vec - Perldoc 瀏覽器 - Perl 繁體中文

函式 / vec

(來源, CPAN)

#vec EXPR,OFFSET,BITS

將 EXPR 中的字串視為由寬度為 BITS 的元素組成的位元向量，並傳回由 OFFSET 指定的元素值，為一個無符號整數。因此，BITS 指定保留給位元向量中每個元素的位元數。這必須是 1 到 32（或 64，如果您的平台支援）的 2 的次方。

如果 BITS 為 8，則「元素」與輸入字串的位元組相同。

如果 BITS 為 16 或更大，則輸入字串的位元組會分組為大小為 BITS/8 的區塊，每個組會轉換為一個數字，如同使用 pack/unpack 與大端格式 n/N（對於 BITS==64 類似）。有關詳細資訊，請參閱 pack。

如果位元為 4 或更少，則字串會分解成位元組，然後每個位元組的位元會分解成 8/BITS 組。位元組的位元會以類似小端的方式編號，例如 0x01、0x02、0x04、0x08、0x10、0x20、0x40、0x80。例如，將單一輸入位元組 chr(0x36) 分解成兩組會產生清單 (0x6, 0x3)；將其分解成 4 組會產生 (0x2, 0x1, 0x3, 0x0)。

vec 也可以被指定，這種情況下需要括號來提供正確的優先順序，就像

vec($image, $max_x * $x + $y, 8) = 3;

如果選定的元素在字串之外，會傳回值 0。如果寫入字串結束後的元素，Perl 會先用足夠多的零位元組來延伸字串。嘗試寫入字串開頭（即負 OFFSET）會發生錯誤。

如果字串碰巧在內部編碼為 UTF-8（因此設定了 UTF8 標記），vec 會嘗試將其轉換為使用每個字元一個位元組的內部表示法。但是，如果字串包含值為 256 或更高的字元，就會發生致命錯誤。

使用 vec 建立的字串也可以使用邏輯運算子 |、&、^ 和 ~ 來處理。當兩個運算元都是字串時，這些運算子會假設需要位元向量運算。請參閱 "perlop 中的「位元字串運算子」。

以下程式碼會建立一個 ASCII 字串，內容為 'PerlPerlPerl'。註解顯示每個步驟後的字串。請注意，此程式碼在大端或小端機器上運作方式相同。

my $foo = '';
vec($foo,  0, 32) = 0x5065726C; # 'Perl'

# $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
print vec($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"

若要將位元向量轉換成字串或 0 和 1 的清單，請使用這些

my $bits = unpack("b*", $vector);
my @bits = split(//, unpack("b*", $vector));

如果你知道確切的位元長度，可以使用它來取代 *。

以下是一個範例，說明位元實際上是如何就位的

#!/usr/bin/perl -wl

print <<'EOT';
                                  0         1         2         3
                   unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
EOT

