6.6.2.13 Bitwise Operations

For the following bitwise functions, negative numbers are treated as infinite precision twos-complements. For instance -6 is bits …111010, with infinitely many ones on the left. It can be seen that adding 6 (binary 110) to such a bit pattern gives all zeros.

Scheme Procedure: logand n1 n2 …

C Function: scm_logand (n1, n2)

Return the bitwise AND of the integer arguments.

(logand) ⇒ -1
(logand 7) ⇒ 7
(logand #b111 #b011 #b001) ⇒ 1

Scheme Procedure: logior n1 n2 …

C Function: scm_logior (n1, n2)

Return the bitwise OR of the integer arguments.

(logior) ⇒ 0
(logior 7) ⇒ 7
(logior #b000 #b001 #b011) ⇒ 3

Scheme Procedure: logxor n1 n2 …

C Function: scm_loxor (n1, n2)

Return the bitwise XOR of the integer arguments. A bit is set in the result if it is set in an odd number of arguments.

(logxor) ⇒ 0
(logxor 7) ⇒ 7
(logxor #b000 #b001 #b011) ⇒ 2
(logxor #b000 #b001 #b011 #b011) ⇒ 1

Scheme Procedure: lognot n

C Function: scm_lognot (n)

Return the integer which is the ones-complement of the integer argument, ie. each 0 bit is changed to 1 and each 1 bit to 0.

(number->string (lognot #b10000000) 2)
   ⇒ "-10000001"
(number->string (lognot #b0) 2)
   ⇒ "-1"

Scheme Procedure: logtest j k

C Function: scm_logtest (j, k)

Test whether j and k have any 1 bits in common. This is equivalent to (not (zero? (logand j k))), but without actually calculating the logand, just testing for non-zero.

(logtest #b0100 #b1011) ⇒ #f
(logtest #b0100 #b0111) ⇒ #t

Scheme Procedure: logbit? index j

C Function: scm_logbit_p (index, j)

Test whether bit number index in j is set. index starts from 0 for the least significant bit.

(logbit? 0 #b1101) ⇒ #t
(logbit? 1 #b1101) ⇒ #f
(logbit? 2 #b1101) ⇒ #t
(logbit? 3 #b1101) ⇒ #t
(logbit? 4 #b1101) ⇒ #f

Scheme Procedure: ash n count

C Function: scm_ash (n, count)

Return floor(n * 2^count). n and count must be exact integers.

With n viewed as an infinite-precision twos-complement integer, ash means a left shift introducing zero bits when count is positive, or a right shift dropping bits when count is negative. This is an “arithmetic” shift.

(number->string (ash #b1 3) 2)     ⇒ "1000"
(number->string (ash #b1010 -1) 2) ⇒ "101"

;; -23 is bits ...11101001, -6 is bits ...111010
(ash -23 -2) ⇒ -6

Scheme Procedure: round-ash n count

C Function: scm_round_ash (n, count)

Return round(n * 2^count). n and count must be exact integers.

With n viewed as an infinite-precision twos-complement integer, round-ash means a left shift introducing zero bits when count is positive, or a right shift rounding to the nearest integer (with ties going to the nearest even integer) when count is negative. This is a rounded “arithmetic” shift.

(number->string (round-ash #b1 3) 2)     ⇒ \"1000\"
(number->string (round-ash #b1010 -1) 2) ⇒ \"101\"
(number->string (round-ash #b1010 -2) 2) ⇒ \"10\"
(number->string (round-ash #b1011 -2) 2) ⇒ \"11\"
(number->string (round-ash #b1101 -2) 2) ⇒ \"11\"
(number->string (round-ash #b1110 -2) 2) ⇒ \"100\"

Scheme Procedure: logcount n

C Function: scm_logcount (n)

Return the number of bits in integer n. If n is positive, the 1-bits in its binary representation are counted. If negative, the 0-bits in its two’s-complement binary representation are counted. If zero, 0 is returned.

(logcount #b10101010)
   ⇒ 4
(logcount 0)
   ⇒ 0
(logcount -2)
   ⇒ 1

Scheme Procedure: integer-length n

C Function: scm_integer_length (n)

Return the number of bits necessary to represent n.

For positive n this is how many bits to the most significant one bit. For negative n it’s how many bits to the most significant zero bit in twos complement form.

(integer-length #b10101010) ⇒ 8
(integer-length #b1111)     ⇒ 4
(integer-length 0)          ⇒ 0
(integer-length -1)         ⇒ 0
(integer-length -256)       ⇒ 8
(integer-length -257)       ⇒ 9

Scheme Procedure: integer-expt n k

C Function: scm_integer_expt (n, k)

Return n raised to the power k. k must be an exact integer, n can be any number.

Negative k is supported, and results in 1/n^abs(k) in the usual way. n^0 is 1, as usual, and that includes 0^0 is 1.

(integer-expt 2 5)   ⇒ 32
(integer-expt -3 3)  ⇒ -27
(integer-expt 5 -3)  ⇒ 1/125
(integer-expt 0 0)   ⇒ 1

Scheme Procedure: bit-extract n start end

C Function: scm_bit_extract (n, start, end)

Return the integer composed of the start (inclusive) through end (exclusive) bits of n. The startth bit becomes the 0-th bit in the result.

(number->string (bit-extract #b1101101010 0 4) 2)
   ⇒ "1010"
(number->string (bit-extract #b1101101010 4 9) 2)
   ⇒ "10110"

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