How to make a 3D Matrix

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Nicholas Deosaran
编辑： Jon 2020-9-14
How to make a 3D Matrix that contains all ones in the first layer, all twos in the second layer and all threes in the third layer?
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Abdolkarim Mohammadi
Abdolkarim Mohammadi 2020-9-3
Size = [5,6,3];
M = ones(Size) .* permute([1;2;3],[3,2,1]);

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Jon
Jon 2020-9-3
编辑：Jon 2020-9-3
You can also do it like this, for example for a 2 by 4 by 3
A = ones(2,4,3).*reshape(1:3,1,1,3)
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Abdolkarim Mohammadi
Abdolkarim Mohammadi 2020-9-11
A simple way to reach the matrix you wanted is to create a m-by-n-by-k array of ones and then multiply it by a 1-by-1-by-k array of 1 to k. The trick is to first create a 1-by-k array and then permute it to ge the so-called 1-by-1-by-k array. You can permute it in different ways, including the permute() function and reshape().
Jon
Jon 2020-9-14
编辑：Jon 2020-9-14
You asked about what the reshape function does.
The reshape(1:3,1,1,3) The first argument 1:3 makes a 3 element row vector with elements [1, 2, 3]the second, third and fourth arguments specify that we want to turn this row vector into a 1 row, by 1 column, by 3 "page" array. So 1 goes in the 1,1,1 location of the first page, 2 goes into th 1,1,2 location on the second page and 3 goes into the 1,1,3 location on the third page. By shaping it this way the array dimension for the multiplication
ones(2,4,3).*reshape(1:3,1,1,3)
become "compatible". Please see https://www.mathworks.com/help/matlab/matlab_prog/compatible-array-sizes-for-basic-operations.html#:~:text=In%20the%20simplest%20cases%2C%20two,element%2Dwise%20operation%20or%20function.
Also type doc reshape on the command line to get documentation on the reshape function for further details

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