Dear All,

sorry my previous post was a bit short and I failed to explain. There are of course lots of solutions. Here are some more:

Select[{{0, 0, 1, 0}, {0, 0, 0, 0}, {0, 1, 0, 1}, {1, 0, 0, 0}}, #[[{1, 3}]] == {0, 0} &]

Here's another one:

Select[{{0, 0, 1, 0}, {0, 0, 0, 0}, {0, 1, 0, 1}, {1, 0, 0, 0}}, #.{1, 0, 1, 0} == 0 &]

Also there is one which is quite close to Mary's original solution:

Select[{{0, 0, 1, 0}, {0, 0, 0, 0}, {0, 1, 0, 1}, {1, 0, 0, 0}}, SubsetQ[Position[#, 0], {{1}, {3}}] &]

Note that there are tiny differences between all of these. For example we can distinguish between approximate numbers and exact numbers:

Select[{{0, 0, 1, 0}, {0, 0, 0, 0}, {0, 1, 0, 1}, {1, 0, 0, 0}}, #[[1]] == 0 && #[[3]] == 0 &]

and

Select[{{0, 0, 1, 0}, {0, 0, 0, 0}, {0, 1, 0, 1}, {1, 0, 0, 0}}, #[[1]] === 0 && #[[3]] === 0 &]

both work, but they react differently to finite precision:

Select[1. {{0, 0, 1, 0}, {0, 0, 0, 0}, {0, 1, 0, 1}, {1, 0, 0, 0}}, #[[1]] === 0 && #[[3]] === 0 &]

and

Select[1. {{0, 0, 1, 0}, {0, 0, 0, 0}, {0, 1, 0, 1}, {1, 0, 0, 0}}, #[[1]] == 0 && #[[3]] == 0 &]

give different results. Same can be achieved with Cases.

Cheers,

Marco

Thanks a lot to both of you! both solutions are sufficient.

What I don't understand yet is why I don't completely understand why the error isn't removed by formating as a list of lists:

Select[{{0,0,1,0},{0,0,0,0},{0,1,0,1},{1,0,0,0}}, SubsetQ[Position[#,0],{{1},{3}}]

Is the problem with the #? Or is it just not ok to call up a list from a list of lists?

Hi Mary,

Select[{{0, 0, 1, 0}, {0, 0, 0, 0}, {0, 1, 0, 1}, {1, 0, 0, 0}}, #[[1]] == 0 && #[[3]] == 0 &]

Cheers,

Marco