create sub arrays from a array

Hello Everyone,

I am new to MATLAB so I am asking a very basic question which I am not able to solve.

I have an array of diemensions 672*9 , I want to create smaller arrays by comapring the value of column 9 only.

So for example my values in column 9 are

4

4

4

3.5

3

3.1

3.2

4

3.5

I specify the threshold value as 3.4 . So as my values in first 4 rows is more than 3.4 I store them in a new array. Then next 3 values in second array and finally last 2 values in third array.

I hope I have framed the question properly.

Answer 1

[Azzi Abdelmalek]

Edit

a=[4,4,4,3.5,3,3.1,3.2,4,3.5];
idx=a>=3.4;
idx2=sort([strfind(idx,[true false]) strfind(idx,[false,true]) numel(a)]);
idx1=[1 idx2(1:end-1)+1];
% pre-allocate;
y=cell(1,numel(idx));
for k=1:numel(idx1)
 y{k}=a(idx1(k):idx2(k));
end
y{1}
y{2}
y{3}

[Azzi Abdelmalek]

Look at edited answer

[Jan]

What about pre-allocating y, because you know the final length initially? When you add this details, it is more likely that the OP will not need to ask this again next week.

It is recommended not to use the loop counter k outside the loop in Matlab.

I'm convinced, that the forum is really a good place to offer good programming practize.

[Azzi Abdelmalek]

Ok, it's done

[Ricky]

Actually the size of my array is 672*9 and I want to evaluate only row 9 for the condition. So I would actually have 25 or 26 arrays with 9 columns. These arrays are the subset of the main array. So if I add all the rows in these 25 or 26 arrays I will get 672 rows of initial array.

[Jan]

@Ricky: I do not understand your explanations. Does the method shown by Azzi help to solve your problem?

[Ricky]

@Jan Simon : Sorry I misunderstood the solution it works for me now.

Answer 2

[Andrei Bobrov]

a=[4,4,4,3.5,3,3.1,3.2,4,3.5];

out = accumarray(cumsum([true; diff(a(:) > 3.4)~=0]),a(:),[],@(x){x});