Generate a list in Mathematica with a conditional tested for each element

Question

Suppose we want to generate a list of primes p for which p + 2 is also prime.

A quick solution is to generate a complete list of the first n

Answer 1

Well, someone has to allocate memory somewhere for the full table size, since it is not known before hand what the final size will be.

In the good old days before functional programming :), this sort of thing was solved by allocating the maximum array size, and then using a separate index to insert to it so no holes are made. Like this

x=Table[0,{100}];  (*allocate maximum possible*)
j=0;
Table[ If[PrimeQ[k+2], x[[++j]]=k],{k,100}];

x[[1;;j]]  (*the result is here *)

{1,3,5,9,11,15,17,21,27,29,35,39,41,45,51,57,59,65,69,71,77,81,87,95,99}