On Thu, Mar 08, 2012 at 12:41:01AM +0800, lina wrote:
> Hi,
>
> $ ./substr_accessing_examples.pl
> Undefined subroutine &main::subst called at
> ./substr_accessing_examples.pl line 15.
>
> #!/usr/bin/env perl
>
> use strict;
> use warnings;
>
> #
> # get a 5-byte string, skip 3 bytes,
> # then grab two 8-byte strings, then the rest;
> #
>
> # (my $leading, my $s1, my $s2, my $trailing) = unpack("A5 x3 A8 A8
> A*", my $data);
>
> my $string = "This is what you have.";
>
> my $first = subst($string,0,1);
>
> print $first, "\n";
>
> I don't know how to find the one like use warnings module contains the substr
>
> Thanks for your time,
Hi Lina
I think you are looking for the substr function in Perl. You can find the
documentation on it using perldoc :
perldoc -f substr
substr EXPR,OFFSET,LENGTH,REPLACEMENT
substr EXPR,OFFSET,LENGTH
substr EXPR,OFFSET
Extracts a substring out of EXPR and returns it. First character
is at offset 0 (or whatever you've set $[ to (but <don't do
that)). If OFFSET is negative (or more precisely, less than $[),
starts that far back from the end of the string. If LENGTH is
omitted, returns everything through the end of the string. If
LENGTH is negative, leaves that many characters off the end of
the string.
my $s = "The black cat climbed the green tree";
my $color = substr $s, 4, 5; # black
my $middle = substr $s, 4, -11; # black cat climbed the
my $end = substr $s, 14; # climbed the green tree
my $tail = substr $s, -4; # tree
my $z = substr $s, -4, 2; # tr
You can use the substr() function as an lvalue, in which case
EXPR must itself be an lvalue. If you assign something shorter
than LENGTH, the string will shrink, and if you assign something
longer than LENGTH, the string will grow to accommodate it. To
keep the string the same length, you may need to pad or chop
your value using "sprintf".
If OFFSET and LENGTH specify a substring that is partly outside
the string, only the part within the string is returned. If the
substring is beyond either end of the string, substr() returns
the undefined value and produces a warning. When used as an
lvalue, specifying a substring that is entirely outside the
string raises an exception. Here's an example showing the
behavior for boundary cases:
my $name = 'fred';
substr($name, 4) = 'dy'; # $name is now 'freddy'
my $null = substr $name, 6, 2; # returns "" (no warning)
my $oops = substr $name, 7; # returns undef, with warning
substr($name, 7) = 'gap'; # raises an exception
An alternative to using substr() as an lvalue is to specify the
replacement string as the 4th argument. This allows you to
replace parts of the EXPR and return what was there before in
one operation, just as you can with splice().
my $s = "The black cat climbed the green tree";
my $z = substr $s, 14, 7, "jumped from"; # climbed
# $s is now "The black cat jumped from the green tree"
Note that the lvalue returned by the three-argument version of
substr() acts as a 'magic bullet'; each time it is assigned to,
it remembers which part of the original string is being
modified; for example:
$x = '1234';
for (substr($x,1,2)) {
$_ = 'a'; print $x,"\n"; # prints 1a4
$_ = 'xyz'; print $x,"\n"; # prints 1xyz4
$x = '56789';
$_ = 'pq'; print $x,"\n"; # prints 5pq9
}
Prior to Perl version 5.9.1, the result of using an lvalue
multiple times was unspecified.
Hope you have more success.
Kind Regards
Lesley
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