Removing entry from @rray

[Robert Valcourt]

Hello,

For some time I have been using two different method for removing an entry
from an @rray. It is my beleif that these methods may be outdated or
erroneous and I wanted to get some opinions. My program reads the contents
of a text file, and places each line into the array. Here are my two
methods:

Using Splicing
=========================

open (ADDRESSES, "<addressbook.txt") or die "Can't open file: $!";
@entries=<ADDRESSES>;
close(ADDRESSES);

$variable = "Smith";

foreach $entry(@entries) {
$location++;
if ($entry eq $variable) {
$location--;
$line = $location;
}
}

splice(@entries, $line, 1);

First of all I understand that many of you will first comment on the means
by which I read the text file and place its content into the array or that
strict isnt used above, and I will be fixing that soon using SLURP and
calling the "use strict" header.

So this method uses the old splice routine. I find this method to be
confusing and im not quite a fan of it. On to the next:

Redefining the @array element inside to loop
=========================

open (ADDRESSES, "<addressbook.txt") or die "Can't open file: $!";
@entries=<ADDRESSES>;
close(ADDRESSES);

$variable = "Smith";

foreach $entry(@entries) {
if ($entry eq $variable) {
$entry = "";
}
}

This method simply remodifies the $entry with a null value killing its data
and the following \n inside the loop (at least I think it does).

Both of these methods do the trick but I have a feeling that neither may be
the most efficient way or worse these methods may have side effects that I
am unaware of. Alot has changed in the world of Perl since I started using
these methods and I wanted to know if there is a better/safer/more efficient
way to do this.

I would also like to ask about terminating a loop ... obviously after a
match is found I would want to terminate the loop so that it doesn't waste
time checking all the other array elements for a match. Suprisingly I was
not able to source this answer. I once read somewhere that a command called
"return" or something is used, thoughts?

I appriciate you time on this, Thanks much,

Robert

 

[John W. Krahn]

For some time I have been using two different method for removing an entry
from an @rray. It is my beleif that these methods may be outdated or
erroneous and I wanted to get some opinions. My program reads the contents
of a text file, and places each line into the array. Here are my two
methods:

Using Splicing
=========================

open (ADDRESSES, "<addressbook.txt") or die "Can't open file: $!";
@entries=<ADDRESSES>;
close(ADDRESSES);

$variable = "Smith";

foreach $entry(@entries) {
$location++;
if ($entry eq $variable) {
$location--;
$line = $location;
}
}

splice(@entries, $line, 1);

It looks like you want something like:

open my $ADDRESSES, '<', 'addressbook.txt'
or die "Can't open 'addressbook.txt' $!";

my @entries = grep $entry ne $variable, <$ADDRESSES>;

close $ADDRESSES;




John

 

[anno4000]

Hello,

For some time I have been using two different method for removing an entry
from an @rray. It is my beleif that these methods may be outdated or
erroneous and I wanted to get some opinions. My program reads the contents
of a text file, and places each line into the array. Here are my two
methods:

Using Splicing
=========================

open (ADDRESSES, "<addressbook.txt") or die "Can't open file: $!";
@entries=<ADDRESSES>;
close(ADDRESSES);

$variable = "Smith";

foreach $entry(@entries) {
$location++;
if ($entry eq $variable) {
$location--;
$line = $location;
}
}

splice(@entries, $line, 1);

First of all I understand that many of you will first comment on the means
by which I read the text file and place its content into the array or that
strict isnt used above, and I will be fixing that soon using SLURP and
calling the "use strict" header.

So this method uses the old splice routine. I find this method to be
confusing and im not quite a fan of it. On to the next:

Redefining the @array element inside to loop
=========================

open (ADDRESSES, "<addressbook.txt") or die "Can't open file: $!";
@entries=<ADDRESSES>;
close(ADDRESSES);

$variable = "Smith";

foreach $entry(@entries) {
if ($entry eq $variable) {
$entry = "";
}
}

This method simply remodifies the $entry with a null value killing its data
and the following \n inside the loop (at least I think it does).

The methods don't do the same thing. The first one actually removes
the entry from the array (the array length shrinks by one). The second
method doesn't change the array but replaces the element with an empty
string. After printing all entries both results look the same but the
arrays are different after each one.

Both of these methods do the trick but I have a feeling that neither may be
the most efficient way or worse these methods may have side effects that I
am unaware of. Alot has changed in the world of Perl since I started using
these methods and I wanted to know if there is a better/safer/more efficient
way to do this.

