Name
vec
Synopsis
vec string, offset, bits
Treats a string as a
vector of unsigned integers and returns the value of the element
specified by offset and
bits. The function may also be assigned
to, which causes the element to be modified. The purpose of the
function is to provide compact storage of lists of small integers.
The integers may be very small—vectors can hold numbers that are
as small as one bit, resulting in a bitstring.
The offset specifies the number
of elements to skip over to find the one you want.
bits is the number of bits per element
in the vector, so each element can contain an unsigned integer in
the range 0..(2**
bits )-1.
bits must be one of 1, 2,
4, 8, 16, or 32. As many elements as possible are
packed into each byte, and the ordering is such that vec($vectorstring,0,1) is guaranteed to
go into the lowest bit of the first byte of the string. To find
the position of the byte in which an element will be placed, you
have to multiply the offset by the
number of elements per byte. When bits
is 1, there are eight elements
per byte. When bits is 2, there are four elements per byte.
When bits is 4, there are two elements (called
nybbles) per byte. And so on.
Regardless of whether your system is big-endian or
little-endian, vec($foo, 0, 8)
always refers to the first byte of string $foo. See select
for examples of bitmaps generated with vec.
Vectors created with vec
can also be manipulated with the logical operators |, &, ^, and ~, which will assume a bit vector operation ...
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