A is a matrix , what does this statement A([1,end],[1,end]) mean ?

Question

Muhammed Thameem 2020-6-30

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https://ww2.mathworks.cn/matlabcentral/answers/557002-a-is-a-matrix-what-does-this-statement-a-1-end-1-end-mean

评论： DGM 2022-8-25

采纳的回答： madhan ravi

The result of this command gives the 4 corners of the matrix, but can't figure out how that command works

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Answer 1

madhan ravi 2020-6-30

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That's equivalent to:

[A(1,1), A(1,end);...
 A(1,end), A(end,end)]  

https://www.mathworks.com/company/newsletters/articles/matrix-indexing-in-matlab.html - read above linear indexing.

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Ceethal Kottakali Piyus 2022-1-20

Hey did you mean its equivalent to:

[A(1,1), A(1,end);...

A(end,1), A(end,end)]

or

[A(1,1), A(1,end);...

A(1,end), A(end,end)] because the second option doesn't giving the 4 corners of the matrix

DGM 2022-8-25

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@Ceethal Kottakali Piyus, you are correct.

A = magic(5) % an example matrix
A = 5×5
    17    24     1     8    15
    23     5     7    14    16
     4     6    13    20    22
    10    12    19    21     3
    11    18    25     2     9
A([1,end],[1,end])
ans = 2×2
    17    15
    11     9
[A(1,1), A(1,end); A(end,1), A(end,end)]   
ans = 2×2
    17    15
    11     9

@RAVIKIRAN YALAMARTHI's answer may also serve as demonstration.

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Answer 2

RAVIKIRAN YALAMARTHI 2020-6-30

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Simple example:

A = [1 2 3;4 5 6;7 8 9]
A([1,end],[1,end])

ans = 2 by 2

1 3

7 9

To call the elements in a matrix, we have to mention the row and column index values.

So, A(1,2) = 2. Since, 1st row and 2nd column element is 2.

similarly, A([1,end],[1,end]) will call the elements of,

1st row & 1st column: A(1,1)

last row & 1st column: A(end,1)

1st row & last column: A(1,end)

last row & last column: A(end,end)

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Gianpiero 2022-8-25

what does it mean (1:end, 1)?

DGM 2022-8-25

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That set of subscripts will address the first column of a matrix.

A = magic(5) % an example matrix
A = 5×5
    17    24     1     8    15
    23     5     7    14    16
     4     6    13    20    22
    10    12    19    21     3
    11    18    25     2     9
A(1:end,1) % this is the same
ans = 5×1
    17
    23
     4
    10
    11
A(:,1) % as this
ans = 5×1
    17
    23
     4
    10
    11