# How to create a vector from two vectors?

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DS Chen 2016 年 8 月 30 日
コメント済み: DS Chen 2016 年 8 月 30 日
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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### 採用された回答

Stephen Cobeldick 2016 年 8 月 30 日

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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DS Chen 2016 年 8 月 30 日
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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