How do I sample a matrix in MATLAB?

Question

I have a matrix in MATLAB from which I want to sample every other entry:

a =

     1     5     9    13
     2     6    10    14
     3     7    11    15
     4     8    12    16

And I want:

How can I do this without a for loop?

score 12 · Accepted Answer · answered Nov 24 '09 at 07:03

12

I don't know of a multi-dimensional way to do it automatically, but Matlab's indexing is good enough if you're happy to specify it for each dimension:

a(1:2:end,1:2:end)

answered Nov 24 '09 at 07:03

Will Robertson

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+1: Somehow I forgot to include the `end` syntax in my answer. Good catch. ;) – gnovice Nov 24 '09 at 19:15

gnovice · Answer 2 · 2009-11-24T06:32:22.563

5

This should work for your specific example:

result = a([1 3],[1 3]);

and more generally:

result = a(1:2:size(a,1),1:2:size(a,2));

For more details about indexing in MATLAB, you can check out the documentation here.

edited Nov 24 '09 at 06:32

answered Nov 24 '09 at 06:21

gnovice

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score 2 · Answer 3 · edited Mar 16 '16 at 22:55

2

I found this today. A is the original matrix, sampled every s element.

Adown=downsample(downsample(A,s)',s)'

It down-samples the matrix vertically, transposes, then samples the orthogonal direction, then transposes back.

edited Mar 16 '16 at 22:55

answered Mar 16 '16 at 22:34

Mike

21
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score 1 · Answer 4 · answered Oct 28 '11 at 17:41

1

samples_x = floor(linspace(1, size(a,1), new_Nx));
samples_y = floor(linspace(1, size(a,2), new_Ny));
new_a = a(samples_x,samples_y)

answered Oct 28 '11 at 17:41

rocky

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+1 and welcome to Stack Overflow! Your answer could be even more useful if you explained in a few words what your code does. Feel free to use the edit link below your question... – Jonas Heidelberg Nov 02 '11 at 21:25

How do I sample a matrix in MATLAB?

4 Answers4