The if-else-then Structure

The commands if, else and then provide a simple control structure.

The commands listed in Table 4-28 control the flow of conditional execution.

Table 4-28 ifelsethen Commands

Command	Stack Diagram	Description
if	( flag -- )	Execute the following code when flag is true.
else	( -- )	Execute the following code when flag is false.
then	( -- )	Terminate ifelsethen.

The format for using these commands is:

---

flag	if
	(do this if true)
then	
(continue normally) 

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or

---

flag	if
	(do this if true)
else
	(do this if false)
then	
(continue normally) 

---

The if command consumes a flag from the stack. If the flag is true (nonzero), the commands following the if are performed. Otherwise, the commands (if any) following the else are performed.

---

ok : testit  ( n -- ) 
] 5 >  if  ." good enough " 
] else  ." too small " 
] then 
] ." Done. "  ; 
ok
ok 8 testit
good enough Done. 
ok 2 testit 
too small Done. 
ok 

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Note -

The ] prompt reminds you that you are part way through creating a new colon definition. It reverts to ok after you finish the definition with a semicolon.

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