Adding information to a cell array without using a loop?

Hi all,

Hopefully there is a built-in function for this that I'm not finding.

I currently have an array of length Nx2. Let's say for example that it looks like this:

Array=

    [1 2
     1 7 
     1 9
     2 3
     2 4
     4 1 
     5 8 
     5 9]

I want to make a cell array of size(max(Array(:,1)),1). So in this case it would be 5x1. Then, I want to cycle through Array and add the number in the 2nd column into the corresponding cell from the first column. In a loop, I would do it like this:

C=cell(max(Array(:,1),1);
for i=1:length(C)
C{i}=Array(Array(:,1)==i,2);
end

The result would look like this:

C=
[2 7 9] [3 4] [ ] [1] [8 9]

My problem is that I have a very big array, about 100k in length. Is there a faster way of doing this without running a loop through? Perhaps using cellfun() or something similar?

Thanks in advance!

Answer 1

[Sean de Wolski]

Accumarray!

C = accumarray(Array(:,1),Array(:,2),[],@(x){x}).'

Azzi's solution is typically the safer approach - safeguarding against non-integers and large values. However, for your application using these explicitly does what you want.

[Jordan]

There it is! Great stuff!

Answer 2

[Azzi Abdelmalek]

[a,b,c]=unique(Array(:,1));
C=accumarray(c,Array(:,2), size(a), @(x)  {x});
celldisp(C)

%You can add

C1=cell(max(a),1)
C1(a)=C
celldisp(C1)

[Jordan]

This seems very close, but what happens in the situation (like above) where I might skip a number? If you look above I want my answer to be this:

C=
[2 7 9] [3 4] [ ] [1] [8 9]

, but this function gives me this, skipping the empty cell:

C=
[2 7 9] [3 4] [1] [8 9]

[Azzi Abdelmalek]

You did not see C1