A major component of computers is being able to make decisions. If
you’ve worked with other programming languages, you’ll be familiar with
common terms like if
and else
.
Let’s look at a quick example to introduce conditionals:
fr> : IS_TWO? ( n -- n ) dup 2 = if ." It's two!" then ;
ok.
fr> 2 is_two?
It's two! ok.
The complete definition is dup 2 = if ." xxx" then
. We
have a few new words we haven’t seen before here, so lets step through
this definition:
-
dup
duplicates the top element of the stack -
2
pushes a 2 onto the stack =
pops the top two values and pushesTRUE
if they’re equal andFALSE
otherwiseif
checks if the top of stack isTRUE
; if not, it skips tothen
-
." xxx"
printsxxx
-
then
signals the end of theif
statement
Thus, we get the following behavior:
fr> 2 is_two?
It's two! ok.
fr> 1 is_two?
ok.
Note how in the second case, we have no output, since the value
pushed is not equal to 2. It would be nice to have some output to tell
us this, which is where else
comes in. else
executes statements only if the if
branch did not execute.
For example:
fr> : IS_TWO? ( n -- n ) dup 2 = if ." It's two!" else ." Not two :(" then ;
ok.
fr> 2 is_two?
It's two! ok.
fr> 1 is_two?
Not two :( ok.
then
statements end the conditional. It’s important to
note that everything after then
will execute regardless
of whether the if
block executed or not. You
must include a then
to close an if
statement, or else the interpreter won’t know where to skip to after the
end of interpretation.
We can also nest if
statements. For this example, we’ll
use <
, which functions exactly like =
except that it pushes TRUE
if a < b
and
FALSE
otherwise.
fr> : is_big? ( a -- a ) dup 10 < if ." Small" else dup 20 < if ." Medium" else ." Big" then then ." number" cr ;
ok.
fr> 5 is_big?
Small number
ok.
fr> 15 is_big?
Medium number
ok.
fr> 25 is_big?
Big number
ok.
Writing long functions like this can be a little annoying. We can use
the \
to break up lines without executing functions, which
can help make them more readable:
fr> : is_big? ( a -- a ) \
+ dup 10 < \
+ if \
+ ." Small" \
+ else dup 20 < \
+ if ." Medium" \
+ else ." Big" \
+ then \
+ then \
+ ." number" ;
ok.
The interpreter will add the +
to signify that it’s
waiting for you to finish the line. If you get stuck, use
CTRL+C
to quit.
So what is happening here? It’s a function that expects a number on the stack and doesn’t consume it.
-
dup
duplicates the number -
10 <
pushesTRUE
if the number less than 10 andFALSE
otherwise - if
TRUE
on the stack, print “Small” and go to (8) -
dup
duplicates the number -
20 <
pushesTRUE
if the number less than 20 andFALSE
otherwise - if
TRUE
on the stack, print “Medium” and go to (8) - print “Big”
- print “number”
Note: Every if
needs exactly one
then
!
There are many comparators available, not just =
and
<
:
-
=
: are the top two elements equal? -
<
: is the top element greater than the first? -
>
: is the top element less than the first? -
<>
: are the top two elements not equal? -
0=
: is the top element zero? -
0<
: is the top element greater than zero? -
0>
: is the top element less than zero? -
<=
,>=
are defined as one would expect
if
uses R’s as.logical
to check if the top
value evaluates to TRUE
. This means that nonzero numbers
will be treated as TRUE
, whereas 0
will
evaluate to false. Things that cannot be converted to logicals
(e.g. 'a'
) will throw an error.
Logical Operators
Just like in other programming languages, froth
supports
a number of logical operators.
-
AND
: pushTRUE
if the top two elements are bothTRUE
-
OR
: pushTRUE
if at least one of the top two elements areTRUE
-
XOR
: pushTRUE
if exactly one of the top two elements isTRUE
-
NOT
: pushTRUE
if the top element isFALSE
and vice-versa
Some words come with built-in checks. For example, ?DUP
duplicates the top value only if it is not zero. For error-handling, you
can use ABORT"
. ABORT"
checks the stack for a
value; if it is TRUE
, it clears the stacks and prints an
error message.
Words in this chapter
-
if
: if top of stack isTRUE
, executes. Else jumps to the nextelse
orthen
block. -
else
: executes commands untilthen
only if the precedingif
did not execute. -
then
: terminates anif
orif...else
block. -
\
: signals to the interpreter that you’re making a newline without running commands -
=
: are the top two elements equal? -
<
: is the top element greater than the first? -
>
: is the top element less than the first? -
<>
: are the top two elements not equal? -
0=
: is the top element zero? -
0<
: is the top element greater than zero? -
0>
: is the top element less than zero? -
<=
: is top element greater than or equal to the second? -
>=
: is top element less than or equal to the second? -
AND
: pushTRUE
if the top two elements are bothTRUE
-
OR
: pushTRUE
if at least one of the top two elements areTRUE
-
XOR
: pushTRUE
if exactly one of the top two elements isTRUE
-
NOT
: pushTRUE
if the top element isFALSE
and vice-versa -
?DUP
: duplicate top value if it is nonzero -
ABORT"
: abort if top value true, print error message (terminated by"
)