Extra brackets of Position command

Question

Command Position always comes with two extra curly brackets {{}} when it's just one index for the location. I use [[1,1]] in front of my position commands. Is there a better way?

Answer 1

No, to my knowledge there isn't a better way, but there is a good reason for this behaviour.

First, the element might appear more than once, so Position must return a list:

Position[{1, 2, 1, 3}, 1]

(* ==> {{1}, {3}} *)

Second, you might be searching a more complicated compound expression than a list, e.g. a matrix, so each list element might need to hold more than one index:

Position[{{1, 2, 1, 3}, {4, 5, 1, 6}}, 1]

(* ==> {{1, 1}, {1, 3}, {2, 3}} *)

Position[{1, {2, 1}}, 1]

(* ==> {{1}, {2, 2}} *)

Each index set that Position returns can be directly used in the Extract function, which is sometimes more convenient to use with Positions's return value than Part.

You might think that it would be better if Position returned a single number as a special case when there's a single match only in a one-dimensional list, such as

Position[{1, 2, 3}, 1]

(* ==> {{1}} *)

If Position did this, programs using it would not be easily generalizable to different inputs and the user would sometimes need to handle this special case.

If you only need the first occurrence of the pattern, you might consider using the fourth argument to your advantage (as Coolwater has pointed out in a comment above):

Position[list, patt, Infinity, 1]

immediately terminates when the first match is found so it is really useful for large redundant lists. If you are only interested in matches of the topmost level, use the third argument:

Position[list, patt, 1, 1]

This is equivalent to passing a vector as list, that only has depth 1.