Repeat element of a vector n times without loop.

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Justin Solomon
Justin Solomon am 28 Aug. 2012
Bearbeitet: DGM am 2 Aug. 2023
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
  1 Kommentar
John
John am 9 Dez. 2015
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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Akzeptierte Antwort

Azzi Abdelmalek
Azzi Abdelmalek am 28 Aug. 2012
Bearbeitet: Azzi Abdelmalek am 28 Aug. 2012
n=3 ; x=(1:3)' % example
r=repmat(x,1,n)';
r=r(:)'
  3 Kommentare
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Azzi Abdelmalek
Azzi Abdelmalek am 29 Aug. 2012
%you mean
r = repmat(x', n, 1)
Jan
Jan am 29 Aug. 2012
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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Weitere Antworten (6)

jack
jack am 23 Nov. 2015
I would use
repelem(X,3,1)
  3 Kommentare
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Arif Billah
Arif Billah am 1 Aug. 2023
This should be chosen as the best 'correct' answer, thanks!
DGM
DGM am 2 Aug. 2023
Bearbeitet: DGM am 2 Aug. 2023
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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Walter Roberson
Walter Roberson am 28 Aug. 2012
kron(x, ones(n,1))
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Abdelrahman Abdeltawab
Abdelrahman Abdeltawab am 13 Dez. 2018
Bearbeitet: Abdelrahman Abdeltawab am 13 Dez. 2018
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
Walter Roberson am 14 Dez. 2018
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 =
1 2
1 2
1 2
3 4
3 4
3 4

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Kevin Moerman
Kevin Moerman am 29 Aug. 2012
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
  2 Kommentare
Walter Roberson
Walter Roberson am 13 Sep. 2021
Ran in:
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

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Justin Solomon
Justin Solomon am 28 Aug. 2012
Thanks guys, these all work perfectly!
Jianshe Feng
Jianshe Feng am 3 Okt. 2016
y = repmat(x,1,3); y = transpose(y); y = y(:);
Jianshe Feng
Jianshe Feng am 3 Okt. 2016
ind = [1;1;1;2;2;2;3;3;3]; x(ind)
  1 Kommentar
Walter Roberson
Walter Roberson am 7 Apr. 2017
Ah, but how do you construct the ind vector for general length n repetitions ?

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