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Finished conditionals section for now.

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HorseMD 2014-11-12 21:56:13 +00:00
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@ -61,7 +61,7 @@ of what is written here should work elsewhere.
\ And so on. \ 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. \ 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 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 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 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 over \ duplicate the second item to the top of the stack
@ -107,7 +107,28 @@ see square \ dup * ; ok
\ ------------------------------ Conditionals ------------------------------ \ ------------------------------ 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 ------------------------------ \ ------------------------------ Loops ------------------------------