for $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__

不論執行於哪種機器架構上，上述範例都應該會印出以下表格

                                  0         1         2         3
                   unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
vec($_, 0, 1) = 1   ==          1 10000000000000000000000000000000
vec($_, 1, 1) = 1   ==          2 01000000000000000000000000000000
vec($_, 2, 1) = 1   ==          4 00100000000000000000000000000000
vec($_, 3, 1) = 1   ==          8 00010000000000000000000000000000
vec($_, 4, 1) = 1   ==         16 00001000000000000000000000000000
vec($_, 5, 1) = 1   ==         32 00000100000000000000000000000000
vec($_, 6, 1) = 1   ==         64 00000010000000000000000000000000
vec($_, 7, 1) = 1   ==        128 00000001000000000000000000000000
vec($_, 8, 1) = 1   ==        256 00000000100000000000000000000000
vec($_, 9, 1) = 1   ==        512 00000000010000000000000000000000
vec($_,10, 1) = 1   ==       1024 00000000001000000000000000000000
vec($_,11, 1) = 1   ==       2048 00000000000100000000000000000000
vec($_,12, 1) = 1   ==       4096 00000000000010000000000000000000
vec($_,13, 1) = 1   ==       8192 00000000000001000000000000000000
vec($_,14, 1) = 1   ==      16384 00000000000000100000000000000000
vec($_,15, 1) = 1   ==      32768 00000000000000010000000000000000
vec($_,16, 1) = 1   ==      65536 00000000000000001000000000000000
vec($_,17, 1) = 1   ==     131072 00000000000000000100000000000000
vec($_,18, 1) = 1   ==     262144 00000000000000000010000000000000
vec($_,19, 1) = 1   ==     524288 00000000000000000001000000000000
vec($_,20, 1) = 1   ==    1048576 00000000000000000000100000000000
vec($_,21, 1) = 1   ==    2097152 00000000000000000000010000000000
vec($_,22, 1) = 1   ==    4194304 00000000000000000000001000000000
vec($_,23, 1) = 1   ==    8388608 00000000000000000000000100000000
vec($_,24, 1) = 1   ==   16777216 00000000000000000000000010000000
vec($_,25, 1) = 1   ==   33554432 00000000000000000000000001000000
vec($_,26, 1) = 1   ==   67108864 00000000000000000000000000100000
vec($_,27, 1) = 1   ==  134217728 00000000000000000000000000010000
vec($_,28, 1) = 1   ==  268435456 00000000000000000000000000001000
vec($_,29, 1) = 1   ==  536870912 00000000000000000000000000000100
vec($_,30, 1) = 1   == 1073741824 00000000000000000000000000000010
vec($_,31, 1) = 1   == 2147483648 00000000000000000000000000000001
vec($_, 0, 2) = 1   ==          1 10000000000000000000000000000000
vec($_, 1, 2) = 1   ==          4 00100000000000000000000000000000
vec($_, 2, 2) = 1   ==         16 00001000000000000000000000000000
vec($_, 3, 2) = 1   ==         64 00000010000000000000000000000000
vec($_, 4, 2) = 1   ==        256 00000000100000000000000000000000
vec($_, 5, 2) = 1   ==       1024 00000000001000000000000000000000
vec($_, 6, 2) = 1   ==       4096 00000000000010000000000000000000
vec($_, 7, 2) = 1   ==      16384 00000000000000100000000000000000
vec($_, 8, 2) = 1   ==      65536 00000000000000001000000000000000
vec($_, 9, 2) = 1   ==     262144 00000000000000000010000000000000
vec($_,10, 2) = 1   ==    1048576 00000000000000000000100000000000
vec($_,11, 2) = 1   ==    4194304 00000000000000000000001000000000
vec($_,12, 2) = 1   ==   16777216 00000000000000000000000010000000
vec($_,13, 2) = 1   ==   67108864 00000000000000000000000000100000
vec($_,14, 2) = 1   ==  268435456 00000000000000000000000000001000
vec($_,15, 2) = 1   == 1073741824 00000000000000000000000000000010
vec($_, 0, 2) = 2   ==          2 01000000000000000000000000000000
vec($_, 1, 2) = 2   ==          8 00010000000000000000000000000000
vec($_, 2, 2) = 2   ==         32 00000100000000000000000000000000
vec($_, 3, 2) = 2   ==        128 00000001000000000000000000000000
vec($_, 4, 2) = 2   ==        512 00000000010000000000000000000000
vec($_, 5, 2) = 2   ==       2048 00000000000100000000000000000000
vec($_, 6, 2) = 2   ==       8192 00000000000001000000000000000000
vec($_, 7, 2) = 2   ==      32768 00000000000000010000000000000000
vec($_, 8, 2) = 2   ==     131072 00000000000000000100000000000000
vec($_, 9, 2) = 2   ==     524288 00000000000000000001000000000000
vec($_,10, 2) = 2   ==    2097152 00000000000000000000010000000000
vec($_,11, 2) = 2   ==    8388608 00000000000000000000000100000000
vec($_,12, 2) = 2   ==   33554432 00000000000000000000000001000000
vec($_,13, 2) = 2   ==  134217728 00000000000000000000000000010000
