3.6 Stack Manipulation

Stack manipulation words rearrange the data on the stack.

     1 .s drop .s
     1 .s dup .s drop drop .s
     1 2 .s over .s drop drop drop
     1 2 .s swap .s drop drop
     1 2 3 .s rot .s drop drop drop

These are the most important stack manipulation words. There are also variants that manipulate twice as many stack items:

     1 2 3 4 .s 2swap .s 2drop 2drop

Two more stack manipulation words are:

     1 2 .s nip .s drop
     1 2 .s tuck .s 2drop drop

Assignment: Replace nip and tuck with combinations of other stack manipulation words.

          Given:          How do you get:
          1 2 3           3 2 1
          1 2 3           1 2 3 2
          1 2 3           1 2 3 3
          1 2 3           1 3 3
          1 2 3           2 1 3
          1 2 3 4         4 3 2 1
          1 2 3           1 2 3 1 2 3
          1 2 3 4         1 2 3 4 1 2
          1 2 3
          1 2 3           1 2 3 4
          1 2 3           1 3
     

     5 dup * .

Assignment: Write 17^3 and 17^4 in Forth, without writing 17 more than once. Write a piece of Forth code that expects two numbers on the stack (a and b, with b on top) and computes (a-b)(a+1).

Reference: Stack Manipulation.