how to increase the rows of an array in each loop?

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saharsahar
saharsahar 2012-2-25
回答： Dmitry Mittov 2015-3-13
Dear all, How to increase the elemnts of a multi dimentional array in each loop ? for example at first A=[], the next loop it will be A=[2 3 5 7] the third A= [ 2 3 5 7; 3 5 3 1 ] and so on. the colomn of A is attributes which is application-dependent.(I preferably want to use : to show all colomns)
Thanks

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回答（3 个）

Jan
Jan 2012-2-25
A = [];
for i = 1:5
A = cat(1, A, rand(1, 4));
% or:
% A = [A; rand(1, 4)];
end
But consider, that this is not efficient, because Matlab has to reserve the memory for the matrix in each iteration again and copy the former values. Look for "pre-allocation" in this forum to see better methods.
Dmitry Mittov
Dmitry Mittov 2015-3-13
sz = [5 4];
cell_A = arrayfun(@(i) rand(sz(2),1), 1:sz(1), 'UniformOutput', false);
A = cell2mat(cell_A)';
In my case I had vector X and wanted to get the matrix of [X; X .^ 2; X .^3; ... X .^ n]'. Not sure that works effective (in case of memory consumption), but looks so functional.
saharsahar
saharsahar 2012-2-26
Thanks Jan, That is great . However I need to have another loop and I want "A" will be different for each loop. I mean, for example there are 6 object (firs loop) which each object has its own "A" matrix in following code I define the Loop j. In this case it gives me problem.
A = [];
for j=1:6
for i = 1:5
A(j) = cat(1, A(j), rand(1, 4));
% or:
% A = [A; rand(1, 4)];
end
end
can you help me please in this specific case? Thanks
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saharsahar
saharsahar 2012-2-26
The colomns of A is actually application dependent. it will be a variable based on the features of inputs. in this example I assume it will be 4 . the rows of A also will be increased in each loop based on a "if condition". so I don know exactly how many rows will be, as it depends to accept if conditin or not. so here i just showing a simple example of A with 4 cols and 5 rows.
However I have 6 Objects which each of them have a A matrix.(See the code plz)
Is that make sense?
Thanks

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