ftell

Get file position indicator

Syntax

• position = ftell(fid)
  

Description

position = ftell(fid) returns the location of the file position indicator for the file specified by fid, an integer file identifier obtained from fopen. The position is a nonnegative integer specified in bytes from the beginning of the file. A returned value of -1 for position indicates that the query was unsuccessful; use ferror to determine the nature of the error.

full

Convert sparse matrix to full matrix

Syntax

• A = full(S)
  

Description

A = full(S) converts a sparse matrix S to full storage organization. If S is a full matrix, it is left unchanged. If A is full, issparse(A) is 0.

Remarks

Let X be an m-by-n matrix with nz = nnz(X) nonzero entries. Then full(X) requires space to store m*n real numbers while sparse(X) requires space to store nz real numbers and (nz+n) integers.

On most computers, a real number requires twice as much storage as an integer. On such computers, sparse(X) requires less storage than full(X) if the density, nnz/prod(size(X)), is less than one third. Operations on sparse matrices, however, require more execution time per element than those on full matrices, so density should be considerably less than two-thirds before sparse storage is used.

Examples

Here is an example of a sparse matrix with a density of about two-thirds. sparse(S) and full(S) require about the same number of bytes of storage.

• S = sparse(+(rand(200,200) < 2/3));
  A = full(S);
  whos
  Name     Size      Bytes   Class
    A    200X200   320000   double array
    S    200X200   318432   double array (sparse)