Finding the position of a specific value in a list

Question

Is it possible to easily find the position of a specific value in a list? For example, if the list is {{x1,y1},{x2,y2},...,{xn,yn}} I would like to know the position of y = 123456.78. I would prefer to not use If.

Answer 1

You can use the Position function. Which returns a list of the positions of the pattern you are searching for.

k = RandomInteger[10, {10, 2}]
(* ->{{2, 10}, {9, 3}, {9, 10}, {8, 1}, {2, 0}, {10, 10}, {10, 0}, {10,6}, {3, 1}, {3, 2}} *)

Position[k, 3]

returns

{{2, 2}, {9, 1}, {10, 1}}

Which is a list of the indices of the three occurrences of 3 in the list, k.

Answer 2

Position[] is the function you want:

la = RandomInteger[{-9, 9}, {8, 2}]
{{-4, -4}, {1, -8}, {3, 0}, {-2, -3}, {-3, -1}, {4, 9}, {2, 4}, {0, -2}}

Position[la, 0]
{{3, 2}, {8, 1}}

(* check results *)
la[[##]] & @@@ %
{0, 0}