vec($_,14, 2) = 2   ==  536870912 00000000000000000000000000000100
vec($_,15, 2) = 2   == 2147483648 00000000000000000000000000000001
vec($_, 0, 4) = 1   ==          1 10000000000000000000000000000000
vec($_, 1, 4) = 1   ==         16 00001000000000000000000000000000
vec($_, 2, 4) = 1   ==        256 00000000100000000000000000000000
vec($_, 3, 4) = 1   ==       4096 00000000000010000000000000000000
vec($_, 4, 4) = 1   ==      65536 00000000000000001000000000000000
vec($_, 5, 4) = 1   ==    1048576 00000000000000000000100000000000
vec($_, 6, 4) = 1   ==   16777216 00000000000000000000000010000000
vec($_, 7, 4) = 1   ==  268435456 00000000000000000000000000001000
vec($_, 0, 4) = 2   ==          2 01000000000000000000000000000000
vec($_, 1, 4) = 2   ==         32 00000100000000000000000000000000
vec($_, 2, 4) = 2   ==        512 00000000010000000000000000000000
vec($_, 3, 4) = 2   ==       8192 00000000000001000000000000000000
vec($_, 4, 4) = 2   ==     131072 00000000000000000100000000000000
vec($_, 5, 4) = 2   ==    2097152 00000000000000000000010000000000
vec($_, 6, 4) = 2   ==   33554432 00000000000000000000000001000000
vec($_, 7, 4) = 2   ==  536870912 00000000000000000000000000000100
vec($_, 0, 4) = 4   ==          4 00100000000000000000000000000000
vec($_, 1, 4) = 4   ==         64 00000010000000000000000000000000
vec($_, 2, 4) = 4   ==       1024 00000000001000000000000000000000
vec($_, 3, 4) = 4   ==      16384 00000000000000100000000000000000
vec($_, 4, 4) = 4   ==     262144 00000000000000000010000000000000
vec($_, 5, 4) = 4   ==    4194304 00000000000000000000001000000000
vec($_, 6, 4) = 4   ==   67108864 00000000000000000000000000100000
vec($_, 7, 4) = 4   == 1073741824 00000000000000000000000000000010
vec($_, 0, 4) = 8   ==          8 00010000000000000000000000000000
vec($_, 1, 4) = 8   ==        128 00000001000000000000000000000000
vec($_, 2, 4) = 8   ==       2048 00000000000100000000000000000000
vec($_, 3, 4) = 8   ==      32768 00000000000000010000000000000000
vec($_, 4, 4) = 8   ==     524288 00000000000000000001000000000000
vec($_, 5, 4) = 8   ==    8388608 00000000000000000000000100000000
vec($_, 6, 4) = 8   ==  134217728 00000000000000000000000000010000
vec($_, 7, 4) = 8   == 2147483648 00000000000000000000000000000001
vec($_, 0, 8) = 1   ==          1 10000000000000000000000000000000
vec($_, 1, 8) = 1   ==        256 00000000100000000000000000000000
vec($_, 2, 8) = 1   ==      65536 00000000000000001000000000000000
vec($_, 3, 8) = 1   ==   16777216 00000000000000000000000010000000
vec($_, 0, 8) = 2   ==          2 01000000000000000000000000000000
vec($_, 1, 8) = 2   ==        512 00000000010000000000000000000000
vec($_, 2, 8) = 2   ==     131072 00000000000000000100000000000000
vec($_, 3, 8) = 2   ==   33554432 00000000000000000000000001000000
vec($_, 0, 8) = 4   ==          4 00100000000000000000000000000000
vec($_, 1, 8) = 4   ==       1024 00000000001000000000000000000000
vec($_, 2, 8) = 4   ==     262144 00000000000000000010000000000000
vec($_, 3, 8) = 4   ==   67108864 00000000000000000000000000100000
vec($_, 0, 8) = 8   ==          8 00010000000000000000000000000000
vec($_, 1, 8) = 8   ==       2048 00000000000100000000000000000000
vec($_, 2, 8) = 8   ==     524288 00000000000000000001000000000000
vec($_, 3, 8) = 8   ==  134217728 00000000000000000000000000010000
vec($_, 0, 8) = 16  ==         16 00001000000000000000000000000000
vec($_, 1, 8) = 16  ==       4096 00000000000010000000000000000000
vec($_, 2, 8) = 16  ==    1048576 00000000000000000000100000000000
vec($_, 3, 8) = 16  ==  268435456 00000000000000000000000000001000
vec($_, 0, 8) = 32  ==         32 00000100000000000000000000000000
vec($_, 1, 8) = 32  ==       8192 00000000000001000000000000000000
vec($_, 2, 8) = 32  ==    2097152 00000000000000000000010000000000
vec($_, 3, 8) = 32  ==  536870912 00000000000000000000000000000100
vec($_, 0, 8) = 64  ==         64 00000010000000000000000000000000
vec($_, 1, 8) = 64  ==      16384 00000000000000100000000000000000
vec($_, 2, 8) = 64  ==    4194304 00000000000000000000001000000000
vec($_, 3, 8) = 64  == 1073741824 00000000000000000000000000000010
vec($_, 0, 8) = 128 ==        128 00000001000000000000000000000000
vec($_, 1, 8) = 128 ==      32768 00000000000000010000000000000000
vec($_, 2, 8) = 128 ==    8388608 00000000000000000000000100000000
vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001