if i have matrix of 20X2048 and i want to make it 20000X2048 how to do it

matrix size = 20 x 2048

new matrix size =20000 x 2048

Answer 1

[John D'Errico]

help repmat

 REPMAT Replicate and tile an array.
    B = REPMAT(A,M,N) or B = REPMAT(A,[M,N]) creates a large matrix B 
    consisting of an M-by-N tiling of copies of A. If A is a matrix, 
    the size of B is [size(A,1)*M, size(A,2)*N].
 
    B = REPMAT(A,N) creates an N-by-N tiling.  
    
    B = REPMAT(A,P1,P2,...,Pn) or B = REPMAT(A,[P1,P2,...,Pn]) tiles the array 
    A to produce an n-dimensional array B composed of copies of A. The size 
    of B is [size(A,1)*P1, size(A,2)*P2, ..., size(A,n)*Pn].
    If A is m-dimensional with m > n, an m-dimensional array B is returned.
    In this case, the size of B is [size(A,1)*P1, size(A,2)*P2, ..., 
    size(A,n)*Pn, size(A, n+1), ..., size(A, m)].
 
    REPMAT(A,M,N) when A is a scalar is commonly used to produce an M-by-N
    matrix filled with A's value and having A's CLASS. For certain values,
    you may achieve the same results using other functions. Namely,
       REPMAT(NAN,M,N)           is the same as   NAN(M,N)
       REPMAT(SINGLE(INF),M,N)   is the same as   INF(M,N,'single')
       REPMAT(INT8(0),M,N)       is the same as   ZEROS(M,N,'int8')
       REPMAT(UINT32(1),M,N)     is the same as   ONES(M,N,'uint32')
       REPMAT(EPS,M,N)           is the same as   EPS(ONES(M,N))
 
    Example:
        repmat(magic(2), 2, 3)
        repmat(uint8(5), 2, 3)
 
    Class support for input A:
       float: double, single
       integer: uint8, int8, uint16, int16, uint32, int32, uint64, int64
       char, logical
 
    See also BSXFUN, MESHGRID, ONES, ZEROS, NAN, INF.

    Documentation for repmat
       doc repmat

    Other uses of repmat

       codistributed/repmat       lti/repmat       symfun/repmat
       gpuArray/repmat            matlab/repmat    tall/repmat
       InputOutputModel/repmat

[ANUSAYA SWAIN]

but this will also change the column dmension. I just want to change the row dimension to 20000 keeping column dimension same as 2048

[Walter Roberson]

NewMatrix = repmat(Matrix, 20000/size(Matrix,1), 1)

or

NewMatrix = repelem(Matrix, 20000/size(Matrix,1), 1)

The difference is whether you want to repeat the first element 1000 times, then the second element 1000 times, and so on (if so, repelem), or if you want to repeat have all 20 values in order, followed by the same 20 in order, then again and again and again 1000 times

[Stephen23]

"but this will also change the column dmension."

Only if you tell it to. If you specify the column replication as 1, then the number of columns won't change.