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MathGroup Archive 2002

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RE: sparse matrix

  • To: mathgroup at
  • Subject: [mg35195] RE: [mg35190] sparse matrix
  • From: "DrBob" <majort at>
  • Date: Mon, 1 Jul 2002 03:34:51 -0400 (EDT)
  • Sender: owner-wri-mathgroup at

If the matrix is sparse, don't STORE it as a matrix -- you waste a lot
of space if you do.  Here's one way to do it, with a short example file
I created:

r = ReadList["sparse.txt", {Number, Number, Number}]

{{2, 3, 12}, {1, 7, 41}}

sparse[line_, col_] := 0
(sparse[#[[1]], #[[2]]] = #[[3]]) & /@ r;


sparse[1, 7] = 41
sparse[2, 3] = 12
sparse[line_, col_] := 0

The function "sparse" is zero for all inputs not found in the file.

If you had a situation where other default values applied, that can be
easily handled too.  For instance, if the default diagonal values are
one, while all others default to zero, define it this way:

sparse[line_, line_] := 1
sparse[line_, col_] := 0
(sparse[#[[1]], #[[2]]] = #[[3]]) & /@ r;

Inputs from the file will override defaults.

When you've defined "sparse" use Clear[r] to save space, or you can read
entries one line at a time, defining "sparse" as you go along, so the
"r" matrix isn't needed at all.

Bobby Treat

-----Original Message-----
From: Louis Patrick O'Carroll [mailto:louis at] 
To: mathgroup at
Subject: [mg35195] [mg35190] sparse matrix


is it possible in mathematica to import from a text file a sparse
The file is written in the form
l c v

where l is line, c is colum and v is a non zero value.

Thanks for the help,


PS: please send the responce to my email address.

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