parsing a list or vector

Question

I have a table of elements that generate a list

q[i_]=i;
B=Table[q[i],{i,1,50}]

I would like to parse the list meaning, take the first 10 elements, then take every other element from 11 to 20, then take every 3rd element from 21 to 50.

Any suggestions? Thank you all

Answer 1

x = Table[i, {i, 1, 50}];

{
  Take[x, 10],
  Take[x, {11, 20, 2}],
  Take[x, {21, 50, 3}]
} // Flatten

{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 15, 17, 19, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48}

Or as oneliner

Take[x, #] & /@ {{1, 10, 1}, {11, 20, 2}, {21, 50, 3}} // Flatten

Same output

Answer 2

x = Array[q, {50}];

In addition to Take suggested in eldo's answer , you can also use Part (x[[..]])

Join @@ x[[#]] & @@@ {{;; 10}, {11 ;; 20 ;; 2}, {21 ;; 50 ;; 3}}
(* {q[1], q[2],q[3],q[4],q[5],q[6],q[7],q[8],q[9],q[10],
    q[11],q[13],q[15],q[17],q[19],
    q[21],q[24],q[27],q[30],q[33],q[36],q[39],q[42],q[45], q[48]} *)

Or Extract

Join @@ Extract[x, {{Range[10]}, {11 ;; 20 ;; 2}, {21 ;; 50 ;; 3}}]
(* same output *)

See also: Undocumented form of Extract