Perl in a Nutshell, 2nd Edition

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 ...