3 dimension array?

Hi, How to write 3 or 4 dimansion array?

Thanks, Henry

Answer 1

[Stephen23]

Any internet search engine will give these pages at the top of the search results:

http://www.mathworks.com/help/matlab/examples/manipulating-multidimensional-arrays.html

http://www.mathworks.com/help/matlab/math/multidimensional-arrays.html

http://www.mathworks.com/help/matlab/math/matrix-indexing.html

MATLAB's documentation is really useful and readable. It tells us how to use MATLAB. It has working examples too. You can search it using your favorite internet search engine: this will find the official, correct and recommended ways of using MATLAB. The first link above includes the title "Creating Multi-Dimensional Arrays" at the top of the page. Perhaps you might like to read this.

Here are three easy ways to create ND arrays:

• indexing:

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

• reshape:

>> X = [9,7,8,6,1,3,2,4];
>> reshape(X,2,2,2)
ans(:,:,1) =
     9     8
     7     6
ans(:,:,2) =
     1     2
     3     4

• cat:

>> X = [1,2;3,4];
>> Y = [9,8;7,6];
>> cat(3,Y,X)
ans(:,:,1) =
     9     8
     7     6
ans(:,:,2) =
     1     2
     3     4

[Henry Buck]

Thanks a lot, Henry

[Luis]

in this example you shows a 2 dimensional array?

[Steven Lord]

Those approaches do start off using 2-dimensional arrays.

The indexing approach assigns those 2-dimensional arrays into segments of a 3-dimensional array.

The reshape approach "folds" the 2-dimensional arrays in the third dimension. [If you've ever seen those toys that are snakes built of plastic or wooden blocks, picture folding one of those into a cube form. I'm thinking of something like a Rubik's Snake.]

The cat approach stacks two 2-dimensional arrays on top of one another, like snapping one Lego block on top of another Lego block.

A fourth approach, that's somewhat related to the indexing approach, is the allocate-and-fill approach. In this you "set up shelves" using a function like zeros, ones, etc. then assign values into the array like putting items on the shelves.

shelves = zeros(2, 3, 4)

shelves = 
shelves(:,:,1) =

     0     0     0
     0     0     0


shelves(:,:,2) =

     0     0     0
     0     0     0


shelves(:,:,3) =

     0     0     0
     0     0     0


shelves(:,:,4) =

     0     0     0
     0     0     0

shelves(1, 2, :) = [1 2 3 4]

shelves = 
shelves(:,:,1) =

     0     1     0
     0     0     0


shelves(:,:,2) =

     0     2     0
     0     0     0


shelves(:,:,3) =

     0     3     0
     0     0     0


shelves(:,:,4) =

     0     4     0
     0     0     0

I've essentially put stuff in the "middle of the back of each shelf" with the assignment statement. This approach makes sure I have the right sized "set of shelves" from the get-go without the trick the indexing approach used, where it put items on the second shelf first (to make sure you got a shelving unit with at least two shelves.)

[Walter Roberson]

To be explicit:

There is no MATLAB notation to directly represent 3 or more dimensions of an array. For example,

A = [[1 2 3], [4 5 6]]

does NOT create a 2 x 1 x 3 array, and there is no notation similar to

A = [1 2 3 ## 4 5 6]

to create a 1 x 3 x 2 array.

The closest MATLAB notation to that is

A = cat(3, 1 2 3, 4 5 6)

The direct MATLAB notation only extends to 2 dimensions.

[Stephen23]

A = cat(3, [1,2,3], [4,5,6])

[Walter Roberson]

Opps,

A = cat(3, [1 2 3], [4 5 6])

it is!