Elegant way to replicate a vector, placing it into each cell in a cell array
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Hi,
I would like to be able to create a cell array, with each cell containing the same 1 by 3 vector that I specify. For example, suppose that I have the following:
A=[1 2 3];
and suppose that I want to obtain the following:
mycell={[1 2 3],[1 2 3],[1 2 3];[1 2 3],[1 2 3],[1 2 3]};
Is there an elegant way to do this? It is as if I would like a sort of "repcell" function, similar to the repmat function. I am not sure if one exists, though.
I could do this:
tic
mycell=cell(2,3);
for i=1:1:size(mycell,1)
for j=1:1:size(mycell,2)
mycell{i,j}=A;
end
end
toc
and I get "Elapsed time is 0.000956 seconds." Or I could do:
tic
myarray=repmat(A,2,3);
mycell=mat2cell(myarray,[1 1],[3 3 3]);
toc
and I get "Elapsed time is 0.029993 seconds."
Either one of these seems like it will work fine for me, even when I want my cell array to be very large. Still, I am wondering, is there a more elegant way to do this (even in terms of code readability)?
Thank you very much for your time.
Andrew DeYoung
Carnegie Mellon University
0 个评论
采纳的回答
Matt Fig
2011-6-7
mycell = repmat({A},2,3)
Your MAT2CELL method properly pre-allocates a cell array, in the sense James Tursa is talking about. It can be written as a one-liner:
mycell = mat2cell(repmat(A,M,N),ones(1,M),ones(1,N)*length(A));
So does this, and it is much faster than the MAT2CELL method:
mycell = cellfun(@double,repmat({A},M,N),'Un',0);
更多回答(2 个)
Fangjun Jiang
2011-6-7
A=[1 2 3];
mycell=cell(2,3);
mycell(:)={A};
4 个评论
James Tursa
2011-6-7
@Andrew: I am still not clear what you are doing downstream with mycell. Are you *replacing* cells with a completely different variables, or are you replacing only certain *elements* of the cell contents with different value? i.e., are you doing something like:
mycell{k} = something_else
Or are you doing something like:
mycell{k}(m) = some_value
If it is the former, then pre-allocating mycell with variable contents ala repmat and the like makes no sense whatsoever, and you would be better off pre-allocating an empty cell array and then filling the contents downstream as you go.
Fangjun Jiang
2011-6-7
I am not trying to win a competition but here is the example. It looks like it will be faster to pre-allocate using hidden reference than using the repmat().
>> clear all;
A=1:3;
tic;
mycell=cell(10000,1);
mycell(:)={A};
for k=length(mycell)
mycell{k}=rand(1,3);
end
toc;
clear all;
A=1:3;
tic;
mycell=repmat({A},10000,1);
for k=length(mycell)
mycell{k}=rand(1,3);
end
toc;
Elapsed time is 0.003057 seconds.
Elapsed time is 0.012636 seconds.
And it is still the case if the loop is mycell{k}(2)=rand;
10 个评论
James Tursa
2011-6-8
The data storage layout for a cell array is *exactly* the same as the data storage layout for all other variables ... the data is contiguous in memory. But the "data" for a cell array in this context is variable pointers. So, yes, the actual content of the cells themselves are not in general contiguous in memory (the variables being pointed to are not contiguous), but the *pointers* to these variables, which is what is physically stored in the cell array data area, *are* contiguous in memory. It is *that* memory that gets constantly copied and re-copied in a loop that increases the size of the cell array if you don't pre-allocate the cell array. The L in your above code has nothing to do with this since it only affects the cell variables and not the physical cell array data area. e.g., to see the effect isolated:
n = 100000;
A = 1:3;
clear mycell1
tic
mycell1 = cell(1,n); % pre-allocated
for k=1:n
mycell1{k} = A; % not growing in a loop
end
toc
clear mycell2
tic
for k=1:n
mycell2{k} = A; % growing in a loop
end
toc
isequal(mycell1,mycell2)
Elapsed time is 0.049412 seconds.
Elapsed time is 27.087694 seconds.
ans =
1
Fangjun Jiang
2011-6-8
Thank you, James! Your comments have enlightened me. And Thank you Sean for the link. I enjoyed reading it.
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