12.2.69 ldb [Accessor]

ldb bytespec integer ⇒ byte

(setf ( ldb bytespec place) new-byte)

Pronunciation::

pronounced ’lid ib or pronounced ’lid e b or pronounced ’el ’d\=e ’b\=e

Arguments and Values::

bytespec—a byte specifier.

integer—an integer.

byte, new-byte—a non-negative integer.

Description::

ldb extracts and returns the byte of integer specified by bytespec.

ldb returns an integer in which the bits with weights 2^(s-1) through 2^0 are the same as those in integer with weights 2^(p+s-1) through 2^p, and all other bits zero; s is (byte-size bytespec) and p is (byte-position bytespec).

setf may be used with ldb to modify a byte within the integer that is stored in a given place.

The order of evaluation, when an ldb form is supplied to setf, is exactly left-to-right.

The effect is to perform a dpb operation and then store the result back into the place.

Examples::

 (ldb (byte 2 1) 10) ⇒  1
 (setq a (list 8)) ⇒  (8)
 (setf (ldb (byte 2 1) (car a)) 1) ⇒  1
 a ⇒  (10)

See Also::

byte , byte-position, byte-size, dpb

Notes::

 (logbitp j (ldb (byte s p) n))
    ≡ (and (< j s) (logbitp (+ j p) n))

In general,

 (ldb (byte 0 x) y) ⇒  0

for all valid values of x and y.

Historically, the name “ldb” comes from a DEC PDP-10 assembly language instruction meaning “load byte.”