diff options
| -rw-r--r-- | forth.html.markdown | 27 |
1 files changed, 24 insertions, 3 deletions
diff --git a/forth.html.markdown b/forth.html.markdown index de0e18c2..bea7cf38 100644 --- a/forth.html.markdown +++ b/forth.html.markdown @@ -61,7 +61,7 @@ of what is written here should work elsewhere. \ And so on. -\ ------------------------------ More Advanced Stack Maniulation ------------------------------ +\ ------------------------------ Stack Maniulation ------------------------------ \ Naturally, as we do so much work with the stack, we'll want some useful methods. @@ -77,7 +77,7 @@ dup * \ square the top item 6 4 5 rot * - \ sometimes we just want to reorganize 4 0 drop 2 / \ add 4 and 0, remove 0 and divide the top by 2 -\ ------------------------------ Extra Stack Manipulation ------------------------------ +\ ------------------------------ More Advanced Stack Manipulation ------------------------------ tuck \ acts like dup, except it duplicates the top item into the 3rd* position in the stack over \ duplicate the second item to the top of the stack @@ -107,7 +107,28 @@ see square \ dup * ; ok \ ------------------------------ Conditionals ------------------------------ -\ TODO +\ Booleans: +\ In forth, -1 is used to represent truth, and 0 is used to represent false. +\ The idea behind this is that -1 is 11111111 in binary, whereas 0 is obviously 0 in binary. +\ However, any non-zero value is usually treated as being true. + +42 42 = / -1 ok +12 53 = / 0 ok + +\ `if` is a compile-only word. This means that it can *only* be used when we're compiling a word. +\ when creating conditionals, the format is <boolean> `if` <stuff to do> `then` <rest of program>. + +: ?>64 ( n -- n ) DUP 64 > if ." Greater than 64!" then ; \ ok +100 ?>64 \ Greater than 64! ok + +\ This unimaginative example displays "Greater than 64!" when the number on the stack is greater +\ than 64. However, it does nothing when the test is false. Let's fix that with the `else` word! + +: ?>64 ( n -- n ) DUP 64 > if ." Greater than 64!" else ." Less than 64!" then ; \ ok +100 ?>64 \ Greater than 64! ok +20 ?>64 \ Less than 64! ok + +\ As you can see, conditionals behave more or less like they do in most programming languages. \ ------------------------------ Loops ------------------------------ |