[GAP Forum] Reading strongly regular graphs into GAP

[Alexander Hulpke]

Dear Forum, Dear Robert Bailey,

> This is perhaps not strictly a GAP question, but anyway....
> 
> I've been working on some computations in GAP (using the GRAPE and other
> packages), testing certain properties of distance-regular and strongly
> [...]
> 
> The graphs are given by their adjacency matrices; however, as given
> these matrices are not immediately readable by GAP.  Does anyone know of
> an easy way to convert them into the right format

As Mathieu Dutour already wrote, the classical method is to coax your favorite P-named language (Perl, Python) to parse these matrices so that you get a valid GAP input file.
However you can also read in the file line by line to GAP and write a small routine to parse the data. Personally (with the bias that I know GAP far better than Perl or Python) I find this easier.
For example the appended program can be used to read in the lists of adjacency matrices as given on this web page, the function call

a:=ReadMatrixList("PathTofilename");;

returns a list of the adjacency matrices over Q. (You can probably take it from there.)

At the moment the function assumes single digits but it should be easy to adapt to more complicated formats.

(This function ought to work also under Windows, but as usual you need to be careful how to specify the path.)

I hope this is of help,

   Alexander Hulpke

# the promised function
ReadMatrixList:=function(file)
local l,f,m,row;
  l:=[];
  f:=InputTextFile(file);
  m:=fail;
  while not IsEndOfStream(f) do
    row:=ReadLine(f);
    if row<>fail then
      row:=Chomp(row); # remove CR,LF
      row:=Filtered(row,x->x<>' '); # remove blanks
      if Length(row)=0 then
        # new matrix
        if m<>fail and Length(m)>0 then Add(l,m);fi;
        m:=[];
      elif IsSubset(DIGITS,row) then
        # new row
        row:=List(row,x->Int([x]));
        Add(m,row);
      else
        Print("#I ",row,"\n"); # non parsed row
      fi;
    fi;
  od;
  CloseStream(f);
  if m<>fail and Length(m)>0 then Add(l,m);fi;
  return l;
end;

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