[GAP Forum] Adding One to a Vector

[Max Neunhoeffer]

Dear Ian,

The following completes in 35 seconds on one of our machines and
probably does what you want:

i := Iterator(GF(2)^24);
while not(IsDoneIterator(i)) do
    v := NextIterators(i);
    # Do something with v
od;

It produces all vectors you want one after another. Unfortunately,
the vectors coming out of this are not yet compressed. So you probably
want to add a "ConvertToVectorRep(v,2)" in the loop, depending on your
applications.

Also you cannot change the order in which the vectors occur.

Note that there is also an enumerator, but this is much slower.

I hope this helps, if not, please do not hesitate to ask again.

Best regards,
  Max.

On Mon, Mar 05, 2007 at 10:44:10AM +0000, Ian McLoughlin wrote:
> Hi everyone,
> 
> I need to loop over all the vectors of length 24 over GF(2) of weight at
> least 8. The current way I'm doing this is the following....
> 
> 
> AddOne := function( vector )
> 
>        local i ;;
> 
>        for i in [24,23..1] do
> 
>                if ( vector[i] = 0*Z(2) ) then
> 
>                        vector[i] := 1*Z(2) ;;
>                        break ;;
> 
>                fi ;;
> 
>                if ( vector[i] = 1*Z(2) ) then
> 
>                        vector[i] := 0*Z(2) ;;
> 
>                fi ;;
> 
>        od ;;
> 
> end ;;
> 
> 
> start      := Z(2) * [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,1,1,1,1,1,1,0]  ;;
> zerovec  := Z(2) * [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]  ;;
> 
> ConvertToVectorRep( start   , GF(2) ) ;;
> ConvertToVectorRep( zerovec , GF(2) ) ;;
> 
> while ( start <> zerovec ) do
> 
>                AddOne( start ) ;;
> 
>                if ( WeightVecFFE( start ) < 8 ) then
>                        continue ;;
>                fi ;;
> 
> od ;;
> 
> 
> 
> Obviously I do some stuff with each new value of start, but I've omitted it
> to make my email less ugly. :)
> The reason I wrote my own method called AddOne was because I couldn't find
> any Gap method that did the equivalent of c's "start++;".
> Apologies for representing the vector backwards(compared to the usual in
> Gap) by the way!
> 
> My question is this: does Gap have an inbuilt method to add one to a binary
> vector as if the whole vector represented a single number(i.e. with
> carrying)?
> If anyone has a suggestion as to how to loop through all binary vectors of
> length 24 and weight at least eight another way I'd love to hear it too! My
> main concern is speed!
> 
> Thanks a lot,
> Ian
> 
> 
> -- 
> Ian McLoughlin
> emailme at ianmcloughlin.com
> _______________________________________________
> Forum mailing list
> Forum at mail.gap-system.org
> http://mail.gap-system.org/mailman/listinfo/forum
> 

-- 
   # #   Max Neunhoeffer <max.neunhoeffer at math.rwth-aachen.de>
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   # #   May the Source be with you!