Dear Laurent,
thanks for your remark. Let me just add a few comments on it:
1) Dependently on the size of matrices and maybe other factors,
another implementation could be more CPU time saving. For
example, the following function swaps i-th and j-th row of a
matrix mat and returns the resulting matrix m:
Swap:=function(mat,i,j)
local m, t;
m := ShallowCopy(mat);
t := m[i];
m[i] := m[j];
m[j] := t;
return m;
end;
gap> e := RandomMat(1000,1000);;
gap> for i in [1..10000] do x:=Permuted(e,(1,2)); od; time;
4534
gap> for i in [1..10000] do x:=Swap(e,1,2); od; time;
211
And you can save even more time, if you can use the destructive
counterpart of this Swap function, changing the argument mat:
SwapDestructive:=function(m,i,j)
local t;
t := m[i];
m[i] := m[j];
m[j] := t;
end;
gap> for i in [1..10000] do SwapDestructive(e,1,2); od; time;
20
2) The function PermutedCols actually belongs to the GUAVA
package, see http://www.gap-system.org/Manuals/pkg/guava/htm/
chap7.html#s3ss8.
So the GUAVA package should be loaded first before calling PermutedCols
Best wishes,
Alexander
On 10 Apr 2006, at 18:24, Laurent Bartholdi wrote:
Dear Alexander, Dear Forum,
GAP does not have standard functions to swap rows or columns,
but these operations can be easily described in GAP. Please
let me know if you need more help to implement them.
In fact, GAP does have such commands -- even though they're not
documented as such. See Permuted() and PermutedCols()
--
Laurent Bartholdi \ laurent.bartholdi<at>gmail<dot>com
EPFL SB SMA IMB MAD \ Téléphone: +41 21-6935458
Station 8 \ Secrétaire: +41 21-6935501
CH-1015 Lausanne, Switzerland \ Fax: +41 21-6930339
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