Extracting data from a table under conditions

Question

I have a very large table with two columns. Let's say the name of first column is E and the name of the second is P. How can I get the E values when P = 0.

Answer 1

  tbl = RandomInteger[{0, 3}, {10, 2}]
  (* {{0, 3}, {0, 0}, {1, 3}, {2, 0}, {2, 0}, {0, 0}, {1, 1}, {2, 2}, {1, 0}, {3, 3}}*)

You have many alternative methods:

  Cases[tbl, {x_, 0} :> x] 
  (* or *) Cases[tbl, {_, 0}][[All, 1]]
  (* or *) DeleteCases[tbl, {_, Except[0]}][[All, 1]]
  (* or *) Select[tbl, Last[#] == 0 &][[All, 1]]
  (* or *) Pick[tbl[[All, 1]], tbl[[All, 2]], 0]
  (* or *) Pick[tbl[[All, 1]], # == 0 & /@ tbl[[All, 2]]]
  (* or *) tbl /. {{x_, 0} :> x, {_, _} :> Sequence[]}
  (* or *) tbl[[Flatten@Position[tbl[[All, 2]], 0], 1]]
  (* or *) tbl[[Flatten@Position[tbl, {_, 0}], 1]]
  (* or *) Extract[tbl[[All, 1]], Position[tbl, {_, 0}]]
  (* or *) Extract[#[[1]], Position[#[[2]], 0]] &@Transpose@tbl
  (* or *) Delete[tbl, Position[tbl, {_, Except[0]}]][[All, 1]]

all give

 {0, 2, 2, 0, 1}

Answer 2

Something like

Pick @@ Transpose@largetable~Join~{0}

might do it. Unless 0 should be the Real number 0. If you have both, then try

Pick @@ Transpose@largetable~Join~{0 | 0.}

Edit: The above is the same as Apply[Pick, Join[Transpose[largetable], {0 | 0.}]] and has the same effect as

With[{columns = Transpose[largetable]},
 Pick[columns[[1]], columns[[2]], 0 | 0.]]

provided largetable is of the form {{E1, P1}, {E2, P2}, ...}.

Edit 2: Example

largetable = Table[{i, RandomInteger[]}, {i, 10}]

Out[1]= {{1, 0}, {2, 0}, {3, 0}, {4, 1}, {5, 0}, {6, 0}, {7, 0}, {8, 1}, {9, 1}, {10, 0}}

With[{columns = Transpose[largetable]}, 
 Pick[columns[[1]], columns[[2]], 0 | 0.]]

Out[2]= {1, 3, 5, 7, 8, 9, 10}