How to access a vector of a 3D array

I have a code :

%%%%%%%%%%%%%%%%%%%%%%%%

for i = 1 : 3

    for j = 1 : 5

        P{i,j} = rand(1,2);

    end
end

P{1,:}(2)

 % I want to use this to access a certain vector from this 3D matrix P
 % But matlab told me "Bad cell reference operation". Can anyone tell me 
 % the reason and how to do it? Thanks a lot!

Answer 1

[Jan]

Using a cell array is an inefficient representation. Better use a 3D array:

P = zeros(3, 5, 2);
for i = 1 : 3
  for j = 1 : 5
    P(i,j,:) = rand(1,2);
  end
end

Now the indexing is trivial:

P(1, :, 2)

[Jianjian]

Yeah, it works now. Thanks!!

Answer 2

[Jianjian]

I know there is a stupid way to access this certain vector when j is small:

[P{1,1}(2), P{1,2}(2), P{1,3}(2), P{1,4}(2), P{1,5}(2)]

But it is not realistic when j is very large. Anyone can tell me a better way?

Answer 3

[Walter Roberson]

P is not a 3D matrix. You could convert it to a 3D matrix by using cell2mat().

The P that you have established cannot be referenced the way you want with a simple operation. Consider using cellfun

cellfun(@(V) V(2), P)

[Jianjian]

Although I still don't understand what does "V" mean after I checked the help file of "cellfun", I copied this line to the code, it works. Thanks!

[Walter Roberson]

The V does not mean anything to cellfun. The V is part of the anonymous function, @(V) V(2) that is being used. That means "This is an anonymous function that accepts a single parameter that we will temporarily refer to as "V"; what we should do when this function is called is to return the second element of the array currently named "V", which is to say the second element of whatever was passed in as the first parameter of the anonymous function."