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matlabmatrixsparse-matrix

What does MATLAB's full() function do with a row parameter?

I have a line of code as follows

nrma2 = full(abs(sum(A.*A',1)));

but sum(...,1) is a row. abs() returns only a row with absolute numbers

So, what will full() do?

Solution

  • full() will make a full matrix out of a sparse one. A sparse matrix can be a row vector, that's just a 1 -by- N matrix. The code you posted presumably has A as a sparse matrix, or includes the full() in case it is not a full matrix already to ensure it becomes a full vector.

    A = sparse([1,0;1,0])
A =
   (1,1)        1
   (2,1)        1
nrma2 = full(abs(sum(A.*A',1)))  % Full row-matrix
nrma2 =
     1     0
abs(sum(A.*A',1))  % Sparse row-matrix
ans =
   (1,1)        1

    Specifically, from the documentation on full():

    full
    Convert sparse matrix to full storage

    (...)

    S — Sparse matrix to convert
    Sparse matrix to convert, specified as a matrix. If S is already a full matrix, then A is identical to S.