Re^2: how internally $#array is working in Perl

From perlvar:

$[

The index of the first element in an array, and of the first character
+ in a 
substring. Default is 0, but you could theoretically set it to 1 to ma
+ke Perl 
behave more like awk (or Fortran) when subscripting and when evaluatin
+g the 
index() and substr() functions. (Mnemonic: [ begins subscripts.)

As of release 5 of Perl, assignment to $[ is treated as a compiler dir
+ective, and 
cannot influence the behavior of any other file. (That's why you can o
+nly assign 
compile-time constants to it.) Its use is deprecated, and by default w
+ill trigger 
a warning.

Note that, unlike other compile-time directives (such as strict), assi
+gnment to 
$[ can be seen from outer lexical scopes in the same file. However, yo
+u can use 
local() on it to strictly bind its value to a lexical block.
[download]

I suspect you can assign 1 to it, but other values are silently ignored. I tried your code with $[=1, and lo and behold:

perl -le '@arr=qw(10 1 2 3 4);$[=1;print $arr[$_],"\t",$_ for 0 .. $#a
+rr;'
10    0
10    1
1    2
2    3
3    4
4    5
[download]

(shrug)

SSF

Comment on Re^2: how internally $#array is working in Perl Select or Download Code

Replies are listed 'Best First'.
Re^3: how internally $#array is working in Perl by DrHyde (Prior) on Aug 13, 2010 at 09:59 UTC
I suspect you can assign 1 to it, but other values are silently ignored You suspect incorrectly. `$ perl -e '@array=(qw(ant bat cat)); $[ = 94; print $array[95]."\n"' bat $ perl -e '@array=(qw(ant bat cat)); $[ = -9; print $array[-8]."\n"' bat` [download]	[reply] [d/l]
Re^4: how internally $#array is working in Perl by ikegami (Patriarch) on Aug 13, 2010 at 13:53 UTC
You're not even limited to positive numbers. `$ perl -wle'$[ = -3; $_ = "abcdef"; print substr($_,0,1)' Use of assignment to $[ is deprecated at -e line 1. d` [download] You could get weird constructs that caused Perl to crash: `$ perl -we'$[ = 3; $_ = "abc"; my $x = substr($_,2,1)' panic: sv_setpvn called with negative strlen at -e line 1.` [download] I fixed it in 5.10.1 or 5.12.0 when fixing another bug: `$ perl -we'$[ = 3; $_ = "abc"; my $x = substr($_,2,1)' Use of assignment to $[ is deprecated at -e line 1. substr outside of string at -e line 1.` [download] Keep in mind `$[` is slated to be removed from Perl.	[reply] [d/l] [select]
Re^5: how internally $#array is working in Perl by DrHyde (Prior) on Aug 16, 2010 at 09:51 UTC
You're not even limited to positive numbers. yes, as I demonstrated with my example that used -9 :-) Keep in mind $[ is slated to be removed from Perl. has `a2p` been considered when making this change?	[reply] [d/l]
Re^6: how internally $#array is working in Perl by Corion (Patriarch) on Aug 16, 2010 at 09:55 UTC
Re^3: how internally $#array is working in Perl by nvivek (Vicar) on Aug 13, 2010 at 08:50 UTC
How internally array holds the value like this way.I have checked the code by using Dumper function in Data::Dumper module. In that, it printed only 10,1,2,3,4 but when I loop through the array.It printed 6 values as 10,10,1,2,3,4.Explain me how it works? Thanks in advance	[reply]
Re^4: how internally $#array is working in Perl by jethro (Monsignor) on Aug 13, 2010 at 09:34 UTC
If you set $[ to 1 you tell perl that array index 0 doesn't exist. Now ask yourself: What should perl do if you then access or print $a[0]? What's the value of an array index that doesn't exist? The answer is: Anything If you write `print 1/0;` you don't expect a meaningful answer, right? Similarly you don't get a meaningful value in $a[0] if you tell perl that $a[0] doesn't exist The correct way to handle access to index 0 would be to print an error message or warning. But that might break some really old scripts. So perl seems to print the same value that is in index 1, the first "legal" index. And a word of advice: Don't change `$[`. And if you want to loop through an array, `foreach (@a) {}` will get you any value, no matter at what index the array begins and ends	[reply] [d/l] [select]


Come for the quick hacks, stay for the epiphanies.
	PerlMonks