Next: , Previous: , Up: Forth Tutorial   [Contents][Index]


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.