Bit Strings (MIT/GNU Scheme 12.1)

9 Bit Strings

A bit string is a sequence of bits. Bit strings can be used to represent sets or to manipulate binary data. The elements of a bit string are numbered from zero up to the number of bits in the string less one, in right to left order, (the rightmost bit is numbered zero). When you convert from a bit string to an integer, the zero-th bit is associated with the zero-th power of two, the first bit is associated with the first power, and so on.

Bit strings are encoded very densely in memory. Each bit occupies exactly one bit of storage, and the overhead for the entire bit string is bounded by a small constant. However, accessing a bit in a bit string is slow compared to accessing an element of a vector or character string. If performance is of overriding concern, it is better to use character strings to store sets of boolean values even though they occupy more space.

The length of a bit string is the number of bits that it contains. This number is an exact non-negative integer that is fixed when the bit string is created. The valid indexes of a bit string are the exact non-negative integers less than the length of the bit string.

Bit strings may contain zero or more bits. They are not limited by the length of a machine word. In the printed representation of a bit string, the contents of the bit string are preceded by ‘#*’. The contents are printed starting with the most significant bit (highest index).

Note that the external representation of bit strings uses a bit ordering that is the reverse of the representation for bit strings in Common Lisp. It is likely that MIT/GNU Scheme’s representation will be changed in the future, to be compatible with Common Lisp. For the time being this representation should be considered a convenience for viewing bit strings rather than a means of entering them as data.

#*11111
#*1010
#*00000000
#*

All of the bit-string procedures are MIT/GNU Scheme extensions.