FAQ

On 2/6/10, Chris Coggins wrote:
Rob Dixon wrote:
Chris Coggins wrote:
I posted a question to this list yesterday from the google groups
interface, asking for help with the following. I have since tried to
post additional details in replies to that message using the google
groups site and none of my posts have shown up on the list so let's
try this again the right way.

I have a datafile with 1 line of data, 30 numbers delimited by
tildes. (Sample code below only uses 4 numbers to save time)

I need to open this file, grab this line of data, split it into
individual numbers, perform some simple math (addition) on each
number, and then put the new values back into the datafile, replacing
the original data that was there.

I neglected to mention in my original post that I need to access
these new values elsewhere in the script to perform additional math
functions with them.

I've tried several variations of the code below, including using
arrays, and none of them got through without the script failing. The
new datafile gets written, but it merely contains a ~. The errors I
get are "use of uninitialized values in such and such line" and
"variables needs explicit package name". When the script does run,
the variables lose their values as soon as I close the file after
inhaling it, making all the rest of the actions in the subroutine
futile.

the datafile used below initially contains 30 values that were
written previously, and this has been verified over and over.

sub newValues {
my($file) = shift;

open(FILE, "<$file") or die("Unable to open data file.");
while (<FILE>) {
my($a,$b,$c,$d) = split(/~/, $_);
$a = $a+$previousarray[4]; # I've tried to perform this math action
after close(FILE) and get the same result -> failure.
$b = $b+$previousarray[7];
$c = $c+$previousarray[0];
}
close(FILE);
open(FILE, ">$file") || die ("unable to open file");
print FILE ("$a~$b~$c \n");
close(FILE);
}
#end sub

Now my datafile just contains a single tilde and no values. I've also
tried to bring the original data from the datafile into my(@array)
and get the same errors.

Keep in mind I need other subroutines to be able use these new
values, but these local variables lose their values once the file is
closed. I've also tried to declare them as global variables but I'm
not doing something right because the script won't execute.

I really need some help figuring this out if somebody would be kind
enough to help a novice.
Hello Chris

Your lexical variables $a, $b, $c are local to the 'while' block, and
do not exist outside it. Error "Global symbol xx requires explicit
package name" is because the variable is not declared. Error "Use of
uninitialized value..." is because Perl uses a value of 'undef' where
variables are missing.

As an aside it is bad form to use meaningless variable names, and $a
and $b are special names that are used by core Perl so should be
avoided. Also it is prefereable to use lexical file handles and the
three-argument form of 'open', and incorporating the standard variable
$! into the die string will give valuable information about why the
operation failed.

Declaring your variables outside the 'while' loop would fix your
problem, but as you are interested only in the first line of the file
it is as well to remove the loop altogether. Take a look at the code
below and see if it meets your requirements.

HTH,

Rob


sub newValues {

my $file = shift;

open my $data, '<', $file or die "Unable to open data file: $!";
my $line = <$data>;
close $data;

my @data = split /~/, $line;
die "Invalid data format" if @data < 3;
$data[0] += $previousarray[4];
$data[1] += $previousarray[7];
$data[2] += $previousarray[0];

open my $out, '>', $file or die "Unable to open output file: $!";
print $out join('~', @data[0,1,2]), "\n";
close $out or die "Failed to close output file: $!";
}
Rob, thank you very much. This worked perfectly, unless my datafile is
empty at the start. Easiest fix was inserting 30 zero-tildes into the file.

I have a couple other snags if yall are willing to tolerate the questions.

1. another script uses data passed into it from an html form. Some of
the fields are empty, and my "print report file" subroutine prints the
empty variables because I don't know how to filter them out, and they
need to be removed from the report. Can someone give me the perfectly
formed "if" statement that lets me skip over the one or two empty
variables during the print action? The data being printed to the file is
stored in an array. See code below.

print FILE ("Report summary: $taskarray[0] \n"); #this value is good
print FILE ("Data for task 3: $taskarray[3] \n"); #this value is good
print FILE ("Data for task 7: $taskarray[7] \n"); #this value is empty,
don't print this data
print FILE ("Data for task 2: $taskarray[2] \n"); #this value is empty,
don't print this data
print FILE ("Data for task 11: $taskarray[11] \n"); #this value is good
but needs to be modified, see #2 below

2. Some of the data being printed above needs to be stripped of some of
its characters in the string. For example, one of the values is a part
number with 27 characters. I only need the last 10 characters of the
string for this report because somebody else is going to fill in the
front 17 characters with a new part code that I have nothing to do
with. The string currently looks like this:
TNA-140ZQ00-170011234567.890. I need it to print to FILE like this:
___-_______-______1234567.890, and I need to do this in the same block
of code above.

Chris

1) You would have to be more specific as to what an 'empty variable'
is. Zero? A blank string? undef?


2)

use strict;
use warnings;
use 5.010;

my $string = 'TNA-140ZQ00-170011234567.890';
my $prefix = '___-_______-______';

my $last_eleven = substr $string, -11;
say "$prefix$last_eleven";

--output:--
___-_______-______1234567.890


By the way, you should know that a period is a character. You are
pretty sloppy in describing your problems. Consider providing *no*
verbal explanation of your problems. Rather try posting, "I have xxxx
and I am trying to get yyyy". Here is an example program
demonstrating what I am trying to do. My *expected* output (or
error) is this. My *actual* output is this."

Also, when you have a problem in your actual program, to try and solve
it you should create another practice program. Try to mimic the
problem with a 10 line example program, and then post your example
program--not your real program.

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postedFeb 5, '10 at 9:39a
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