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How to write efficiently a large matrix of real numbers to an ASCII file

  • To: mathgroup at
  • Subject: [mg7266] How to write efficiently a large matrix of real numbers to an ASCII file
  • From: Martin Heimann <martin.heimann at>
  • Date: Tue, 20 May 1997 02:59:18 -0400 (EDT)
  • Organization: mpi für meteorologie
  • Sender: owner-wri-mathgroup at

Dear fellow Mathematica users,

This seems to be a very basic question and must have been asked many
times before, maybe it is in a FAQ, but I could not find it.

How to write efficiently a matrix of real numbers to an ASCII file such
that it can be read by a normal program (e.g. in Fortran or C)?

WRI suggests to use WriteMatrix:

WriteMatrix[filename_String,data_List,separator_String:"\t"] :=
  With[{myfile = OpenWrite[filename]},

which, however, does not write exponents in the standard "e" format.
Furthermore, WriteMatrix is very slow (try it on a matrix of 1000 x 10
real numbers!).

The exponents are written correctly if one uses FortranForm to write the
individual numbers:

WriteFMatrix[filename_String,data_List,separator_String:"\t"] :=
  With[{myfile = OpenWrite[filename]},

This is still very slow, however. Has anybody a faster suggestion? Maybe
by using Compile[]? 

I'd appreciate very much your suggestions.

Martin Heimann (martin.heimann at

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