Requested array exceeds the maximum possible variable size.

I am trying to get the mean value of all pixels in 5D array (arr):

let (f) a 4D array that I want to search for a value in it:

size of arr= (800,800,3,9,9)

size of f = (800,800,9,9)

[x, y, z, w] = ind2sub(size(f),find(f == value));

result = mean(arr(x, y ,3, z ,w),'all');

I receive this error :

Requested array exceeds the maximum possible variable size. when using mean!

any help is appreciated,

thanks

Answer 1

[Steven Lord]

Let's say you want to extract the two elements equal to 1 in this matrix.

A = [1 2 3; 4 5 6; 7 8 1]

A = 3×3

1 2 3 4 5 6 7 8 1

You can do this with find and ind2sub.

locations = find(A == 1)

locations = 2×1

1 9

[row, col] = ind2sub(size(A), locations)

row = 2×1

1 3

col = 2×1

1 3

Now let's extract the 1's from A.

B = A(row, col)

B = 2×2

1 3 7 1

Why are the 3 and 7 in there? Because B contains all the elements at the intersections of rows in the variable row and columns in the variable col. So even though you may have expected B to have only two elements because row and col each had two elements, it actually has two times two or four elements.

Now let's look at a slightly different matrix:

C = [1 1 3; 4 5 6; 7 8 1];
locationsC = find(C == 1);
[rowC, colC] = ind2sub(size(C), locationsC)

rowC = 3×1

1 1 3

colC = 3×1

1 2 3

D = C(rowC, colC)

D = 3×3

1 1 3 1 1 3 7 8 1

These are the elements at the intersections of rows 1, 1, and 3 and columns 1, 2, and 3. Given that knowledge, even though arr is of size (800,800,3,9,9) the array arr(x, y ,3, z ,w) may be much larger.

If you just want the mean of elements in an array corresponding to locations of a specific value in another array, don't use subscripted indexing. Use logical indexing.

justThreeElements = A(C == 1)

justThreeElements = 3×1

1 2 1

mean(justThreeElements)

ans = 1.3333

The fact that you have that other dimension in arr is a little tricky, but you could extract just the third page to a separate variable.

[MatlabUser]

Thank you so much for your explanation, it is really helpful !

so it means the output value is not the correct one since it will contain the intersections as well,

thanks !

[MatlabUser]

Does the same story apply to sum() as well?

[Walter Roberson]

Yes. It has to do with how indexing works, and the indexing is done before the outer function such as sum() or mean() is called.