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Bug in MatrixEntries #164

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wilfwilson opened this issue Apr 22, 2016 · 2 comments
Closed

Bug in MatrixEntries #164

wilfwilson opened this issue Apr 22, 2016 · 2 comments
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@wilfwilson
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MatrixEntries returns the set of elements contained in the matrix of a Rees (0-)matrix semigroup.

This is not documented, and so the expected behaviour is not specified.

Currently MatrixEntries returns Union(Matrix(R){Columns(R)}{Rows(R)});. The function Union returns a sorted list, however it is not always possible to compare certain elements with 0 (which may be a matrix entry), leading to an error:

gap> S := Semigroup(SymmetricInverseMonoid(2));;
gap> id := Identity(S);;
gap> R := ReesZeroMatrixSemigroup(S, [[id], [0]]);;
gap> MatrixEntries(R);
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `<' on 2 arguments at /Users/Wilf/GAP/lib/methsel2.g:241 called from
Set( Concatenation( tounite )
 ) at /Users/Wilf/GAP/lib/coll.gi:2788 called from
Union( Matrix( x ){Columns( x )}{Rows( x )}
 ) at /Users/Wilf/GAP/pkg/semigroups/gap/semigroups/semirms.gi:340

This bug is present in stable-2.7 and unstable-3.0.

I will submit a pull request to fix this problem in each branch, by calling Unique instead of Union to remove duplicates without sorting. Another option would be to always place 0 first (if present) and then sort the remaining entries (which should all be comparable, as they come from the same semigroup) and append them to the end of [0].
@wilfwilson wilfwilson self-assigned this Apr 22, 2016
@wilfwilson wilfwilson added the bug Label for issues or PR which report or fix bugs label Apr 22, 2016
wilfwilson added a commit to wilfwilson/Semigroups that referenced this issue Apr 22, 2016
@wilfwilson
Fix MatrixEntries (Issue semigroups#164)
863f9e6 
MatrixEntries fails for some Rees 0-matrix semigroups whose
matrices contain 0, since MatrixEntries removes duplicates and
sorts using Union, however 0 is not comparable with all
semigroup elements.

This issue is resolved by using Unique to remove duplicates,
but not sorting the list of matrix entries.

Resolves: semigroups#164
wilfwilson added a commit to wilfwilson/Semigroups that referenced this issue Apr 22, 2016
@wilfwilson
Fix MatrixEntries (Issue semigroups#164)
c9e6ecd 
MatrixEntries fails for some Rees 0-matrix semigroups whose
matrices contain 0, since MatrixEntries removes duplicates and
sorts using Union, however 0 is not comparable with all
semigroup elements.

This issue is resolved by using Unique to remove duplicates,
but not sorting the list of matrix entries.

Resolves: semigroups#164
This was referenced Apr 22, 2016
Merged
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@james-d-mitchell
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james-d-mitchell commented Apr 28, 2016 •
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@wilfwilson I think the last thing you suggest sounds like the best idea. Sort the non-zero entries, then append them to [0]. Can you please amend the PRs to do this?

wilfwilson added a commit to wilfwilson/Semigroups that referenced this issue Apr 28, 2016
@wilfwilson
Fix MatrixEntries (Issue semigroups#164)
0acc7b6 
MatrixEntries fails for some Rees 0-matrix semigroups whose
matrices contain 0, since MatrixEntries removes duplicates and
sorts using Union, however 0 is not comparable with all
semigroup elements.

This issue is resolved by using Unique to remove duplicates,
putting 0 (if present) at the start of the list, and then sorting
the remaining elements (which are all from the same
semigroup, and are hence sortable).

Resolves: semigroups#164
wilfwilson added a commit to wilfwilson/Semigroups that referenced this issue Apr 28, 2016
@wilfwilson
Fix MatrixEntries (Issue semigroups#164)
8de3c31 
MatrixEntries fails for some Rees 0-matrix semigroups whose
matrices contain 0, since MatrixEntries removes duplicates and
sorts using Union, however 0 is not comparable with all
semigroup elements.

This issue is resolved by using Unique to remove duplicates,
putting 0 (if present) at the start of the list, and then sorting
the remaining elements (which are all from the same
semigroup, and are hence sortable).

Resolves: semigroups#164
@wilfwilson
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@james-d-mitchell I've amended the PRs as requested.

wilfwilson added a commit to wilfwilson/Semigroups that referenced this issue May 1, 2016
@wilfwilson
Fix MatrixEntries (Issue semigroups#164)
890ea24 
MatrixEntries fails for some Rees 0-matrix semigroups whose
matrices contain 0, since MatrixEntries removes duplicates and
sorts using Union, however 0 is not comparable with all
semigroup elements.

This issue is resolved by using Unique to remove duplicates,
putting 0 (if present) at the start of the list, and then sorting
the remaining elements (which are all from the same
semigroup, and are hence sortable).

Resolves: semigroups#164
wilfwilson added a commit to wilfwilson/Semigroups that referenced this issue May 1, 2016
@wilfwilson
Fix MatrixEntries (Issue semigroups#164)
c6cbdc1 
MatrixEntries fails for some Rees 0-matrix semigroups whose
matrices contain 0, since MatrixEntries removes duplicates and
sorts using Union, however 0 is not comparable with all
semigroup elements.

