How to take Matrix as arguments in user defined function?

I am trying yo write a function that takes matrix as argument and returns a matrix.

function B=rpt(A))
j=length(A);
for i=1:j
  B(2*i,:)=A(i,:);
  B((2*i)-1,:)=A(i,:);
end

But It doesn't work. Can anyone explain why?

[Azzi Abdelmalek]

What "it doesn't work" means? is there any error messages? And please give an example that allows to test your code, you can also explain what is the expected result

[dpb]

More specifically, as written your function

a) Doesn't check which dimension is the greater but uses result of length() on the input argument as the size of the first dimension. Use

size(A,1)

instead. Depending on the input this won't matter but if you pass an array w/ more columns than rows it will error on a range error.

b) While not fatal, it doesn't preallocate B so the overhead in building B will be sizable as it reallocates over and over the new rows. Use

B=zeros(2*j,size(A,2));

before the loop

C) Stylistically, and occasionally the cause of actual errors or debugging grief, i and j are predefined in Matlab as the imaginaries of the same name in common mathematical use. Just be aware if complex variables are ever around of aliasing same.