Bug in CertainRows/Columns

[kamalsaleh]

Hallo,
The output of the command CertainRows/CertainColumns applied on a matrix (for which HasEvalUnionOfRows/Columns is set to be true) may be wrong.

Bellow is an example of this situation.

LoadPackage( "rings" );
R := HomalgFieldOfRationals();
A := HomalgMatrix( "[ 1,2,3,4,5,6 ]", 2, 3, R );
B := HomalgMatrix( "[ 1,2,2,2,1,1, 1,1,1,0,2,4 ]", 4, 3, R );
u1 := UnionOfColumns( A, HomalgZeroMatrix( 2, 3, R ) );
u2 := UnionOfColumns( HomalgZeroMatrix( 4, 3, R ), B );
u := UnionOfRows( u1, u2 );
Display( u );
[ [ 1, 2, 3, 0, 0, 0 ],
[ 4, 5, 6, 0, 0, 0 ],
[ 0, 0, 0, 1, 2, 2 ],
[ 0, 0, 0, 2, 1, 1 ],
[ 0, 0, 0, 1, 1, 1 ],
[ 0, 0, 0, 0, 2, 4 ] ]
Display( CertainRows( u, [ 2, 1 ] ) );
[ [ 4, 5, 6, 0, 0, 0 ],
[ 1, 2, 3, 0, 0, 0 ] ]
Display( CertainRows( u, [ 3, 1 ] ) );
[ [ 1, 2, 3, 0, 0, 0 ],
[ 0, 0, 0, 1, 2, 2 ] ]

You can find a solution to this in my master branch (not sure if it is the best possible):
kamalsaleh@4879149

[sebasguts]

The command CertainRows in this case of a stacked matrix calls the method

function ( M, plist )
    local  e, A, B, a, rowsA, rowsB, plistA, plistB;
    Info( InfoCOLEM, 2, COLEM.color, "\033[01mCOLEM\033[0m ", COLEM.color, "CertainRows( UnionOfRows )", "\033[0m" );
    e := EvalUnionOfRows( M );
    A := e[1];
    B := e[2];
    a := NrRows( A );
    rowsA := [ 1 .. a ];
    rowsB := [ 1 .. NrRows( B ) ];
    plistA := Filtered( plist, function ( x )
            return x in rowsA;
        end );
    plistB := Filtered( plist - a, function ( x )
            return x in rowsB;
        end );
    return UnionOfRows( CertainRows( A, plistA ), CertainRows( B, plistB ) );
end

As you see from the method, first all rows from the upper matrix are extracted, then all rows from the lower one.
This means all rows from the first matrix are always above the rows from the second matrix.

@kamalsaleh s solution is valid, but produces a lot of matrices in between.