The simplest method (then and now) to remove an entry is grep:

@entries = grep $_ ne $entry, @entries;

I would also like to ask about terminating a loop ... obviously after a
match is found I would want to terminate the loop so that it doesn't waste
time checking all the other array elements for a match. Suprisingly I was
not able to source this answer. I once read somewhere that a command called
"return" or something is used, thoughts?

perldoc -f last.

You can also use List::Util::first to find the entry efficiently
(first() leaves the loop after the first match):

my $i = List::Util::first { $_ eq $entry } @entries;
splice @entries, $i, 1 if defined $i;

This should be faster if the lists are very long. Just how long
"very long" is can only be decided by benchmarks.

Anno

 

[Mirco Wahab]

My program reads the contents
of a text file, and places each line into the array.
Here are my two methods:

Using Splicing
=========================
...
Redefining the @array element inside to loop
=========================

Aside from Anno's and John's comments,
I'd like to make another remark.

Both of your variants won't work,
becvause you can't simply 'eq' a
variable against a string read from
a file (which may at least contain a
'\n' and probably a lot more)

Suppose we have a simple "address file":

Smarf Boomtown Boomstreet 23
Smith Huntington Hunstreet 12
Solch Berlin Berlstrasse 123

Then you could read all except "Smith ..."
(as the other posts suggested => grep) by
"grep negated regex":

...
my $variable = 'Smith';
open (my $addresses, '<', 'addressbook.txt') or die "without remorse: $!";
my @entries = grep !/\b$variable\b/, <$addresses>;
...

(\bWORD\b) means: WORD must be delimited by word boundaries,
so "Smithnwesson" woudn't match.

I would also like to ask about terminating a loop ... obviously after a
match is found I would want to terminate the loop so that it doesn't waste
time checking all the other array elements for a match. Suprisingly I was
not able to source this answer. I once read somewhere that a command called
"return" or something is used, thoughts?

That's called 'last':

...
...
for my $index (0..@entries-1) {
if( $entries[$index] =~ /\b$variable\b/ ) {
splice @entries, $index, 1;
last;
}
}
...


Regards

M.

 

[xhoster]

Hello,

For some time I have been using two different method for removing an
entry from an @rray. It is my beleif that these methods may be outdated
or erroneous and I wanted to get some opinions. My program reads the
contents of a text file, and places each line into the array. Here are my
two methods:

Using Splicing
=========================

open (ADDRESSES, "<addressbook.txt") or die "Can't open file: $!";
@entries=<ADDRESSES>;
close(ADDRESSES);

$variable = "Smith";

foreach $entry(@entries) {
$location++;
if ($entry eq $variable) {
$location--;
$line = $location;
}
}

splice(@entries, $line, 1);

First of all I understand that many of you will first comment on the
means by which I read the text file and place its content into the array
or that strict isnt used above, and I will be fixing that soon using
SLURP and calling the "use strict" header.

So this method uses the old splice routine.

Old? I prefer the term "mature". Anyway, is there any reason you
don't use a hash rather than an array? What happens if $variable doesn't
occur at all in the list?

I find this method to be
confusing and im not quite a fan of it. On to the next:

Redefining the @array element inside to loop
=========================

open (ADDRESSES, "<addressbook.txt") or die "Can't open file: $!";
@entries=<ADDRESSES>;
close(ADDRESSES);

$variable = "Smith";

foreach $entry(@entries) {
if ($entry eq $variable) {
$entry = "";
}
}

This method simply remodifies the $entry with a null value killing its
data and the following \n inside the loop (at least I think it does).

This "removes" all matching entries, the first method removes only the last
one. But it sounds like you anticipate there will be exactly one match?

Both of these methods do the trick but I have a feeling that neither may
be the most efficient way

Is that really a concern?

or worse these methods may have side effects
that I am unaware of. Alot has changed in the world of Perl since I
started using these methods and I wanted to know if there is a
better/safer/more efficient way to do this.
grep


I would also like to ask about terminating a loop ... obviously after a
match is found I would want to terminate the loop so that it doesn't
waste time checking all the other array elements for a match.
last

Suprisingly
I was not able to source this answer. I once read somewhere that a
command called "return" or something is used, thoughts?

return can be used to break out of loops (as long as the loop is contained
in a subroutine call or other "return"able construct) but one should only
do that if your goal is both leave the loop and return from the outer
construct. If you just want to leave the loop, use last.

Xho