MATLAB: Finding Missing elements in a vector

Question

I have a vector P =

     1
     2
     3
     4
     5
     6
     7
     8
     9
    10
    11
    12
    13
    14
    15
    16
    17
     2
    19
    20
    21
     3
    23
    24
     6
    26
    27
    28
    29
    30
    31
    32
As per the ascending order (1 2...15..32) of vector P, the elements 18,25,28 are missing and their position is occupied by the elements 2,3,6. now i want a vector which indicates this numbers like n=[2 3 6]. if elements are in order perfectly (1:10), I don't want new vector n.

Answer 1

This will do the trick

P(~(P==1:length(P)))

just for clarification

   P==1:length(P)

returns a logical array of the position occupied by the right number.

   ~(P==1:length(P))

is the logical array of the position occupied by the WRONG number

   P(~(P==1:length(P)))

get the values in those positions.