# Extend a vector by extending its elements

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f10w on 29 May 2014
Commented: f10w on 29 May 2014
I have a vector of dimension d: (1,2,...,d) (it's (x_1,x_2,...,x_n) but I removed the 'x_' for simplicity). Now I would like to extend it (in a fast manner) to obtain a d*d vector of the form:
(1,1,...,1,2,...,2,...,d,d,...,d)
where each element repeat d times.
I looked at this page: http://www.mathworks.fr/fr/help/wavelet/ref/wextend.html but it seems to me that the function does not handle this kind of extension.

Udit Gupta on 29 May 2014
Edited: Udit Gupta on 29 May 2014
reshape(repmat(X,d,1),1,[])
should do the trick.
Example -
>> X = [1 2 3 4];
>> d=4;
>> reshape(repmat(X,d,1),1,[])
ans =
1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4
f10w on 29 May 2014
Thanks, Udit!

George Papazafeiropoulos on 29 May 2014
Edited: George Papazafeiropoulos on 29 May 2014
If X is a column vector:
% initial data
d=10;
% engine
X=(1:d)';
ind=ones(d^2,1);
ind(d+1:d:d^2)=1-d;
% result
output_vector=X(cumsum(ind))
If X is a row vector:
% initial data
d=10;
% engine
X=1:d;
ind=ones(d^2,1);
ind(d+1:d:d^2)=1-d;
% result
output_vector=X(cumsum(ind))
without the need to reshape or repmat!
f10w on 29 May 2014
Thanks, George. I will compare with Udit's solution and get back to you.

Jos (10584) on 29 May 2014
Edited: Jos (10584) on 29 May 2014
A slower but nice alternative to repmat:
X = [10 20 30 40]
d = 4
Y = kron(X, ones(1,d))

Jos (10584) on 29 May 2014
Edited: Jos (10584) on 29 May 2014
A fast and also nice alternative:
X = [10 ; 20 ; 30 ; 40]
d = 3
Y = X(ceil((1:d*numel(X))/d))
f10w on 29 May 2014
Thanks, Jos. I will compare with Udit's solution and get back to you.