The do Loop

A do loop (also called a counted loop) is used when the number of iterations of the loop can be calculated in advance. A do loop normally exits just before the specified ending value is reached.

Table 4-24 lists commands to control the execution of counted loops.

Table 4-24 do (Counted) Loop Commands

Command	Stack Diagram	Description
+loop	( n -- )	End a do+loop construct; add n to loop index and return to do (if n < 0, index goes from start to end inclusive).
?do	( end start -- )	Begin ?doloop to be executed 0 or more times. Index goes from start to end-1 inclusive. If end = start, loop is not executed.
?leave	( flag -- )	Exit from a doloop if flag is non-zero.
do	( end start -- )	Begin a doloop. Index goes from start to end-1 inclusive. Example: 10 0 do i . loop (prints 0 1 2...d e f).
i	( -- n )	Leaves the loop index on the stack.
j	( -- n )	Leaves the loop index of the next outer enclosing loop on the stack.
leave	( -- )	Exit from doloop.
loop	( -- )	End of doloop.

This screen shows several examples of the ways in which loops are used.

---

ok 10 5 do  i .  loop 
5 6 7 8 9 a b c d e f
ok
ok 2000 1000 do i .  i c@ . cr   i c@ ff = if leave then  4 +loop 
1000 23
1004 0
1008 fe
100c 0
1010 78
1014 ff
ok : scan ( byte -- ) 
]    6000 5000     (Scan memory 5000 - 6000 for bytes not equal to the specified byte)
]    do dup i c@ <> (  byte error? )  
]      if i . then  ( byte ) 
]    loop 
]    drop ( the original byte was still on the stack, discard it ) 
]  ; 
ok 55 scan 
5005 5224 5f99 
ok 6000 5000 do i i c! loop     (Fill a region of memory with a stepped pattern)
ok       
ok 500 value testloc 
ok : test16 ( -- ) 1.0000 0 ( do 0-ffff )     (Write different 16-bit values to a location)
]     do i testloc w! testloc w@ i <> ( error? )     (Also check the location)
]       if ." Error - wrote " i . ." read " testloc w@ . cr 
]        leave ( exit after first error found )     (This line is optional)
]       then 
]     loop 
]  ; 
ok test16 
ok 6000 to testloc 
ok test16 
Error - wrote 200 read 300 
ok 

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