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:
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:
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
;
CASE
Some versions of Forth include support for case statements. This is not part of the standard FlashForth build, but you can add it yourself...
\ *********************************************************************
\ Case for FlashForth *
\ Filename: case.txt *
\ Date: 26.01.2014 *
\ FF Version: 5.0 *
\ Copyright: Mikael Nordman *
\ Author: Mikael Nordman *
\ *********************************************************************
\ FlashForth is licensed acording to the GNU General Public License*
\ *********************************************************************
\ A case implementation posted by Jenny Brien on c.l.f.
\ Modified to use for..next instead of do..loop
-case
marker -case
hex ram
\ of compare
: (of) ( n1 n2 -- n1 flag )
inline over
inline -
0=
;
: case ( -- #of )
0
; immediate
: of ( #of -- #of orig )
postpone (of) ( copy and test case value)
postpone if ( add orig to control flow stack )
postpone drop ( discard case value if case is matching )
; immediate
: default ( #of -- #of orig )
postpone true ( Force to take the default branch )
postpone if ( add orig to control flow stack )
postpone drop ( discard case value )
; immediate
: endof ( orig1 -- orig2 #of )
postpone else
swap 1+
; immediate
: endcase ( orig1..orign #of -- )
postpone drop ( discard case value )
for
postpone then ( resolve of branches )
next
; immediate
As an example of how to use case:
: case-test
case
2 of ." two " 2222 endof
3 of ." three " 3333 endof
default ." default " 9999 endof
endcase
u.
;
2 case-test
3 case-test
8 case-test
: case-test2
case
11 of endof
default endof
endcase
;
Now, let's learn how to create loops in Forth.