Why is MyCellArray{:} different than x = MyCellArray{:}?

I have a 32x1 cell array, each cell is a 600x1 matrix. I originally wanted to average the matrix elements across the 32 cells, the result being a 600x1 mean matrix. Purely by accident I discovered that

MyCellArray{:}

results in the desired mean matrix, while

x = MyCellArray{:}

results in the contents of the first array. I would like to know why they are different, and how can I automatically assign the ans from MyCellArray{:}?

Answer 1

[Zachary]

Thank you for your responses but never mind this question. I feel sort of ridiculous about this, but my code for calculating the mean matrix was wrong. The ans that resulted from MyCellArray{:} was simply the last matrix, and x = MyCellArray{:} results in the first matrix.

For those of you who wish to calculate the mean of each matrix element across the 32 cells, assuming the size of each cell is the same (600x1 in this case) without using loops, I figured out the following code:

meanArray = arrayfun(@(y) mean(cellfun(@(x) x(y), MyCellArray)), [1:600])'

[Cedric]

Then what about the following:

 meanArray = mean(cat(3, MyCellArray{:}), 3) ;

It could be 100s of times faster than a solution mixing ARRAYFUN and CELLFUN.

Answer 2

[Cedric]

MyCellArray{:} is a Comma Separated List (CSL). It is the information that you are missing I guess. The x on the left hand side (LHS) is another one with one element. MATLAB assigns values to elements of the LHS in the order they appear in the RHS.

EDIT: to illustrate

Define a cell array with basic rand arrays rand, 10+rand, 20+rand (that you can recognize):

 >> C = {rand(2), 10+rand(2), 20+rand(2)} ;

Full CSL:

 >> C{:}
 ans =
    0.3404    0.2238
    0.5853    0.7513
 ans =
   10.2551   10.6991
   10.5060   10.8909
 ans =
   20.9593   20.1386
   20.5472   20.1493

Assign first element to single element LHS

 >> x = C{:}
 x =
    0.3404    0.2238
    0.5853    0.7513

Assign first two elements to two elements of the LHS:

 >> [x,y] = C{:}
 x =
    0.3404    0.2238
    0.5853    0.7513
 y =
   10.2551   10.6991
   10.5060   10.8909

Assign first three elements to three elements of LHS:

 >> [x,y,z] = C{:}
 x =
    0.3404    0.2238
    0.5853    0.7513
 y =
   10.2551   10.6991
   10.5060   10.8909
 z =
   20.9593   20.1386
   20.5472   20.1493

Answer 3

[James Tursa]

MyCellArray{:} is a comma separated list. It is equivalent to typing the following:

MyCellArray{1},MyCellArray{2},MyCellArray{3},...,MyCellArray{32}  % <-- typing all 32 elements

x = MyCellArray{:} is an assignment with a comma separated list on the right hand side. E.g.,

x = MyCellArray{1},MyCellArray{2},MyCellArray{3},...,MyCellArray{32}  % <-- typing all 32 elements

So in the second case, you basically have 32 different expressions that you have asked MATLAB to evaluate. The first expression assigns MyCellArray{1} to x, and the rest simply return the remaining 32 elements (and promptly get disgarded).

Answer 4

[Azzi Abdelmalek]

a={rand(600,1) ;rand(600,1)}
b=cellfun(@mean,a)

[Zachary]

This averages each array, i was hoping for an average of each element across the 32 arrays. See below.