Removing an ordered pair from a list of ordered pairs if the second element in the list is Less than a Value

Question

I have a list of ordered pairs from a piezoelectric sensor sampled at 2kH. The first element is time and the second, amplitude. I'm trying to remove all ordered pairs in the list of ordered pairs when the amplitude is less than 20mV.

I've been working with the Select Function

 data4cutoff = Select[data4[[All, 2]], # > 20 &] 

but this only returns a list of amplitudes. Is there a way to keep the ordered pairs with amplitudes over 20?

Answer 1

You have several alternatives to select the rows that satisfy the condition:

data = RandomReal[{0, 50}, {10, 2}]
(* {{46.6745, 22.4118}, {3.45111, 44.7088}, {23.3653, 39.9679},
    {14.3158, 17.5128}, {28.1736, 17.5333}, {49.1356, 30.1345},
    {22.2756, 2.92285}, {26.3554, 37.4685}, {32.4265, 21.1889},
    {45.3501, 38.5635}} *)

In addition to

sel1 = Select[data, #[[2]] > 20 &];

you can also use

sel2 = Cases[data, {_, _?(# > 20 &)}];
sel3 = DeleteCases[data, {_, _?(# <= 20 &)}];
sel4 = Pick[data, #[[2]] > 20 & /@ data];
sel5 = Pick[data, # > 20 & /@ data[[All, 2]]];
sel6 = data /. {{x_, y_} /; (y > 20) :> {x, y}, {_, _} :> Sequence[]};
sel7 = data /. {_, y_} /; (y <= 20) :> Sequence[]
(* {{46.67449041915749`,22.411774029202064`}, {3.451110536527459`,44.708769225023616`},
    {23.365342865421695`,39.96791941543168`}, {49.13560113208372`,30.134498577575656`},
    {26.355391411430162`,37.46849312606298`}, {32.42652381672336`,21.188851959547975`},
   {45.350061059853985`,38.56354675834069`}} *)
sel1 == sel2 == sel3 == sel4 == sel5== sel6== sel7
(* True *)