How to average first two dimension of 3D array/matrix?

Hi all,

I am calculating the average of first two dimension of a 3D matrix but it is showing following error:

Error using sum

Dimension argument must be a positive integer scalar within indexing range.

Error in mean (line 116)

y = sum(x, dim, flag) ./ size(x,dim);

Also, I have used a simplest example given in topic 'Mean of Array Page' in https://in.mathworks.com/help/matlab/ref/mean.html but I am getting same error.

I am using MATLAB 2018a

Please help!

Thanks in advance

[KSSV]

Show us the example which you have tried and got error.

[UTKARSH VERMA]

I have used a simplest example given in topic 'Mean of Array Page' in https://in.mathworks.com/help/matlab/ref/mean.html and I got an error:-

""Error using sum

Dimension argument must be a positive integer scalar within indexing range.

Error in mean (line 116)

y = sum(x, dim, flag) ./ size(x,dim);""

I guess my Matlab version is older (i.e. 2018a)

Answer 1

[Chunru]

a = randi(3, 3, 4, 2)

a = 
a(:,:,1) =

     3     3     1     3
     3     1     2     3
     3     1     3     1


a(:,:,2) =

     3     1     1     2
     1     1     2     2
     1     3     1     1

c = mean(reshape(a, [], size(a,3)))

c = 1×2

2.2500 1.5833

[UTKARSH VERMA]

Thank you Chunru!

It worked perfectly.

Answer 2

[Wan Ji]

Just use mean function to average first two dimension of 3D matrix

a = rand(2,3,4); % I just use rand function so the result is randomly displayed
b = mean(a,1:2) % 1:2 means the first two dimension

The result is

b = mean(a,1:2)

b(:,:,1) =

    0.5258


b(:,:,2) =

    0.2759


b(:,:,3) =

    0.6214


b(:,:,4) =

    0.4449
    

If you want to get array of b

Then

b = squeeze(b) % or b = b(:);

So

b =

    0.5258
    0.2759
    0.6214
    0.4449

[UTKARSH VERMA]

Thank you for replying.

I guess my Matlab version is older (i.e. 2018a) due to which I am not able to get my result and it's showing same problem.

Answer 3

[Steven Lord]

The ability to specify a vector of dimensions over which to sum an array was introduced in release R2018b as stated in the Release Notes.

[UTKARSH VERMA]

Thanks for sharing this information. I didn't know that.