What is a(:,2:3)?

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Bella
Bella 2016-5-17
评论： Kelvin 2023-2-7
a = [ 1 2 3 4 5; 2 3 4 5 6; 3 4 5 6 7; 4 5 6 7 8]
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Bella
Bella 2016-5-17
what?
Kelvin
Kelvin 2023-2-7
1 2 3 4 5 2 3 4 5 6 3 4 5 6 7 4 5 6 7 7 It’s a matrix

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Todd Leonhardt
Todd Leonhardt 2016-5-17
编辑：Todd Leonhardt 2016-5-17
a(:,2:3) is saying "give me the submatrix of a which consists of all rows and the 2nd and 3rd columns".
If you don't have MATLAB you can get a free evaluation, at least for academic purposes. You can also do many of the things MATLAB can do using open-source Octave (though there are significant limitations). Get one of those and then try to do this stuff yourself so you actually learn something ;-)
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Bella
Bella 2016-5-17
hmm, well thank you!
Image Analyst
Image Analyst 2016-5-17
What do you mean "it said it was wrong"? +*What*_ said that? MATLAB? If so, post the error message here. If you got that you must have left off the top row and done this:
a = [
2 3 4 5 6;
3 4 5 6 7;
4 5 6 7 8]
a(:,2:3)
However there is nothing in MATLAB that says anything is wrong about that - no error message or anything.

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Greg Biyu
Greg Biyu 2019-3-12
what is (:,:,3)?
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Walter Roberson
Walter Roberson 2021-3-10
@Anu Sebastian
You were dealing with
he = imread('1.jpg');
which reads the image file and stores the result as an array.
If 1.jpg had been a color image, then the resulting array he would have been a 3D array, with the first index being for rows (vertical height), and the second index being for columns (horizontal distance), and for the third index being for color panes. The first color pane for an RGB image holds the Red component. The second color pane for an RGB image holds the Green component. The third color pane for an RGB image holds the Blue component.
Thus, if 1.jpg had been an RGB image, then he(:,:,2) would be a 2D array containing just the green component of the color information for each pixel.
However, your 1.jpg was not an RGB image; it was a grayscale image, and there is no red or green or blue component. If you needed specifically the green information (for example you were examining leaves) then you would not be able to proceed using that image. If, though, the green component was being used to approximate brightness (eyes are more sensitive to green), then you could just use the data you received instead. For example,
if ndims(he) > 2
G = he(:,:,2);
else
G = he;
end
end
Anu Sebastian
Anu Sebastian 2021-3-10
Thank you sir

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Maria Celeste
Maria Celeste 2022-11-2
What are the answers to these? >> B(2:5) 20. >> A(4:8) 21. >> A(:,3) 22. >> B(:,3:4) 23. >> A(2:3,:) 24. >> B(2:3, 2:4) 25. >> length (A)
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Walter Roberson
Walter Roberson 2022-11-2
B(2:5) is 'TAIF'
A(4:8) is [81 84 83 72 76]
A(:,3) is [84; 83]
B(:,3:4) is ['II'; 'XN'; 'YX']
A(2:3,:) is an error because A does not have 3 or more rows.
B(2:3, 2:4) is ['FXN'; 'KYX']

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