Beefy Boxes and Bandwidth Generously Provided by pair Networks
"be consistent"
 
PerlMonks  

perlfunc:vec

by gods
on Aug 24, 1999 at 22:42 UTC ( #249=perlfunc: print w/replies, xml ) Need Help??

vec

See the current Perl documentation for vec.

Here is our local, out-dated (pre-5.6) version:


vec - test or set particular bits in a string



vec EXPR,OFFSET,BITS



Treats the string in EXPR as a vector of unsigned integers, and returns the value of the bit field specified by OFFSET. BITS specifies the number of bits that are reserved for each entry in the bit vector. This must be a power of two from 1 to 32. 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;

Vectors created with vec() can also be manipulated with the logical operators |, &, and ^, which will assume a bit vector operation is desired when both operands are strings.

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'
    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 array of 0's and 1's, use these:

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

If you know the exact length in bits, it can be used in place of the *.


Log In?
Username:
Password:

What's my password?
Create A New User
Chatterbox?
[james28909]: and i need to format those special instances into an absolute date instead of "yesterday"
[stevieb]: ask a question on SoPW, and include at least a half-dozen examples of the input, and at least one example of expected output
[davido]: Exactly: SoPW. This isn't going to be solved easily in the CB.
[james28909]: in need "yesterday" and so on, to be absolute like "1" or "31"
[stevieb]: ...and throw some of your existing code into the equation as well, just so readers know you've given a try at it ;)
[james28909]: ok
[stevieb]: davido thanks for the link ;) I was being the typical lazy
[davido]: date parsing is hard. The more examples you can provide of the input (within reason) and expected output, the better.
[stevieb]: agreed. That's why I said at least a half-dozen. If enough of the different formats are present, the date/time folk may not have to request more. If they do, then at least there was a decent base to start with
[stevieb]: I do date and time transformations in both Perl and Python, but not frequently enough to not have to search for the format params etc ;)

How do I use this? | Other CB clients
Other Users?
Others meditating upon the Monastery: (5)
As of 2017-04-29 02:38 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?
    I'm a fool:











    Results (531 votes). Check out past polls.