Get the length of a list.

nbtrap has asked for the wisdom of the Perl Monks concerning the following question:

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Re: Get the length of a list. by LanX (Saint) on Jul 27, 2013 at 15:36 UTC
see so called goatse (pseudo-)operator also known as Not suitable for work operator `DB<100> $length =()= (1, 2, 3, 4, 3, 2, 1) => 7` [download] Cheers Rolf ( addicted to the Perl Programming Language)	[reply] [d/l]
Re: Get the length of a list. by farang (Chaplain) on Jul 27, 2013 at 16:35 UTC
Another way, though not necessarily better than the original. `print scalar map $_, (1, 2, 3, 4, 3, 2, 1);` [download] But if other processing is going to be done with the list, might as well just make a named array out of it from the start.	[reply] [d/l]
Re: Get the length of a list. by Anonymous Monk on Jul 27, 2013 at 15:18 UTC
Assuming that you have: `@arr=(1, 2, 3, 4, 3, 2, 1);` [download] you can do: `$length_of_array=@arr; print $length_of_array."\n";` [download]	[reply] [d/l] [select]
Re: Get the length of a list. by zork42 (Monk) on Jul 28, 2013 at 04:44 UTC
Evalutating a list in scalar context returns the last element in the list, not its length: `print scalar(1, 2, 3, 4, 3, 2, 1);` will print "1". ~~Nope, evaluating a list in scalar context returns the length of the list.~~ () Apologies, I was getting lists and arrays confused there. I was talking about arrays, not lists. That should have read: "Evaluating an array* in scalar context returns the length of the array." However, I hope my following explaination of why `scalar(list)` does not return the length of the list is still useful. The documentation for scalar says: Because scalar is a unary operator, if you accidentally use a parenthesized list for the EXPR, this behaves as a scalar comma expression, evaluating all but the last element in void context and returning the final element evaluated in scalar context. This is seldom what you want. So what is actually happening here is `(1, 2, 3, 4, 3, 2, 1)` evaluates to `1`, the last element in the list. And `scalar 1` means evaluate `1` in scalar context, which is drum roll `1` ! :) `use warnings;` would have detected this. Example code: `use strict; use warnings; print "A: ", scalar(10, 20, 30, 40, 50, 60, 70), "\n"; # = scalar +(70) = scalar 70 = 70 print "B: ", scalar(70), "\n"; # + = scalar 70 = 70 print "C: ", scalar 70, "\n"; # + = 70 my $x = scalar(10, 20, 30, 40, 50, 60, 70); # = scalar +(70) = scalar 70 = 70 print "D: ", $x, "\n"; $x = (10, 20, 30, 40, 50, 60, 70); # = +(70) = 70 print "E: ", $x, "\n"; my @a = (10, 20, 30, 40, 50, 60, 70); print "F: ", scalar @a, "\n"; # = 7, the + length of @a` [download] Output: Useless use of a constant (10) in void context at D:\stuart\play\temp\ +pp.pl line 4. Useless use of a constant (20) in void context at D:\stuart\play\temp\ +pp.pl line 4. Useless use of a constant (30) in void context at D:\stuart\play\temp\ +pp.pl line 4. Useless use of a constant (40) in void context at D:\stuart\play\temp\ +pp.pl line 4. Useless use of a constant (50) in void context at D:\stuart\play\temp\ +pp.pl line 4. Useless use of a constant (60) in void context at D:\stuart\play\temp\ +pp.pl line 4. Useless use of a constant (10) in void context at D:\stuart\play\temp\ +pp.pl line 10. Useless use of a constant (20) in void context at D:\stuart\play\temp\ +pp.pl line 10. Useless use of a constant (30) in void context at D:\stuart\play\temp\ +pp.pl line 10. Useless use of a constant (40) in void context at D:\stuart\play\temp\ +pp.pl line 10. Useless use of a constant (50) in void context at D:\stuart\play\temp\ +pp.pl line 10. Useless use of a constant (60) in void context at D:\stuart\play\temp\ +pp.pl line 10. Useless use of a constant (10) in void context at D:\stuart\play\temp\ +pp.pl line 13. Useless use of a constant (20) in void context at D:\stuart\play\temp\ +pp.pl line 13. Useless use of a constant (30) in void context at D:\stuart\play\temp\ +pp.pl line 13. Useless use of a constant (40) in void context at D:\stuart\play\temp\ +pp.pl line 13. Useless use of a constant (50) in void context at D:\stuart\play\temp\ +pp.pl line 13. Useless use of a constant (60) in void context at D:\stuart\play\temp\ +pp.pl line 13. A: 70 B: 70 C: 70 D: 70 E: 70 F: 7 [download] UPDATE: (a few minutes later) added comments to code, and 1 extra example *UPDATE #2: As choroba pointed out in the following post, I was getting lists and arrays confused. See () above.**	[reply] [d/l] [select]
Re^2: Get the length of a list. by choroba (Cardinal) on Jul 28, 2013 at 10:29 UTC
Nope, evaluating a list in scalar context returns the length of the list. You seem to confuse lists and arrays. لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ	[reply]
Re^3: Get the length of a list. by zork42 (Monk) on Jul 29, 2013 at 04:11 UTC
You are correct choroba. Thanks for pointing out my error :) perldata - Context highlights the difference between `scalar(array)` and `scalar(list)`: If you evaluate an array in scalar context, it returns the length of the array. (Note that this is not true of lists, which return the last value, like the C comma operator, nor of built-in functions, which return whatever they feel like returning.)	[reply] [d/l] [select]
Re: Get the length of a list. by mtmcc (Hermit) on Jul 27, 2013 at 16:07 UTC
Also, you can find the last index in the array like this: `my @list = (1, 2, 3, 4, 3, 2, 1); print STDERR "Last index: $#list\n";` [download] It's clearly not the length, but it is handy in some settings.	[reply] [d/l]
Re: Get the length of a list. by Anonymous Monk on Jul 27, 2013 at 16:09 UTC
if all you want is the length of an array, why not? `print $#;`	[reply] [d/l]
Re^2: Get the length of a list. by LanX (Saint) on Jul 27, 2013 at 16:41 UTC
Lists are not arrays! see FAQ#What is the difference between a list and an array? for instance the easiest way to know how many values a function or a `grep` returned is using a goatse: `DB<101> sub tst {a..z} DB<102> $length =()= tst() => 26 DB<107> grep { $_ % 2 } 1..10 => (1, 3, 5, 7, 9) DB<108> $n =()= grep { $_ % 2 } 1..10 => 5` [download] Cheers Rolf ( addicted to the Perl Programming Language)	[reply] [d/l] [select]