Problem Statement

Write a program which counts up from   1,   displaying each number as the multiplication of its prime factors. For the purpose of this task,   1   (unity)   may be shown as itself.

2   is prime,   so it would be shown as itself.       6   is not prime;   it would be shown as

2 × 3

{\displaystyle 2\times 3}

. 2144   is not prime;   it would be shown as

2 × 2 × 2 × 2 × 2 × 67

{\displaystyle 2\times 2\times 2\times 2\times 2\times 67}

.

Let's start with the solution:

: .factors ( n -- )
  2
  begin  2dup dup * >=
  while  2dup /mod swap
         if   drop  1+ 1 or    \ next odd number
         else -rot nip  dup . ." x "
         then
  repeat
  drop . ;

: main ( n -- )
  ." 1 : 1" cr
  1+ 2 ?do i . ." : " i .factors cr loop ;

15 main bye