Cell array summation

Hi. I'am working on SVD++ algorithm for my course project and I've recently faced a huge bottleneck it the code which I am unable to solve by myself.

the code which consumes approximately 70% of time is:

    for item=1:length(UserDataClear2)
    Ys=Ys+ItemInfo{UserDataClear2{user}(item),4};
    end

It does the following:

for each user there is a vector of items he rated (just numbers). It finds corresponding vector ItemInfo(x,4) where x stand for item's number (which i get from user profile, so set of items changes for each user).

I need to find a sum of all these vectors (ItemInfo(x,4)) - which seems to be the hardest part, as these vectors are stored in cell array.

Other variants such as

    for item=1:length(UserDataClear2)
    Ys{item}=ItemInfo{UserDataClear2{user}(item),4};
    end

    YsSum=sum(cat(2,Ys{:}),2)

work much slower.

In other words: I need to sum vectors from cell arrays, whereas vectors these vectors are different for each user; all of them are stored in huge cell array.

Any ideas how this code can be imporved?

Sincerely, Alex.

Answer 1

[Andrew Newell]

The slowdown seems to be caused by the use of cell data for indices into ItemInfo. Try this:

Ys = sum([ItemInfo{UserDataClear2{user},4}])

This accesses the array UserDataClear2 only once.

[Alexey Zanin]

Brilliant and simple, thanks a lot!

The following code does summation allong the correct dimension of cell array:

Ys=sum([ItemInfo{UserDataClear2{user}(1:length(UserDataClear{user}(:,1))),4}],2);

[Andrew Newell]

Yikes!