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Add outline of loops.

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HorseMD 2014-11-12 22:33:04 +00:00
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1 changed files with 37 additions and 3 deletions
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@ -110,12 +110,12 @@ see square \ dup * ; ok
\ Booleans: \ Booleans:
\ In forth, -1 is used to represent truth, and 0 is used to represent false. \ 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. \ 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. \ However, any non-zero value is usually treated as being true:
42 42 = / -1 ok 42 42 = / -1 ok
12 53 = / 0 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. \ `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>. \ 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 : ?>64 ( n -- n ) DUP 64 > if ." Greater than 64!" then ; \ ok
@ -132,7 +132,41 @@ see square \ dup * ; ok
\ ------------------------------ Loops ------------------------------ \ ------------------------------ Loops ------------------------------
\ TODO \ `do` is like `if` in that it is also a compile-only word, though it uses `loop` as its
\ terminator.
: myloop ( -- ) 5 0 do cr ." Hello!" loop ; \ ok
test
\ Hello!
\ Hello!
\ Hello!
\ Hello!
\ Hello! ok
\ `do` expects two numbers before it: the end number and the index number, respectively.
\ (cr means carraige-return, essentially it a newline). This is equivalent to a for-loop
\ in other languages, with a definite number of times to loop.
\ So what if we want to get the value of the index as we loop? We use `i`.
: one-to-15 ( -- ) 15 0 do i . loop ; \ ok
one-to-15 \ 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ok
: squares ( -- ) 10 0 do i DUP * . loop ; \ ok
squares \ 0 1 4 9 16 25 36 49 64 81 ok
\ Thidly, we can also change how large the step is between each loop iteration with `+loop`.
\ `+loop` reads the number on the top of the stack for how far to move each iteration.
: threes ( -- ) 15 0 do i . 3 +loop ; \ ok
threes \ 0 3 6 9 12 ok
\ Finally, while loops:
: death ( -- ) begin ." Are we there yet?" 0 until ;
\ Will print "Are we there yet?" forever. While loops are constructed in the format
\ of `begin` <stuff to do> <flag> `until`. The loop will run until flag is a
\ truthy value (not 0).
\ ------------------------------ The Return Stack ------------------------------ \ ------------------------------ The Return Stack ------------------------------