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::

@xref{byte; byte-size; byte-position} , byte-position, byte-size, section dpb [Function]

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."

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