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What is the meaning of A (: ,: ,1) in matlab? where A is a matrix

Asked by bh dhouha on 16 Feb 2015
Latest activity Commented on by Walter Roberson
on 27 Jan 2016
What is the meaning of A (: ,: ,1) in matlab? where A is a matrix .thanks

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Answer by Image Analyst
on 16 Feb 2015
 Accepted Answer

If the badly-named "A" represents a color image, then A(:, :, 1) would mean all rows and all columns in the first image plane - in other words, the red channel of the image. The code mean(mean(A(:,:,1))) would mean the average intensity of the red channel:
meanRed = mean(mean(A(:,:,1)));
Stephen and I are guessing because the author just used a single letter of the variable rather than some descriptive name would would give us some context as to what "A" represents. I hate code that just looks like a random alphabet soup of single letter variables - it makes it very hard to understand, follow, and maintain. For example if the variable were named rgbImage, then we'd know what "A" represented. As of now, we don't. You didn't supply any context either so we're left to speculate (or just ignore the question which is probably what most people did).

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No need to be so rude bro
That response is not rude: it makes educated guesses about the real meaning of the question, and then it describes the difficulties with how the question was phrased, offering alternatives.
Any code that is intended to last should be written with the thought that someone else may have to read it, so variable names should be self-explanatory or follow conventions that shape the meaning (e.g., 'x' and 'y' might not really explain what the data is for, but the convention is well enough ingrained that it is easy to follow.)

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Answer by Stephen Cobeldick on 16 Feb 2015
Edited by Stephen Cobeldick on 16 Feb 2015

A(:,:,1) means: all rows and all columns of A that are in its first page.
(The third dimension is referred to in the MATLAB documentation as a "page", just as the first dimension is "row" and the second is "column").
In MATLAB all arrays can be multidimensional, and the contents can be referred to using indexing . In your example the variable A has three dimensions, and they are referred to in this way:
A(:, % all rows
:, % all columns
1) % on the first page
You should read about the colon operator too.
Although you write that A is a matrix, actually it might not be: a matrix (by definition) only has two dimensions (all higher dimensions are one), which means that if the third dimension has size two or more, then it will be an array, rather than a matrix. You might like to review matrix indexing as well.

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what do you mean by first page? So does this code mean(mean(A(:,:,1))) give the mean of all the matrix A? Thanks for your help :)
That depends on the size of A. The behavior of mean changes depending on the size of the array that is passed to it, so you will have to tell us size(A) before we can answer that question.
If you want the mean of all elements of an array, regardless of its size or number of dimensions, then you can use mean(A(:)).
Note that mean also has an optional argument that specifies which dimensions it calculates along. We can use this for example, if A has three dimensions, to get the mean of each page using the following code:
mean(mean(A,1),2)
Thanks a lot Stephen :)

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