[SciPy-user] repmat

[Travis Oliphant]

Aarre Laakso wrote:

>Hi,
>
>Can anyone give me a clue about how I might implement a fast workalike
>for MATLAB's repmat function?
>
>http://www.mathworks.com/access/helpdesk/help/techdoc/ref/repmat.html
>
>The simple use case B = repmat(A,m,n) where A is an array and m,n are
>positive integers would be sufficient for my needs.
>
>  
>
I think the kronecker product is what you want:

kron(ones(m,n),b)

should be the same as

repmat(b, m,n)

Here is the implementation of kron in scipy.linalg  (full scipy --- I 
think it should come over to scipy core, though).

 >>> utils.source(scipy.linalg.kron)
In file: /usr/lib/python2.4/site-packages/scipy/linalg/basic.py

def kron(a,b):
    """kronecker product of a and b

    Kronecker product of two matrices is block matrix
    [[ a[ 0 ,0]*b, a[ 0 ,1]*b, ... , a[ 0 ,n-1]*b  ],
     [ ...                                   ...   ],
     [ a[m-1,0]*b, a[m-1,1]*b, ... , a[m-1,n-1]*b  ]]
    """
    if not a.flags['CONTIGUOUS']:
        a = reshape(a, a.shape)
    if not b.flags['CONTIGUOUS']:
        b = reshape(b, b.shape)
    o = outerproduct(a,b)
    o=o.reshape(a.shape + b.shape)
    return concatenate(concatenate(o, axis=1), axis=1)


-Travis