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How do I determine the position of rows whose entries all satisfy some condition (e.g., all elements of the row are positive)? I've found out how to use Select to extract the correct rows, but I haven't been able to find out how to use Position to do the equivalent.

A = {{-1, 2, 3}, {4, 5, 6}, {-7, 8, 9}}
Select[A, And @@ Thread[# > 0] &]

gives

{{4, 5, 6}}

but I want to know the position of that row (i.e. {2}).

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    $\begingroup$ Position[And @@ Thread[# > 0] & /@ A, True]? $\endgroup$
    – kglr
    Feb 4 '15 at 22:54
  • $\begingroup$ wonderful. thanks! $\endgroup$
    – user12734
    Feb 5 '15 at 0:11
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Use the function VectorQ,

A = {{-1, 2, 3}, {4, 5, 6}, {-7, 8, 9}};
Position[A, x_ /; VectorQ[x, # > 0 &], {1}]

or

Position[VectorQ[#, # > 0 &] & /@ A, True, {1}]

As Mr.Wizard points out that you should use a levelspec {1} to avoid testing subexpressions.

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    $\begingroup$ A good choice as VectorQ has early exit behavior, and does not unpack packed arrays. +1 (Edit: however, you should use a levelspec {1} to avoid testing subexpressions and losing this benefit.) $\endgroup$
    – Mr.Wizard
    Feb 5 '15 at 8:31
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    $\begingroup$ @Mr.Wizard is there any nice thread what can unpack? $\endgroup$
    – Kuba
    Feb 5 '15 at 8:31
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    $\begingroup$ @Kuba You mean this? $\endgroup$
    – Mr.Wizard
    Feb 5 '15 at 8:32
  • $\begingroup$ @Mr.Wizard it seems so, thanks :) $\endgroup$
    – Kuba
    Feb 5 '15 at 8:33
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aA = {{-1, 2, 3}, {4, 5, 6}, {-7, 8, 9}};

Position[And @@ Thread[# > 0] & /@ aA, True]
(* {{2}} *)

Or, using the third argument of Position to set the levelspec to 1:

Position[aA, _?(And @@ Thread[# > 0] &), 1]
(* {{2}} *)
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Without #, yeah:

Position[A, {__?Positive}, {1}]
{{2}}
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