Treats the string in EXPR as a bit vector made up of elements of width BITS and returns the value of the element specified by OFFSET as an unsigned integer. BITS therefore specifies the number of bits that are reserved for each element in the bit vector. This must be a power of two from 1 to 32 (or 64, if your platform supports that).
If BITS is 8, "elements" coincide with bytes of the input string.
If BITS is 16 or more, bytes of the input string are grouped into chunks of size BITS/8, and each group is converted to a number as withpack/unpack with big-endian formatsn/N (and analogously for BITS==64). Seepack for details.
If bits is 4 or less, the string is broken into bytes, then the bits of each byte are broken into 8/BITS groups. Bits of a byte are numbered in a little-endian-ish way, as in0x01,0x02,0x04,0x08,0x10,0x20,0x40,0x80. For example, breaking the single input bytechr(0x36) into two groups gives a list(0x6, 0x3); breaking it into 4 groups gives(0x2, 0x1, 0x3, 0x0).
vec may also be assigned to, in which case parentheses are needed to give the expression the correct precedence as in
vec($image, $max_x * $x + $y, 8) = 3;If the selected element is outside the string, the value 0 is returned. If an element off the end of the string is written to, Perl will first extend the string with sufficiently many zero bytes. It is an error to try to write off the beginning of the string (i.e., negative OFFSET).
If the string happens to be encoded as UTF-8 internally (and thus has the UTF8 flag set),vec tries to convert it to use a one-byte-per-character internal representation. However, if the string contains characters with values of 256 or higher, a fatal error will occur.
Strings created withvec can also be manipulated with the logical operators|,&,^, and~. These operators will assume a bit vector operation is desired when both operands are strings. See"Bitwise String Operators" in perlop.
The following code will build up an ASCII string saying'PerlPerlPerl'. The comments show the string after each step. Note that this code works in the same way on big-endian or little-endian machines.
my $foo = '';vec($foo, 0, 32) = 0x5065726C; # 'Perl'# $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bitsprint vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')vec($foo, 2, 16) = 0x5065; # 'PerlPe'vec($foo, 3, 16) = 0x726C; # 'PerlPerl'vec($foo, 8, 8) = 0x50; # 'PerlPerlP'vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"vec($foo, 21, 4) = 7; # 'PerlPerlPer' # 'r' is "\x72"vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"vec($foo, 94, 1) = 1; # 'PerlPerlPerl' # 'l' is "\x6c"To transform a bit vector into a string or list of 0's and 1's, use these:
my $bits = unpack("b*", $vector);my @bits = split(//, unpack("b*", $vector));If you know the exact length in bits, it can be used in place of the*.
Here is an example to illustrate how the bits actually fall in place:
#!/usr/bin/perl -wlprint <<'EOT'; 0 1 2 3 unpack("V",$_) 01234567890123456789012345678901------------------------------------------------------------------EOTfor $w (0..3) { $width = 2**$w; for ($shift=0; $shift < $width; ++$shift) { for ($off=0; $off < 32/$width; ++$off) { $str = pack("B*", "0"x32); $bits = (1<<$shift); vec($str, $off, $width) = $bits; $res = unpack("b*",$str); $val = unpack("V", $str); write; } }}format STDOUT =vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>$off, $width, $bits, $val, $res.__END__Regardless of the machine architecture on which it runs, the example above should print the following table:
0 1 2 3 unpack("V",$_) 01234567890123456789012345678901------------------------------------------------------------------vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001Perldoc Browser is maintained by Dan Book (DBOOK). 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