how to replicate an array

Question

I want to make a function like this

>> matdup([1 2],3,4)            %or any other input that user wish to enter
ans= 

 1     2     1     2     1     2     1     2
 1     2     1     2     1     2     1     2
 1     2     1     2     1     2     1     2

I am stuck in my code. My logic:

m = matdup(input,row,col)
for i = 1:row
        for j = 1:col
            m(i, j)= input;

This is producing this:

>> matdup(1,2,2)
ans=

      1 1
      1 1

But failed at this:

>> matdup([1 2],3,4)

error at console:

Subscripted assignment dimension mismatch.

    Error in ==> matdup at 6
                m(i, j)= input

Any idea?

Divakar · Accepted Answer · 2014-04-03 20:29:45Z

3

Method 1: Are you allowed to use ones? Try this -

A = [1 2]

rowIdx = [1 : size(A,1)]';
colIdx = [1 : size(A,2)]';

out = A(rowIdx(:, ones(3,1)), colIdx(:, ones(4,1)))

Output

out =

     1     2     1     2     1     2     1     2
     1     2     1     2     1     2     1     2
     1     2     1     2     1     2     1     2

Method 2: Are you allowed to use bsxfun and permute? Try this for the same result -

A = [1 2]
row_mapped = bsxfun(@plus,A,zeros(3,1))
out = reshape(bsxfun(@plus,row_mapped,permute(zeros(4,1),[3 2 1])),[3 8])

edited Apr 3, 2014 at 20:29

answered Apr 3, 2014 at 19:52

Divakar

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1 Comment

Divakar Over a year ago

@user3440716 Added one more method!

Rafael Monteiro · Accepted Answer · 2014-04-03 19:45:30Z

2

Matlab has a funcion called repmat that does the same.

If you want to create a similar function, you could do something like this:

function B = matdup(A, M, N)
    [nr, nc] = size(A);
    B = zeros([nr nc] .* [M N]);
    for r = 1:M
        for c = 1:N
            rr = (r - 1) * nr + 1;
            cc = (c - 1) * nc + 1;
            B(rr:rr + nr - 1, cc:cc + nc - 1) = A;
        end
    end
end

Note this function is restricted to 2D matrices.

edited Apr 3, 2014 at 19:45

answered Apr 3, 2014 at 19:26

Rafael Monteiro

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user3440716 Over a year ago

i am not allowed to do so. any other thing. i ve idea but not convinced for example taking the size and implementing. but i cant think further what to do next. is it possible to look at library how they have made function?

chappjc · Accepted Answer · 2014-04-03 22:45:15Z

2

Try kron:

matdup = @(x,m,n) kron(ones(m,n),x)

Demonstration:

>> A = [5 6 7];
>> out = matdup(A,3,2)
out =
     5     6     7     5     6     7
     5     6     7     5     6     7
     5     6     7     5     6     7

Note that you can switch the inputs to kron to effectively replicate elements rather than the whole matrix:

repel = @(x,m,n) kron(x,ones(m,n));

Demonstration:

>> A = [5 6 7];
>> out = repel(A,3,2)
out =
     5     5     6     6     7     7
     5     5     6     6     7     7
     5     5     6     6     7     7

edited Apr 3, 2014 at 22:45

answered Apr 3, 2014 at 20:05

chappjc

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Comments

Luis Mendo · Accepted Answer · 2014-04-04 23:34:56Z

2

The replication can be done easily using mod:

function R = matdup(A, M, N)
    [m n]= size(A);
    R = A(mod(0:m*M-1,m)+1, mod(0:n*N-1,n)+1)

answered Apr 4, 2014 at 23:34

Luis Mendo

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1 Comment

chappjc Over a year ago

Clever way to generate steps at a certain period. +1

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