How to create a vector from two vectors?

Question

DS Chen 2016-8-30

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/301370-how-to-create-a-vector-from-two-vectors

评论： DS Chen 2016-8-30

采纳的回答： Stephen23

I have two vectors with the same length:

A = [2 4 5 4 9 8 1 6];

B = [3 6 2 5 4 4 3 5];

Each element in vector B defines how many times the corresponding element in vector A duplicates itself. For example, here I want the final vector C to have 3 twos, 6 fours, 2 fives, 5 fours, 4 nines, 4 eights, 3 ones, and 5 sixes:

C = [2 2 2, 4 4 4 4 4 4, 5 5, 4 4 4 4 4, 9 9 9 9, 8 8 8 8, 1 1 1, 6 6 6 6 6];

I can do it with a for-loop. However in practice, vectors A and B are long. So is there a way to do this without a loop.

Thanks!

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Answer 1

Stephen23 2016-8-30

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/301370-how-to-create-a-vector-from-two-vectors#answer_233138

编辑：Stephen23 2016-8-30

在 MATLAB Online 中打开

If you have MATLAB 2015a or more recent then use repelem:

repelem(A,B)

otherwise:

cell2mat(arrayfun(@(a,b)repmat(a,1,b),A,B,'Uni',0))

although a loop may well be faster.

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Stephen23 2016-8-30

编辑：Stephen23 2016-8-30

在 MATLAB Online 中打开

Important: do NOT expand the array in the loop! Use a preallocated cell array:

>> C = cell(size(A));
>> for k = 1:numel(C), C{k} = repmat(A(k),1,B(k)); end
>> cell2mat(C)

Or a preallocated numeric array (this is faster):

 >> X = cumsum([0,B]);
 >> D = NaN(1,X(end));
 >> for k = 1:numel(A), D(1+X(k):X(k+1)) = A(k); end

DS Chen 2016-8-30

Good Point!

Using preallocated cell array is 40 times faster than expanding array in the loop.

I also test the repelem(A, B) on the other machine. repelem(A,B) is indeed the fastest! It's at least 10 times faster than for-loop. I hope I can use it later:)

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