Auto Squeeze Problem in Array Indexing

Hello everyone,

I have a question regarding array indexing.

I want to have a 3 dimensional array even though the last index of my array is maximum 1.

To be clear, let me give an example. Suppose I want to declare an array;

myArray(1,1,1) = 5;
myArray(1,1,2) = 6;
myArray(1,1,3) = 7;
myArray(1,2,2) = 8;

This gives me perfectly fine 1x2x3 double.

However, if I don't define the second entry in the last index, it squeezes automatically. For instance;

myNewArray(1,1,1) = 5;
myNewArray(1,2,1) = 6;
myNewArray(1,3,1) = 7;
myNewArray(2,2,1) = 8;

Above example gives 2x3 array. (2 dimensional)

But, I want it to be 2x3x1 array. (3 dimensional)

How will I achieve that?

Thanks in advance.

Answer 1

[Walter Roberson]

You cannot do that that in matlab. Matlab always removes trailing singular dimensions. You should be writing your size() with three outputs to capture the trailing dimension as 1, or if you use size with a single output then extend the vector of results with as many trailing 1 as is needed.

Answer 2

[Guillaume]

What is the reason behind your request? It's something odd to want.

I don't think it's possible. Matlab automatically removes trailing singleton dimensions. You can still access the elements of a 2d array as if it were 3d and query the size as if it were 3d:

a = ones(5); %2d array
a(3,4,1) %still valid to query using 3d
[rows, cols, pages] = size(a) %pages is 1

[Ozgun Savas]

Thank you for your answer.

The reason behind my request is to have consistency in my script.

I have 3 consecutive for loops. Depending on the input files, the loops run. In some cases, one of the for loops run only once. And if that is the last index, the array squeezes.

But, I want it to be always 3D since later in my code I perform a check which requires my array to be 3D. And, my check has to be designed for 3D arrays to cover all the possibilities. But, if the array squeezes automatically, I cannot perform that check.

I don't wish to change any data in the array at all. I just want MATLAB to see the size of my array as 3 dimensional array.

[Guillaume]

" I perform a check which requires my array to be 3D" How?

The thing is for matlab, a 2d array is also a 3d array (it's also a 4d, 5d, etc. array).

There is no reason for a function to fail if it expects a 3d array of arbitrary size. As I shown in my example, you can still get the elements of a 2D array using 3d coordinates, and you can still query its size as a 3D array.

[Ozgun Savas]

Let me get into more detail.

My script generates some 3D and 4D arrays. Later, I have to check that my array is whether one the 3D ones or 4D ones.

But if my data is not sufficient and array squeezes,

some 3D arrays turn into 2D,

some 4D arrays turn into 3D,

some 4D arrays turn even into 2D.

And, therefore, I could not perform that check.

[Walter Roberson]

If you need to know whether the array was "originally" 4D then you need to pass that information on specifically. If you took a subarray of an array and ended up with a trailing singular dimension then the subarray will have a lower ndims and there is no way of avoiding that. The kind of array it was before it was processed is a piece of state information that cannot in all circumstances be deduced by looking at the array size afterwards.

Exception: possibly if you were to use a mex routine to build the array you could enter a specific trailing 1 in the dimensions and have it reflected when you ask for the size(), but not necessarily if you ask for ndims(). However, as soon as the array was copied or worked with in arithmetic then it would definitely "collapse" into the lower number of dimensions.

[Ozgun Savas]

I came to the conclusion that thing that I asked cannot be done in MATLAB and I have to pass that information specifically, as you suggested.

So, I changed my code in that manner.

Thank you.