Position in an array

[bernd]

Hello folks,

just for confirmation: Is my assumption correct that there is no
"implicit" method to know the current index of an array (means: the
index of the item which is stored in $_ in a foreach-loop for example)
without using an "external" counter variable?

In the following I want to traverse the array @testarr and want to
treat the very first item (but it could be any other arbitrary item as
well) specially. Is there a way of "knowing" the index without using
the explicitly defined variable $count?

$count = 0 ;
foreach ( @testarr ) {

if ( $count == 0 ) { print $_ }

# same code for all items ....

$count++ ;
}

So, can I get rid of $count in some way (by using an implicit
mechanism) or do I have to live with this, IMO, long winded way.

Cheers


Bernd

 

[Robert 'phaylon' Sedlacek]

$count = 0 ;
foreach ( @testarr ) {

if ( $count == 0 ) { print $_ }

# same code for all items ....

$count++ ;
}

So, can I get rid of $count in some way (by using an implicit
mechanism) or do I have to live with this, IMO, long winded way.

How about

for my $x (0 .. $#testarr) {
print if $x == 0;
}

 

[Mirco Wahab]

for my $x (0 .. $#testarr) {
print if $x == 0;
}

foreach ( @testarr ) {
print if $# == 0;
}

perl6?


Regards

Mirco

 

[Robert 'phaylon' Sedlacek]

for my $x (0 .. $#testarr) {
print if $x == 0;
}

Of course that should be...

for my $x (0 .. $#testarr) {
print $testarr[ $x ] if $x == 0;
}

 

[anno4000]

Hello folks,

just for confirmation: Is my assumption correct that there is no
"implicit" method to know the current index of an array (means: the
index of the item which is stored in $_ in a foreach-loop for example)
without using an "external" counter variable?
Right.


In the following I want to traverse the array @testarr and want to
treat the very first item (but it could be any other arbitrary item as
well) specially. Is there a way of "knowing" the index without using
the explicitly defined variable $count?

$count = 0 ;
foreach ( @testarr ) {

if ( $count == 0 ) { print $_ }

# same code for all items ....

$count++ ;
}

So, can I get rid of $count in some way (by using an implicit
mechanism) or do I have to live with this, IMO, long winded way.

Alternatively you can loop over the index set in the first place:

for my $count ( 0 .. $#testarr ) {
my $el = $testarr[ $count];
print $el unless $count;

# etc.
}

or variants thereof.

Anno

 

[tfe]

Robert 'phaylon' Sedlacek a écrit :

How about

for my $x (0 .. $#testarr) {
print if $x == 0;
}

Might be
for my $x (0 .. $#testarr) {
print $testarr[$x] if $x == 0;
}
or you should do $_ = $test[$x] at the start of the loop...

 

[anno4000]

Robert 'phaylon' Sedlacek a écrit :

How about

for my $x (0 .. $#testarr) {
print if $x == 0;
}

Might be
for my $x (0 .. $#testarr) {
print $testarr[$x] if $x == 0;
}
or you should do $_ = $test[$x] at the start of the loop...

Doing it like that is a very bad idea. $_ is often aliased to
another variable. If it is when this code is called, "$_ = $test[$x]"
will assign the value to an unrelated variable somewhere in the
program, which can be a hard-to-find bug.

Before assigning to $_ you should at least localize the variable:

local $_ = $test[$x];

though a bug in Perl makes even that unsafe in some (rare) situations.
An alternative is another level of aliasing, through a one-shot do
(untested):

for my $x (0 .. $#testarr) {
for ( $testarr[ $x] = {
print unless $x;
# etc.
}
}

Anno

 

[bernd]

@Robert: What You suggested is, basically, what I wanted to avoid (Your
$x is my $count). I think Anno is right: There is no more elegant way
(o.k., this thread made me sure at least ;-).

Thanks to all!

Bernd

 

[RedGrittyBrick]

$count = 0 ;
foreach ( @testarr ) {
if ( $count == 0 ) { print $_ }
# same code for all items ....
$count++ ;
}

Isn't that the same as this?

print $testarr[0];
for (@testarr) {
# ...
}

 

[bernd]

Not at all, I guess. But what is Your intention?

$count = 0 ;
foreach ( @testarr ) {
if ( $count == 0 ) { print $_ }
# same code for all items ....
$count++ ;
}Isn't that the same as this?

print $testarr[0];
for (@testarr) {
# ...



}- Hide quoted text -- Show quoted text -

 

[J. Gleixner]

bernd wrote:

Fixed top-post...

$count = 0 ;
foreach ( @testarr ) {
if ( $count == 0 ) { print $_ }
# same code for all items ....
$count++ ;
}Isn't that the same as this?

print $testarr[0];
for (@testarr) {

Not at all, I guess. But what is Your intention?

I think what the person is showing is, if you only want to print the
first element in @testarr, you can do that by

print $testarr[0];

In other words, there's no need to have a for loop, if all you want to
do is print the first element. I don't think that has anything
to do with your initial question, however based on the code
above that seems to be a possible point to make.

 

[RedGrittyBrick]

bernd top-posted:

Not at all, I guess. But what is Your intention?

I was wondering what yours was.


C:\>perl bernd.pl
--- bernd begins ---
fee
--- bernd ends ---

--- RGB begins ---
fee
--- RGB ends ---



C:\>type bernd.pl
#!perl

@testarr = qw(fee fie foe fum);

print "--- bernd begins ---\n";
#
# bernd wrote
#
$count = 0 ;
foreach ( @testarr ) {
if ( $count == 0 ) { print $_ }
# same code for all items ....
$count++ ;
}
print "\n--- bernd ends ---\n\n";

print "--- RGB begins ---\n";
#
# RGB wrote
# "Isn't that the same as this?"
#
print $testarr[0];
for (@testarr) {
# ...
}
print "\n--- RGB ends ---\n";

 

[Dr.Ruud]

(e-mail address removed)-berlin.de schreef:

$_ is often aliased to
another variable. If it is when this code is called, "$_ = $test[$x]"
will assign the value to an unrelated variable somewhere in the
program, which can be a hard-to-find bug.

"how to solidly spoil the value of $_"

 

[Mark]

Is there a way of "knowing" the index without using
the explicitly defined variable $count?

$count = 0 ;
foreach ( @testarr ) {

if ( $count == 0 ) { print $_ }

# same code for all items ....

$count++ ;
}

So, can I get rid of $count in some way

This following doesn't solve the problem of "'knowing' the index", but
it can detect when the loop is at the desired iteration:

foreach ( @testarr ) {
print if \$_ == \$testarr[0] ;
}