Creating a Matrix with for loop

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Philipp 2020-4-30

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https://ww2.mathworks.cn/matlabcentral/answers/521958-creating-a-matrix-with-for-loop

编辑： Stephen23 2020-5-1

采纳的回答： Rik

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Hi together,

I have a question about creating a matrix. For me, at the moment, it is impossible to solve, I just don't get it.

I've got a matrix X

2x9 double

X = [2 1 2 10 3 0 0 0 0,
       2 2 3 20 5 0 0 0 0]

and I have a vector z=[2 3];

Now I would like to create a matrix T , which looks like this

5x9 double

T= [2 1 2 10 3 0 0 0 0,
2 3 10 3 0 0 0 0,
3 4 20 5 0 0 0 0,
4 5 20 5 0 0 0 0,
5 6 20 5 0 0 0 0];

So this means 2x (z(1)) the first row of X and 3x (z(2)) the 2nd row of X.

In the end, I don't know the final size of X and z and don't know the content, but this is just a small example.

Hope anybody can help. Thank you.

Cheers,

Philipp

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Rik 2020-4-30

How do you determine what values should be changed?

Philipp 2020-4-30

It is a beam divided in 5 Elements, so 6 nodes. X is created by input data and z also, so 10 and 20 is the outer diameter and 3 and 5 the inner diameter.

The first Element before is X(1,:) and it should be divided by z(1) and the second Element is X(2,:) and should be divided by z(2)

Is this the answer to your question?

Thanks,

Philipp

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Answer 1

Rik 2020-4-30

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This code should get you close to what you need. Adapt as needed.

X =[2 1 2 10 3 0 0 0 0;
    2 2 3 20 5 0 0 0 0];
z=[2 3];
if numel(z)~=size(X,2)
    error('mismatch in input sizes')
end
X2=mat2cell(X,ones(size(X,1),1),size(X,2));
z2=num2cell(z);z2=reshape(z2,size(X2));
X2=cellfun(@(data,sz) repelem(data,sz,1),X2,z2,'UniformOutput',false);
T=cell2mat(X2);
clc,disp(T)

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Philipp 2020-4-30

Thank you very much, really helpful

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Answer 2

Stephen23 2020-4-30

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Simpler, no data duplication, fewer intermediate variables with less memory footprint:

>> X = [2,1,2,10,3,0,0,0,0;2,2,3,20,5,0,0,0,0]
X =
     2     1     2    10     3     0     0     0     0
     2     2     3    20     5     0     0     0     0
>> z = [2,3];
>> fun = @(r,n) r*ones(1,n);
>> idx = cell2mat(arrayfun(fun,1:numel(z),z,'uni',0));
>> T = X(idx,:)
T =
     2     1     2    10     3     0     0     0     0
     2     1     2    10     3     0     0     0     0
     2     2     3    20     5     0     0     0     0
     2     2     3    20     5     0     0     0     0
     2     2     3    20     5     0     0     0     0

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Stephen23 2020-4-30

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"...it seems our solutions are fairly similar."

Ummmm... apart from the fact that they work in quite different ways:

Your answer splits up the data array into a cell array (thus duplicating the data in memory), applies repelem to those data arrays (thus duplicating the data again), before finally joining it all back together via cell2mat. The output of the anonymous function is the duplicated data. You used mat2cell and num2cell and cellfun.
My answer generates an index vector which is applied to the original data array (without any intermediate data duplication). As the index is just a vector (of row subscripts) it has a much smaller memory footprint. The data is not duplicated anywhere, nor is repelem used. The output of the anonymous function is a vector of subscripts. I used arrayfun.

I cannot see a lot of similarity between them, except perhaps on a very abstract level: "create some things of the right size, then concatenate them together". However what they create, how they create it, and what they do with it afterwards are all very different. Also note that creating the index (as well as having a smaller memory footprint) means that the index can be repeatedly reapplied to other arrays, whereas your approach requires re-running the entire code for each data array.

"Am I missing something where a condensed version of my code would have a larger memory footprint than yours?"

Lets have a look at the sizes of the intermediate arrays of your code:

>> X2=mat2cell(X,ones(size(X,1),1),size(X,2));
>> whos
  Name      Size            Bytes  Class     Attributes
  X         2x9               144  double              
  X2        2x1               368  cell                
  z         1x2                16  double              
>> z2=num2cell(z);z2=reshape(z2,size(X2));
>> X2=cellfun(@(data,sz) repelem(data,sz,1),X2,z2,'UniformOutput',false);
>> whos
  Name      Size            Bytes  Class     Attributes
  X         2x9               144  double              
  X2        2x1               584  cell                
  z         1x2                16  double              
  z2        2x1               240  cell 

Note that the cell array X2 contains the duplicated data (and so its bytes will scale with the number of elements the input data array). Because the numeric arrays inside X2 are replaced with other numeric arrays, there will also be a point where all of those arrays will need to be in memory (this is not shown here, you would need to use your OS or perhaps the profiler).

Vs. my code:

>> fun = @(r,n) r*ones(1,n);
>> idx = cell2mat(arrayfun(fun,1:numel(z),z,'uni',0));
>> whos
  Name      Size            Bytes  Class              Attributes
  X         2x9               144  double                       
  fun       1x1                32  function_handle              
  idx       1x5                40  double                       
  z         1x2                16  double 

Note the size of array idx (and also those in the temporary cell array) will scale with the number of rows of the input data array (i.e. bytes proportional to rows). It is clear that with the original two-row input array my code has around 10% of the memory footprint of your code (32+40 bytes vs. 584+240 bytes). However we can easily try it with a larger input array and you will see how your variables' memory consumption increases faster than for my code. For example, simply replicating the input arrays 1000 times:

>> X = repmat(X,1000,1);
>> z = repmat([2,3],1,1000);
>> whos X
  Name         Size             Bytes  Class     Attributes
  X         2000x9             144000  double 
>> X2=mat2cell(X,ones(size(X,1),1),size(X,2));
>> whos X2
  Name         Size             Bytes  Class    Attributes
  X2        2000x1             368000  cell               
>> z2=num2cell(z);z2=reshape(z2,size(X2));
>> X2=cellfun(@(data,sz) repelem(data,sz,1),X2,z2,'UniformOutput',false);
>> whos X2 z2
  Name         Size             Bytes  Class    Attributes
  X2        2000x1             584000  cell 
  z2        2000x1             240000  cell

Vs. my code (variable bytes scaling with the number of data rows):

>> idx = cell2mat(arrayfun(fun,1:numel(z),z,'uni',0));
>> whos idx fun
  Name      Size              Bytes  Class              Attributes
  fun       1x1                  32  function_handle              
  idx       1x5000            40000  double

The difference is now my code requires just 5% of the memory that your code does... and so it continues :)

"Apart from how the rows are duplicated..."

The original question is entirely about how to duplicate rows. That is rather the point of it.

Rik 2020-4-30

Thanks for your thorough reply.

It does seem a bit unfair to count X2 in the memory footprint, but not count arrayfun(fun,1:numel(z),z,'uni',0) (not that it is going to matter a lot), but I see what you mean now.

Stephen23 2020-5-1

编辑：Stephen23 2020-5-1

"It does seem a bit unfair to count X2 in the memory footprint, but not count arrayfun(fun,1:numel(z),z,'uni',0)..."

The intermediate cell array has 264 bytes for the original data, it scales with the size and contents of z.

Due to the transient nature of these intermediate arrays, and the unknown sequence in which the JIT compiler might create and destroy them, the only way to really know the actual memory consumption is to measure it: please feel free to do some tests and post the results here.

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