How to duplicate rows of a matrix if the number of copies for each row are different?

Hi all,

I want to duplicate each rows of a matrix according to the numbers given in a vector.

For example, I have matrix A and vector , I want to duplicate the first row of A 2 times, second row by 4 times and third row by 5 times.

Is there any way to do this?

Thank you for your help.

Answer 1

[Stephen23]

out = repelem(A,b,1)

[Hung Dao]

Thanks Stephen,

I tried your suggestion but got this error:

Error using repmat, Replication factors must be a row vector of integers or integer scalars.

I tried repmat(A,int64(b),1) but it did not work neither.

Any advice?

[Stephen23]

Use REPELEM, not REPMAT. Assuming that your matrix A has as many rows as b has elements:

A = randi(9,3,4)

A = 3×4

1 5 2 6 5 9 8 6 2 2 7 4

b = [2,4,5];
out = repelem(A,b,1)

out = 11×4

1 5 2 6 1 5 2 6 5 9 8 6 5 9 8 6 5 9 8 6 5 9 8 6 2 2 7 4 2 2 7 4 2 2 7 4 2 2 7 4

This is much simpler and more efficient than using a WHILE or FOR loop.

[Hung Dao]

Thank you very much. This is exactly what I need.

Answer 2

[KSSV]

Read about repmat.

b = [2 4 5] ;
A = repmat(b,2,1) 

A = 2×3

2 4 5 2 4 5

B = repmat(b,3,1)

B = 3×3

2 4 5 2 4 5 2 4 5

[Hung Dao]

Thanks for your answer but that is not really what I am asking for.

Answer 3

[Sulaymon Eshkabilov]

It can be done in a few different ways, e.g.:

A = magic(3)

A = 3×3

8 1 6 3 5 7 4 9 2

b = [2, 4, 5];
AA = [repmat(A(1,:), b(1),1);repmat(A(2,:), b(2),1); repmat(A(3,:), b(3),1)]

AA = 11×3

8 1 6 8 1 6 3 5 7 3 5 7 3 5 7 3 5 7 4 9 2 4 9 2 4 9 2 4 9 2

% Alternative way:
A = magic(3);
b = [2, 4, 5];
AA =[];
for ii = 1:numel(b)
AA = [AA; repmat(A(ii,:), b(ii),1)];
end
AA

AA = 11×3

8 1 6 8 1 6 3 5 7 3 5 7 3 5 7 3 5 7 4 9 2 4 9 2 4 9 2 4 9 2

[Hung Dao]

Thank you very much.

I am wondering if there is a more efficient way than using a forloop?

[Sulaymon Eshkabilov]

I don't believe there is an alternative more efficient way than using [for .. end] or [while .. end] loop.