if ... else 
​

Like other programming languages, Forth has if ... else ... , but the structure is quite different to that with which you are used to. if treats the top-most stack entry as a boolean, and conditionally executes the code that follows. (Any non-zero value is considered as true.) then goes at the end of the code to be conditionally executed.

Here is some example code that shows you how to use if:

: test ( n -- , tests if n is true )
   if
      ." That is true"
      cr
   else
      ." That’s not true"
      cr
   then
;

Note that the if statement is terminated by then. This is somewhat unusual if you’re used to other programming languages. To show how this new word works, if we typed:

​5 test

we’d get That is true but, if we typed:

0 test

we’d then get That’s not true.
​
For a simple if statement with no need of else, the format is simply:

​: true? ( n -- , prints if n is true )
   if
      ." That is true"
      cr
   then
;

---

Now, let's learn how to create loops in Forth.