Repeat element of a vector n times without loop.

Question

Justin Solomon 2012-8-28

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编辑： DGM 2023-8-2

Say I have a column vector x=[a;b;c]. I want to repeat each element n times to make a long length(x)*n vector. For example, for n=3, the answer would be:

ans=
a
a
a
b
b
b
c
c
c

Can anyone think of an elegant way to do this without looping?

Thanks,

Justin

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John 2015-12-9

U can use repmat it not exactly elegant but it will do the job

x=[a;b;c]; n=3;

newx = [repmat(x(1),n,1);repmat(x(2),n,1);repmat(x(3),n,1)]

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Answer 1

Azzi Abdelmalek 2012-8-28

16
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编辑：Azzi Abdelmalek 2012-8-28

在 MATLAB Online 中打开

 n=3 ; x=(1:3)' % example
 r=repmat(x,1,n)';
 r=r(:)'

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Azzi Abdelmalek 2012-8-29

在 MATLAB Online 中打开

%you mean
r = repmat(x', n, 1)

Jan 2012-8-29

I guess, you are right. repmat(1:3, 1, 2) = [1,2,3,1,2,3] but the OP wants [1,1,2,2,3,3]. Then r = repmat(1:3, 2, 1); r = r(:) avoid the expensive transposition of the matrix. Well, I admit that even reading this message will waste more time then millions of matrix transpositions will cost...

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Answer 2

jack 2015-11-23

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I would use

repelem(X,3,1)

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Arif Billah 2023-8-1

This should be chosen as the best 'correct' answer, thanks!

DGM 2023-8-2

编辑：DGM 2023-8-2

This is probably the more accepted answer today (hence the upvotes), but repelem() was not available until after the question was originally answered (R2015a).

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Answer 3

Walter Roberson 2012-8-28

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在 MATLAB Online 中打开

kron(x, ones(n,1))

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Abdelrahman Abdeltawab 2018-12-13

编辑：Abdelrahman Abdeltawab 2018-12-13

Dear Walter Roberson,

why you did not use outer product and you chosen kronecker ( just curious ) because the guy's question was having vectors ?

Walter Roberson 2018-12-14

在 MATLAB Online 中打开

The * matrix multiplication operator cannot by itself repeat elements. You would need something like

(x.' * repmat(eye(length(x)), 1, n)).'

if you wanted to use the * operator to duplicate elements -- forcing you to call upon repmat() to duplicate elements.

Using the kronecker is a known idiom for duplicating data. It can be used for non-vectors too.

>> kron([1 2;3 4], ones(3,1))
ans =
   2
   2
   2
   4
   4
   4

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Answer 4

Kevin Moerman 2012-8-29

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在 MATLAB Online 中打开

There is several others ways of doing it which in some cases are more efficient. Have a look at what the size of your vector is and compare the methods. Below I compare speeds and it appears that on my computer the third and fourth methods are mostly faster for large arrays.

n=100000; x=1:3; 
a=zeros(n,numel(x)); b=a; c=a; d=a; %memory allocation
tic; a=repmat(x, n, 1); t1=toc; %Repmat method
tic; b=kron(x, ones(n,1)); t2=toc; %kron method
tic; c=x(ones(1,n),:); t3=toc; %indexing method
tic; d=ones(n,1)*x; t4=toc; %multiplication method

Kevin

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Vinay Chakravarthi 2015-1-20

Thx Man..

Walter Roberson 2021-9-13

在 MATLAB Online 中打开

format long g
n=100000; x=1:3; 
a=zeros(n,numel(x)); b=a; c=a; d=a; %memory allocation
tic; a=repmat(x, n, 1); t1=toc %Repmat method
t1 = 
                  0.000543
tic; b=kron(x, ones(n,1)); t2=toc %kron method
t2 = 
                  0.006106
tic; c=x(ones(1,n),:); t3=toc %indexing method
t3 = 
                  0.002276
tic; d=ones(n,1)*x; t4=toc %multiplication method
t4 = 
                  0.001798