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

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bh dhouha
on 16 Feb 2015

Edited: Alexander Vassilev
on 27 Oct 2022

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

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### Accepted Answer

Image Analyst
on 16 Feb 2015

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).

##### 3 Comments

Alexander Vassilev
on 27 Oct 2022

Edited: Alexander Vassilev
on 27 Oct 2022

### More Answers (1)

Stephen23
on 16 Feb 2015

Edited: Stephen23
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

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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