3.19 Counted loops

     : ^ ( n1 u -- n )
     \ n = the uth power of n1
       1 swap 0 u+do
         over *
       loop
       nip ;
     3 2 ^ .
     4 3 ^ .

U+do (from compat/loops.fs, if your Forth system doesn't have it) takes two numbers of the stack ( u3 u4 -- ), and then performs the code between u+do and loop for u3-u4 times (or not at all, if u3-u4<0).

You can see the stack effect design rules at work in the stack effect of the loop start words: Since the start value of the loop is more frequently constant than the end value, the start value is passed on the top-of-stack.

You can access the counter of a counted loop with i:

     : fac ( u -- u! )
       1 swap 1+ 1 u+do
         i *
       loop ;
     5 fac .
     7 fac .

There is also +do, which expects signed numbers (important for deciding whether to enter the loop).

Assignment: Write a definition for computing the nth Fibonacci number.

You can also use increments other than 1:

     : up2 ( n1 n2 -- )
       +do
         i .
       2 +loop ;
     10 0 up2
     
     : down2 ( n1 n2 -- )
       -do
         i .
       2 -loop ;
     0 10 down2

Reference: Counted Loops.