This issue is resolved by using Unique to remove duplicates,
putting 0 (if present) at the start of the list, and then sorting
the remaining elements (which are all from the same
semigroup, and are hence sortable).

Resolves: semigroups#164
wilfwilson added a commit to wilfwilson/Semigroups that referenced this issue May 1, 2016
@wilfwilson
Fix MatrixEntries (Issue semigroups#164)
e02608d 
MatrixEntries fails for some Rees 0-matrix semigroups whose
matrices contain 0, since MatrixEntries removes duplicates and
sorts using Union, however 0 is not comparable with all
semigroup elements.

This issue is resolved by using Unique to remove duplicates,
putting 0 (if present) at the start of the list, and then sorting
the remaining elements (which are all from the same
semigroup, and are hence sortable).

Resolves: semigroups#164
wilfwilson added a commit to wilfwilson/Semigroups that referenced this issue May 1, 2016
@wilfwilson
Fix MatrixEntries (Issue semigroups#164)
a8b5198 
MatrixEntries fails for some Rees 0-matrix semigroups whose
matrices contain 0, since MatrixEntries removes duplicates and
sorts using Union, however 0 is not comparable with all
semigroup elements.

This issue is resolved by using Unique to remove duplicates,
putting 0 (if present) at the start of the list, and then sorting
the remaining elements (which are all from the same
semigroup, and are hence sortable).

Resolves: semigroups#164
wilfwilson added a commit to wilfwilson/Semigroups that referenced this issue May 1, 2016
@wilfwilson
Fix MatrixEntries (Issue semigroups#164)
a48001a 
MatrixEntries fails for some Rees 0-matrix semigroups whose
matrices contain 0, since MatrixEntries removes duplicates and
sorts using Union, however 0 is not comparable with all
semigroup elements.

This issue is resolved by using Unique to remove duplicates,
putting 0 (if present) at the start of the list, and then sorting
the remaining elements (which are all from the same
semigroup, and are hence sortable).

Resolves: semigroups#164
wilfwilson added a commit to wilfwilson/Semigroups that referenced this issue May 5, 2016
@wilfwilson
Fix MatrixEntries (Issue semigroups#164)
1c40557 
MatrixEntries fails for some Rees 0-matrix semigroups whose
matrices contain 0, since MatrixEntries removes duplicates and
sorts using Union, however 0 is not comparable with all
semigroup elements.

This issue is resolved by using Unique to remove duplicates,
putting 0 (if present) at the start of the list, and then sorting
the remaining elements (which are all from the same
semigroup, and are hence sortable).

Resolves: semigroups#164
wilfwilson added a commit to wilfwilson/Semigroups that referenced this issue May 5, 2016
@wilfwilson
Fix MatrixEntries (Issue semigroups#164)
fee7a4f 
MatrixEntries fails for some Rees 0-matrix semigroups whose
matrices contain 0, since MatrixEntries removes duplicates and
sorts using Union, however 0 is not comparable with all
semigroup elements.

This issue is resolved by using Unique to remove duplicates,
putting 0 (if present) at the start of the list, and then sorting
the remaining elements (which are all from the same
semigroup, and are hence sortable).

Resolves: semigroups#164
wilfwilson added a commit to wilfwilson/Semigroups that referenced this issue May 5, 2016
@wilfwilson
Fix MatrixEntries (Issue semigroups#164)
45fc79d 
MatrixEntries fails for some Rees 0-matrix semigroups whose
matrices contain 0, since MatrixEntries removes duplicates and
sorts using Union, however 0 is not comparable with all
semigroup elements.

This issue is resolved by using Unique to remove duplicates,
putting 0 (if present) at the start of the list, and then sorting
the remaining elements (which are all from the same
semigroup, and are hence sortable).

Resolves: semigroups#164
wilfwilson added a commit to wilfwilson/Semigroups that referenced this issue May 5, 2016
@wilfwilson
Fix MatrixEntries (Issue semigroups#164)
44c5748 
MatrixEntries fails for some Rees 0-matrix semigroups whose
matrices contain 0, since MatrixEntries removes duplicates and
sorts using Union, however 0 is not comparable with all
semigroup elements.

This issue is resolved by using Unique to remove duplicates,
putting 0 (if present) at the start of the list, and then sorting
the remaining elements (which are all from the same
semigroup, and are hence sortable).

Resolves: semigroups#164
wilfwilson added a commit to wilfwilson/Semigroups that referenced this issue May 5, 2016
@wilfwilson
Fix MatrixEntries (Issue semigroups#164)
39c97b6 
MatrixEntries fails for some Rees 0-matrix semigroups whose
matrices contain 0, since MatrixEntries removes duplicates and
sorts using Union, however 0 is not comparable with all
semigroup elements.

This issue is resolved by using Unique to remove duplicates,
putting 0 (if present) at the start of the list, and then sorting
the remaining elements (which are all from the same
semigroup, and are hence sortable).

Resolves: semigroups#164
james-d-mitchell added a commit that referenced this issue May 5, 2016
@james-d-mitchell
Merge pull request #166 from wilfwilson/fix-matrixentries-stable-2.7
5356c47 
Fix MatrixEntires in stable-2.7 (Issue #164)
james-d-mitchell added a commit that referenced this issue May 5, 2016
@james-d-mitchell
Merge pull request #165 from wilfwilson/fix-matrixentries-unstable-3.0
6d74b20 
Fix MatrixEntires in unstable-3.0 (Issue #164)
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