MATLAB: Matrix logical indexing

MATLABmatrix logical indexing

I have a matrix A with dimensions nxm. I want to extract all values x that are between two values a and b, that is: (a<x<b). How can I do it in one step?

I know i can do it in two steps: A1 = A(A>a); A2 = A1(A1<b);

how do I combine the two logical statements?

Best Answer

A1=A(A>a & A<b)

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MATLAB: How to replace zeros with other value in a big matrix fast

Don't double call these functions (MAX,MIN,SUM,MEAN,etc.). Learn to use the colon, it is your friend!

A(~A(:))=mean(A(:));

This will be somewhat faster. If you need even more speed, replace the call to mean with its definition.

A(~A(:)) = sum(A(:))/numel(A);

The thing is, with a very large array, it might just take some time!

Here is how I timed these, BTW. You can play around with other options as well! Run the function 3 times (the first time warms it up after a save). The time will print to the command window.

function [] = write_ones()
T1 = 0;  % Time two different approches
T2 = 0;
for ii = 1:5
    A = round(rand(3320))>.75;
      tic
      A(A==0)=mean(mean(A));
      T1 = T1 + toc;
      clear A
      A = round(rand(3320))>.75;  % A new A.
      tic
      A(~A(:)) = sum(A(:))/numel(A);
      T2 = T2 + toc;
  end
[T1 T2]

MATLAB: Is SUM not multithreaded on a dual core machine in MATLAB 7.8 (R2009a)

SUM is memory bound. The issue directly relates to throughput on the CPU(s). The following results illustrate this:

1. Two cores share one BUS

>> x = rand(1000);
>> tic; mean(mean(x)); toc
Elapsed time is 0.002357 seconds.

2. Two cores with two BUSES

>> x = rand(1000);
>> tic; mean(mean(x)); toc
Elapsed time is 0.001802 seconds.

3. Using ‘-singleCompThread’:

>> x = rand(1000);
>> tic; mean(mean(x)); toc
Elapsed time is 0.002660 seconds.

Best Answer

Related Solutions

MATLAB: How to replace zeros with other value in a big matrix fast

MATLAB: Is SUM not multithreaded on a dual core machine in MATLAB 7.8 (R2009a)

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