# Repeating the rows of an array by a number given by another array

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Saeid on 17 Feb 2022
Commented: Saeid on 19 Feb 2022
Consider the array:
x=[linspace(1,4,4)' linspace(5,20,4)']
x = 4×2
1 5 2 10 3 15 4 20
I would like to repeat each row by a number given by another array such as
I=[2 3 2 1]'
I = 4×1
2 3 2 1
so that at the end the first row of x is repeat twice, the second row is repeated 3 times and so on.
Is this possible without using a loop?

Jan on 19 Feb 2022
Edited: Jan on 19 Feb 2022
Use repelem, which is the built-in efficient solution for repeating elements:
x = [linspace(1,4,4)' linspace(5,20,4)']
x = 4×2
1 5 2 10 3 15 4 20
I = [2 3 2 1];
y = repelem(x, I, 1)
y = 8×2
1 5 1 5 2 10 2 10 2 10 3 15 3 15 4 20
A speed comparison:
x = rand(1e4, 2);
I = repmat(1:10, 1, 1000);
tic
for k = 1:100
C = cell(length(I),1) ;
for i = 1:length(C)
C{i} = repmat(x(i,:),I(i),1) ;
end
C = cell2mat(C);
end
toc
tic
for k = 1:100 % Slightly modified for I is a row vector:
idx = arrayfun(@(x,y)x*ones(1,y),1:numel(I),I,'UniformOutput',false);
C = x([idx{:}],:);
end
toc
tic
for k = 1:100
C = repelem(x, I, 1);
end
toc
% R2018b:
% Elapsed time is 3.676185 seconds. loop
% Elapsed time is 6.848004 seconds. arrayfun
% Elapsed time is 0.038391 seconds. repelem
##### 2 CommentsShowHide 1 older comment
Saeid on 19 Feb 2022
Wow, so there IS a difference! Thanks Jan!

### More Answers (1)

KSSV on 17 Feb 2022
x=[linspace(1,4,4)' linspace(5,20,4)'] ;
I=[2 3 2 1]' ;
C = cell(length(I),1) ;
for i = 1:length(C)
C{i} = repmat(x(i,:),I(i),1) ;
end
C = cell2mat(C)
C = 8×2
1 5 1 5 2 10 2 10 2 10 3 15 3 15 4 20
Saeid on 17 Feb 2022
Thanks, Mike!

R2021b

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