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Re: Get the length of a list.

by zork42 (Monk)
on Jul 28, 2013 at 04:44 UTC ( #1046707=note: print w/ replies, xml ) Need Help??


in reply to Get the length of a list.

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
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


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.


Comment on Re: Get the length of a list.
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Re^2: Get the length of a list.
by choroba (Abbot) 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
      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.)

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