt response! I tried your idea out. However, randperm works only on integers, and consequently all values in the vector n are approximated to the nearest integer. Any other thoughts? Navaneethan Loren,

Thank you for the prompt response! I tried your idea out. However, randperm works only on integers, and consequently all values in the vector n are approximated to the nearest integer. Any other thoughts?

Navaneethan

]]> By: Loren http://blogs.mathworks.com/loren/2007/02/16/reordering-arrays/#comment-29593 Loren Wed, 16 Jul 2008 19:48:51 +0000 http://blogs.mathworks.com/loren/?p=77#comment-29593 Navaneethan- Take a look at the function randperm. I think it will do what you need. Something like this: <pre><code> A = [1.2 3.8 5.7 90 63.7 88.65] n = randperm(length(A)) ind = n(1:3) </code> </pre> and repeat the last 2 lines as often as you need. --Loren Navaneethan-

Take a look at the function randperm. I think it will do what you need. Something like this:


A = [1.2 3.8 5.7 90 63.7 88.65]
n = randperm(length(A))
ind = n(1:3)

and repeat the last 2 lines as often as you need.

–Loren

]]> By: Navaneethan Santhanam http://blogs.mathworks.com/loren/2007/02/16/reordering-arrays/#comment-29592 Navaneethan Santhanam Wed, 16 Jul 2008 19:38:33 +0000 http://blogs.mathworks.com/loren/?p=77#comment-29592 Loren, I'm trying to run a simulation in which a vector needs to be reordered randomly during each iteration of the simulation. I've looked up MATLAB's help documentation, and can't seem to find anything. To give you an example: A = [1.2 3.8 5.7 90 63.7 88.65] and I need to pick 3 numbers from the array randomly without replacement. I'd like to randomly reorder the array and pick the first 3 each time. Is there a built-in function to accomplish the randomising part? Also, is this the most efficient way to do it? Navaneethan Loren,

I’m trying to run a simulation in which a vector needs to be reordered randomly during each iteration of the simulation. I’ve looked up MATLAB’s help documentation, and can’t seem to find anything.

To give you an example:
A = [1.2 3.8 5.7 90 63.7 88.65] and I need to pick 3 numbers from the array randomly without replacement. I’d like to randomly reorder the array and pick the first 3 each time. Is there a built-in function to accomplish the randomising part? Also, is this the most efficient way to do it?

Navaneethan

]]> By: Loren http://blogs.mathworks.com/loren/2007/02/16/reordering-arrays/#comment-16073 Loren Tue, 20 Feb 2007 16:57:34 +0000 http://blogs.mathworks.com/loren/?p=77#comment-16073 Dan- The flipping will work if the data came from nchoosek. It won't always work. I'm just trying to point people to various tools. --Loren Dan-

The flipping will work if the data came from nchoosek. It won’t always work. I’m just trying to point people to various tools.

–Loren

]]> By: Dan K http://blogs.mathworks.com/loren/2007/02/16/reordering-arrays/#comment-16072 Dan K Tue, 20 Feb 2007 16:36:39 +0000 http://blogs.mathworks.com/loren/?p=77#comment-16072 Loren, To make sure I am understanding. This analysis will then only work in the specific case where the rows are already in order, just backwards. These approaches don't act to order the data in terms of lowest high value first, that was already done (albeit backwards) by nchoosek. Is this correct? Dan Loren,
To make sure I am understanding. This analysis will then only work in the specific case where the rows are already in order, just backwards. These approaches don’t act to order the data in terms of lowest high value first, that was already done (albeit backwards) by nchoosek. Is this correct?
Dan

]]> By: Loren http://blogs.mathworks.com/loren/2007/02/16/reordering-arrays/#comment-16071 Loren Tue, 20 Feb 2007 16:11:47 +0000 http://blogs.mathworks.com/loren/?p=77#comment-16071 Dan- Flipping IS reordering. It's not deeper than that. --Loren Dan-

Flipping IS reordering. It’s not deeper than that.

–Loren

]]> By: Dan K http://blogs.mathworks.com/loren/2007/02/16/reordering-arrays/#comment-16070 Dan K Tue, 20 Feb 2007 15:57:05 +0000 http://blogs.mathworks.com/loren/?p=77#comment-16070 Hi Loren, I'm mightily confused as to how using rot90 is sorting the array so that the cards with the lowest high value are coming up first. Doesn't rot90(X,2) just flip the array by 180 degrees, preserving the order of the rows? Likewise the flip-flip method seems to do the same thing. Where is the re-ordering occurring? -Dan Hi Loren,
I’m mightily confused as to how using rot90 is sorting the array so that the cards with the lowest high value are coming up first. Doesn’t rot90(X,2) just flip the array by 180 degrees, preserving the order of the rows? Likewise the flip-flip method seems to do the same thing. Where is the re-ordering occurring?
-Dan

]]> By: Loren http://blogs.mathworks.com/loren/2007/02/16/reordering-arrays/#comment-16065 Loren Mon, 19 Feb 2007 20:25:25 +0000 http://blogs.mathworks.com/loren/?p=77#comment-16065 Steve- Good catch and sorry about that. I have updated the blog to show the right version now. --Loren Steve-

Good catch and sorry about that. I have updated the blog to show the right version now.

–Loren

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