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getting smaller arrays in sequence from a big array

Asked by Suleyman Deveci on 16 Jan 2013

Hello all

I have an array which contains more than 1000 elements (lets say 'Array A'). I want to use this array to get smaller size arrays. For example 'Array 1' should include first 'n' elemets of the 'Array A' and 'Array 2' should include the next n elements of the 'Array A' and so on...

How can I do this with a for loop?

Thanks for the answers...

Regards

Suleyman

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Suleyman Deveci

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3 Answers

Answer by Pedro Villena on 16 Jan 2013
Accepted answer
 A = randi(100,1,1000); %matrix of 1x1000
 n = 100;
 for i=1:floor(numel(A)/n),
    eval(sprintf('A%d=A(%d:%d);',i,(i-1)*n+1,i*n));
 end

3 Comments

Suleyman Deveci on 16 Jan 2013

Thank you for your help Pedro... Result is exactly what I need.

José-Luis on 16 Jan 2013

eval is evil and you should try to avoid using it.

Jan Simon on 16 Jan 2013

eval is evil and you should avoid using it.

I've ommitted the "try to", because there is always a better method (except for one counter-example mentioned by Daniel).

There are many reasons to avoid eval and they are discussed frequently in this forum. There is no reason to create an indirekt method to create variables dynamically, if you need equivalent complicated and indirekt methods to access them later on.

Pedro Villena
Answer by Jing on 16 Jan 2013

Hi Suleyman,

There're many different ways to achieve what you want. I'm not sure what is the best one for your purpose, which depends on what kind of smaller arrays do you want. Following is my code, I prefer to use cell to store related arrays. A is the large array, and B has ten 10*10 array in it. You can index into B like B{2}(10,5), which is the A(10,15) originally.

   A=rand(10,100);
   n=10;
   for i=1:(size(A,2)/n)
      B{i}=A(:,(i-1)*n+1:i*n);
   end

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Jing
Answer by Jan Simon on 16 Jan 2013

If you have the data in one compact array, why do you want to split it into several arrays, which contain an index in the name of the variable? It is much more convenient and flexible to use another array, e.g. a cell.

A = rand(1000, 1);
n = 100;
B = reshape(A, n, []);

Now B(:, i) is what you call "Array 1".

If you really need a FOR loop, here an example with a cell:

n   = 100;
Len = numel(A);
Num = ceil(Len / n);
B   = cell(1, Num);
for ii = 1:Num
  ini   = 1 + (ii - 1) * n;
  fin   = min(ii * n, Len);
  B{ii} = A(ini:fin);
end

1 Comment

Suleyman Deveci on 16 Jan 2013

Dear Jan, thank you for your reply. reshape function works well also for my case. I need use this arrays to produce some graphs, that is why I need to split the data.

Jan Simon

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