Create a ring from a list. If the list isn’t specified an empty ring is created. The current element of the ring is set to the first element of the list.

procedure
( ring->list a-ring)→list?
a-ring:ring?

Creates a list of the element in the ring starting from the current one.

Examples:

> (define the-ring(ring '(abcd)))
> (ring->list the-ring)

'(a b c d)

procedure
( ring-size a-ring)→exact-nonnegative-integer?
a-ring:ring?

Returns the number of elements of the ring.

procedure
( ring-current a-ring)→any/c
a-ring:ring?

Returns the current element of the ring. If the ring is empty an exception is raised.

Examples:

> (define the-ring(ring ))
> (ring-size the-ring)

0
> (ring-current the-ring)

ring-current: the ring is empty
> (define the-ring(ring '(abcd)))
> (ring-size the-ring)

4
> (ring-current the-ring)

'a

procedure
( ring-next! a-ring)→any/c
a-ring:ring?

Returns the next element of the ring and sets the current element to this element. If the ring is empty an exception is raised.

procedure
( ring-prev! a-ring)→any/c
a-ring:ring?

Returns the previous element of the ring and sets the current element to this element. If the ring is empty an exception is raised.

Examples:

> (define the-ring(ring '(abcd)))
> (ring-current the-ring)

'a
> (ring-next! the-ring)

'b
> (ring-current the-ring)

'b
> (ring->list the-ring)

'(b c d a)
> (ring-prev! the-ring)

'a
> (ring-prev! the-ring)

'd
> (ring-current the-ring)

'd
> (ring->list the-ring)

'(d a b c)

procedure
( ring-remove-current! a-ring)→void?
a-ring:ring?

Removes the current element of the ring and sets the current element to the next element. If the ring is empty an exception is raised.

procedure
( ring-set-current! a-ringelem)→void?
a-ring:ring?
elem:any/c

Sets the current element to elem with set! . If the ring is empty an exception is raised.

Examples:

> (define the-ring(ring '(abcd)))
> (ring-set-current! the-ring'A)
> (ring->list the-ring)

'(A b c d)
> (ring-remove-current! the-ring)
> (ring->list the-ring)

'(b c d)

procedure
( ring-add-next! a-ringelem)→void?
a-ring:ring?
elem:any/c

Adds a new elem after the current one and sets the new elem as current.

procedure
( ring-add-prev! a-ringelem)→void?
a-ring:ring?
elem:any/c

Adds a new elem before the current one and sets the new elem as current.

Examples:

> (define the-ring(ring '(abcd)))
> (ring-add-prev! the-ring1)
> (ring-add-prev! the-ring2)
> (ring-add-prev! the-ring3)
> (ring->list the-ring)

'(3 2 1 a b c d)
> (set! the-ring(ring '(abcd)))
> (ring-add-next! the-ring1)
> (ring-add-next! the-ring2)
> (ring-add-next! the-ring3)
> (ring->list the-ring)

'(3 b c d a 1 2)

The procedure ring->list can be quite confusing. I use it mainly for debugging purposes.

top ← prev up next →