comparing two arrays in perl

[bcdixit]

Hi,
I have two arrays, for example,

@array = ("john","larry","kevin","peter","mike")
@exclude = ("john","kevin")

i want to delete from @array , values that exist in @exclude.
i.e. i want my @array to look like

@array("","larry","","peter","mike")

i have written this code but it doesnot seem to work.

foreach$i(0..$#exclude)
{
foreach$x(0..$#array)
{
if($array[$x] eq $exclude[$i])
{
delete($array[$x]);
next;
}
}
}

am I missing something? is there someother better way to do it in
perl?

thanks
-bd

 

[Peter Makholm]

@array = ("john","larry","kevin","peter","mike")
@exclude = ("john","kevin")

i want to delete from @array , values that exist in @exclude.
i.e. i want my @array to look like

What you need is an easy way to check if an value is in @exclude. This
can be done by converting it to a hash where easch element from
@exclude is an key with an true value:

This can be done this way

%exclude = map { $_ => 1 } @exclude;

or this way:

$exclude{$_} = 1 for @exclude;

(and probbaly many other ways).

Then you just need to iterate over @array one time.

//Makholm

 

[Paul Lalli]

I have two arrays, for example,

@array = ("john","larry","kevin","peter","mike")
@exclude = ("john","kevin")

i want to delete from @array , values that exist in @exclude.

Your Question (or at least a variant of it) is Frequently Asked...
$ perldoc -q "difference of two arrays"
Found in /opt/perl/lib/5.6.1/pod/perlfaq4.pod
How do I compute the difference of two arrays? How do I
compute the intersection of two arrays?

i.e. i want my @array to look like

@array("","larry","","peter","mike")

i have written this code but it doesnot seem to work.

Does not work is the worst possible error description. *HOW* does it
not work? What results does it produce that you did not expect?

foreach$i(0..$#exclude)
{
foreach$x(0..$#array)
{
if($array[$x] eq $exclude[$i])
{
delete($array[$x]);

delete(), when used on array elements, will undefine internal
elements, and will remove the last element. This is generally not a
good thing, and also doesn't match your desired output. I think you
instead want:
$array[$x] = "";

next;

"next" being the last statement in a block is a no-op. It simply says
to go up to the beginning of the current loop and start the next
iteration. That's what would happen anyway. I think you meant "last"
here.

}
}
}

am I missing something?

You tell me. The code (after being modified as I suggest) does
exactly what you say you want. If you disagree, post a short-but-
complete script with desired output and actual output.

is there someother better way to do it in perl?

Yes, use hashes, as Peter and the FAQ both suggested.

Paul Lalli

 

[Jürgen Exner]

I have two arrays, for example,

@array = ("john","larry","kevin","peter","mike")
@exclude = ("john","kevin")

i want to delete from @array , values that exist in @exclude.
i.e. i want my @array to look like

@array("","larry","","peter","mike")

i have written this code but it doesnot seem to work.

foreach$i(0..$#exclude) {
foreach$x(0..$#array) {
if($array[$x] eq $exclude[$i]){
delete($array[$x]); [...]
am I missing something?

Yes. It is very bad to change the array (add or remove elements) while
looping through it using foreach().

is there someother better way to do it in perl?

Like explained in the FAQ: "How do I compute the difference of two arrays?
How do I compute the intersection of two arrays?"

jue

 

[Paul Lalli]

bcdixit wrote:

foreach$i(0..$#exclude) {
foreach$x(0..$#array) {
if($array[$x] eq $exclude[$i]){
delete($array[$x]); [...]
am I missing something?

Yes. It is very bad to change the array (add or remove elements)\
while looping through it using foreach().

The only element that could be deleted from that array is the last
one. I don't see how that could affect anything, as it would also
therefore be the last iteration of the loop.

Paul Lalli

 

[jkstill]

Hi,
I have two arrays, for example,

@array = ("john","larry","kevin","peter","mike")
@exclude = ("john","kevin")

i want to delete from @array , values that exist in @exclude.
i.e. i want my @array to look like

@array("","larry","","peter","mike")

i have written this code but it doesnot seem to work.

foreach$i(0..$#exclude)
{
foreach$x(0..$#array)
{
if($array[$x] eq $exclude[$i])
{
delete($array[$x]);
next;
}
}
}

am I missing something? is there someother better way to do it in
perl?

thanks
-bd

This looks promising:

http://www.perlmonks.org/index.pl?node=429761

 

[jkstill]

Hi,
I have two arrays, for example,

@array = ("john","larry","kevin","peter","mike")
@exclude = ("john","kevin")

i want to delete from @array , values that exist in @exclude.
i.e. i want my @array to look like

i have written this code but it doesnot seem to work.

foreach$i(0..$#exclude)
{
foreach$x(0..$#array)
{
if($array[$x] eq $exclude[$i])
{
delete($array[$x]);
next;
}
}
}

am I missing something? is there someother better way to do it in
perl?

thanks
-bd

This looks promising:

http://www.perlmonks.org/index.pl?node=429761

My bad, replied to the wrong thread.