Stack Words II

Here are a few more useful built-in words for the stack:

Word Action

.S

Displays the number and contents of the stack without altering it.

OVER

1 2 OVER .S
becomes
<3> 1 2 1

NIP

1 2 NIP .S
becomes
<1> 2

TUCK

1 2 TUCK .S
becomes
<3> 2 1 2

DEPTH

Puts the number of items on the stack. For example,
"Hello" "Smojo" DEPTH .S
becomes
<3> "Hello" "Smojo" 2

Word	Action
.S	Displays the number and contents of the stack without altering it.
OVER	1 2 OVER .S becomes <3> 1 2 1
NIP	1 2 NIP .S becomes <1> 2
TUCK	1 2 TUCK .S becomes <3> 2 1 2
DEPTH	Puts the number of items on the stack. For example, "Hello" "Smojo" DEPTH .S becomes <3> "Hello" "Smojo" 2

The Return Stack

In addition to the main stack, Smojo also has a "return" stack. This is often used for temporary storage of items. Some words:

Word Action

.R

Displays the number and contents of the return stack without altering it.

>R

Pops the top of the stack and pushes it onto the return stack.

R>

Pops the top of the return stack and pushes it onto the main stack.

R@

Pushes the top of the return stack (without popping it) onto the main stack.

RDROP

Drops an item from the top of the return stack.

Word	Action
.R	Displays the number and contents of the return stack without altering it.
>R	Pops the top of the stack and pushes it onto the return stack.
R>	Pops the top of the return stack and pushes it onto the main stack.
R@	Pushes the top of the return stack (without popping it) onto the main stack.
RDROP	Drops an item from the top of the return stack.

Below are some illustration on what happens in the main stack and return stack for

>R

R>

R@

and

RDROP

Quiz

Question 1

Rewrite the advanced stack words:

NIP

OVER

TUCK

R@

and

RDROP

in terms of the basic stack and return stack words.

		: NIP swap drop ;  

		: OVER >R dup R> swap ;

		: TUCK swap >R dup R> swap ;

		: R@ R> dup >R ;

		: RDROP R> drop ;

Question 2

Consider the word

ROT

("rotate") that behaves this way:

1 2 3 ROT

becomes

2 3 1

Write this word in terms of the words you know.

Question 3

Consider the word

-ROT

"undoes"

ROT

2 3 1 -ROT

becomes

1 2 3

Write this word in terms of the words you know.

		: rot >R swap R> swap ;

		: -rot swap >R swap R> ;
		
		1 2 3 rot
		.s  \ 2 3 1
		
		-rot
		.s  \ 1 2 3

Question 4

Write the words

{{

and

}}

that counts the number of items between them, without altering the stack. For example:

{{ 1 2 "mini" 3 }}

becomes

1 2 "mini" 3 4

		: {{ depth >R ;

		: }} depth R> - ;

Next: Do Loops