Select elements of a list based on membership in another list

Question

I have two lists:

a={{"2021-01-04", 12.48, 12.49, 10.53, 11.01, 11.01,35826600}, {"2021-01-05", 10.7, 14.3, 10.69, 13.59, 13.59,61984100}, {"2021-01-06", 14.58, 17.58, 14.25, 16.96, 16.96,83119900}, {"2021-01-07", 20.18, 24.35, 18.55, 22.36, 22.36,118963600}, {"2021-01-08", 25.06, 28.37, 23.8, 26.39, 26.39,97428200}, {"2021-01-11", 20., 25.44, 19.6, 23.36, 23.36,93551100}, {"2021-01-12", 24., 26.92, 22.81, 26.15, 26.15,57414200}, {"2021-01-13", 22.39, 23.66, 21.65, 23., 23.,69463000}, {"2021-01-14", 25.91, 26.55, 23.73, 24.44, 24.44,55343600}};

and

b={{"2021-01-01", 28994., 29600.6, 28803.6, 29374.2, 29374.2,40730301359}, {"2021-01-02", 29376.5, 33155.1, 29091.2, 32127.3,32127.3, 67865420765}, {"2021-01-03", 32129.4,34608.6,32052.3,32782., 32782., 78665235202}, {"2021-01-04", 32810.9, 33440.2,28722.8, 31971.9, 31971.9, 81163475344}, {"2021-01-05", 31977.,34437.6, 30221.2, 33992.4, 33992.4, 67547324782}, {"2021-01-06",34013.6, 36879.7, 33514., 36824.4, 36824.4,75289433811}, {"2021-01-07", 36833.9, 40180.4, 36491.2, 39371.,39371., 84762141031}, {"2021-01-08", 39381.8, 41946.7, 36838.6,40797.6, 40797.6, 88107519480}, {"2021-01-09", 40788.6, 41436.4,38980.9, 40254.5, 40254.5, 61984162837}}

I would like to select the elements of b where the first element is a member of the first column of a; i.e., a[[All,1]].

Answer 1

Is this what you mean?

Select[b, MemberQ[a[[All, 1]], #[[1]]] &] 

Answer 2

1. Find the intersection of the first column of a and b called alist and blist respectively.
2. Find the position of each of the common items in the first column of b.
3. Extract the corresponding rows from b.

    alist = a[[All, 1]];
    blist = b[[All, 1]];
    res1 = Flatten[#, 1] &@
      Extract[b, Position[blist, #] & /@ Intersection[alist, blist]]

---

1. Pick the row from b if the member in the first column of b is in the first column of a

    res2 = Pick[b, (MemberQ[alist, #] & /@ blist)]

---

    res3 = Cases[b, {x_String, __} /; MemberQ[a[[All, 1]], x]]

---

Result

$${{{"2021-01-04", 32810.9, 33440.2, 28722.8, 31971.9, 31971.9, 81163475344}}, {{"2021-01-05", 31977., 34437.6, 30221.2, 33992.4, 33992.4, 67547324782}}, {{"2021-01-06", 34013.6, 36879.7, 33514., 36824.4, 36824.4, 75289433811}}, {{"2021-01-07", 36833.9, 40180.4, 36491.2, 39371., 39371., 84762141031}}, {{"2021-01-08", 39381.8, 41946.7, 36838.6, 40797.6, 40797.6, 88107519480}}}$$

---

res1 == res2 == res3

True

Answer 3

Using Table and IntersectingQ:

First[#]&/@DeleteCases[Table[If[IntersectingQ[a[[i]], b[[j]]] === True, b[[j]], Nothing], {i, 1, Length[a]}, {j, 1, Length[b]}], {}]