RE: confused by use of 'implied' variable
From: perl-win32-users-boun...@listserv.activestate.com
[mailto:perl-win32-users-boun...@listserv.activestate.com] On Behalf Of Greg
VisionInfosoft
Sent: 05 April 2013 21:16
To: Perl-Win32-Users@listserv.ActiveState.com
Subject: confused by use of 'implied' variable
im butchering a public script i found on the internet for purpose of doing a
CGI file upload.
Be very careful, there is a lot of poor quality scripts out there.
theres one excerpt from the script that ive never used before. the few
lines...
while ( $upload_filehandle ) {
print UPLOADFILE;
}
if it were me, i would not write code this way, i write in a way that make it
easier for me to quickly
understand what i was trying to do. if it were me coding this in my 'lame'
(for idiots) way, it would look
more like...
while ( $data = $upload_filehandle ) {
print UPLOADFILE $data;
}
Not unreasonable, but you should restrict the scope of your variable to the
while loop.
while (my $data = $upload_filehandle ) {
etc.
but now that ive seen the code, as originally presented, it does cause me to
ask the question...
how does one know under what set of circumstances can such 'abbreviated' code
be written? in other words, how can one know what kinds of features or
operations can use the implied variable @_ (which I assume is the
variable that would be used in this case).
The implied variable here is $_, not @_.
specifically, if i wanted to append to a variable $aggregated_file_contents,
each new block of the file as it was being read and output...
my thought was to try:
while ( $upload_filehandle ) {
print UPLOADFILE;
$aggregated_file_contents.=;
You have to be explicit here. That is...
$aggregated_file_contents .= $_;
}
as opposed to what i would have normally done ($aggregated_file_contents .=
$data;)
yes, i know i can simply try it, to see if it works - but is there a more
general rule one can follow that
tells them when this kind of thing can normally be used, versus when not?
Start with 'perldoc perlvar'. It is the first one listed under General
Variables.
HTH
--
Brian Raven
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confused by use of 'implied' variable
im butchering a public script i found on the internet for purpose of doing
a CGI file upload.
theres one excerpt from the script that ive never used before. the few
lines...
while ( $upload_filehandle ) {
print UPLOADFILE;
}
if it were me, i would not write code this way, i write in a way that make
it easier for me to quickly understand what i was trying to do. if it were
me coding this in my 'lame' (for idiots) way, it would look more like...
while ( $data = $upload_filehandle ) {
print UPLOADFILE $data;
}
but now that ive seen the code, as originally presented, it does cause me
to ask the question...
how does one know under what set of circumstances can such 'abbreviated'
code be written? in other words, how can one know what kinds of features
or operations can use the implied variable @_ (which I assume is the
variable that would be used in this case).
specifically, if i wanted to append to a variable
$aggregated_file_contents, each new block of the file as it was being read
and output...
my thought was to try:
while ( $upload_filehandle ) {
print UPLOADFILE;
$aggregated_file_contents.=;
}
as opposed to what i would have normally done ($aggregated_file_contents .=
$data;)
yes, i know i can simply try it, to see if it works - but is there a more
general rule one can follow that tells them when this kind of thing can
normally be used, versus when not?
thanks alot for any insights here
greg
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Re: confused by use of 'implied' variable
Greg --
The shortcuts us the perl variable $_. That is, when no specific argument
is supplied, $_ is used by default.
s/x/y/;
while ( ) {...}
print;
All the above and more expect a value to work with, and if none is given,
perl uses $_ by default.
You get into trouble when you have a loop that refers to $_ which then
calls a subroutine that does something else with $_...
See http://perldoc.perl.org/perlvar.html#General-Variables for details on
$_.
Your $aggregated_file_contents example wouldn't work, by the way: there's
no default variable in that context. You'd have to do this:
$aggregated_file_contents .= $_ ;
Hope this helps!
On Friday, April 5, 2013, Greg VisionInfosoft wrote:
im butchering a public script i found on the internet for purpose of doing
a CGI file upload.
theres one excerpt from the script that ive never used before. the few
lines...
while ( $upload_filehandle ) {
print UPLOADFILE;
}
if it were me, i would not write code this way, i write in a way that make
it easier for me to quickly understand what i was trying to do. if it were
me coding this in my 'lame' (for idiots) way, it would look more like...
while ( $data = $upload_filehandle ) {
print UPLOADFILE $data;
}
but now that ive seen the code, as originally presented, it does cause me
to ask the question...
how does one know under what set of circumstances can such 'abbreviated'
code be written? in other words, how can one know what kinds of features
or operations can use the implied variable @_ (which I assume is the
variable that would be used in this case).
specifically, if i wanted to append to a variable
$aggregated_file_contents, each new block of the file as it was being read
and output...
my thought was to try:
while ( $upload_filehandle ) {
print UPLOADFILE;
$aggregated_file_contents.=;
}
as opposed to what i would have normally done ($aggregated_file_contents
.= $data;)
yes, i know i can simply try it, to see if it works - but is there a more
general rule one can follow that tells them when this kind of thing can
normally be used, versus when not?
thanks alot for any insights here
greg
--
Will Trillich :: 812.454.6431
“Grading takes away all the fun from failing. And a huge part of education
is about failure.” -- Shimon Schocken
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