array element return syntax

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

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Re: array element return syntax by ikegami (Patriarch) on Mar 29, 2006 at 15:32 UTC
Close. You need parens around the list expression being indexed. `$hour = (localtime(time))[ 2 ];` [download] You can also extract multiple elements: `my ($y, $m, $d) = (localtime(time))[ 5, 4, 3 ];` [download] This feature is called a "list slice", and it's documented in perldata.	[reply] [d/l] [select]
Re^2: array element return syntax by Lyndley (Novice) on Mar 29, 2006 at 15:42 UTC
`H:\scripts>perl -e "print (localtime(time))[2]" syntax error at -e line 1, near ")[" Execution of -e aborted due to compilation errors.` [download] 8(	[reply] [d/l]
Re^3: array element return syntax by davorg (Chancellor) on Mar 29, 2006 at 15:50 UTC
`-w` will help you understand what's going on. `$ perl -we "print (localtime(time))[2]" print (...) interpreted as function at -e line 1. syntax error at -e line 1, near ")[" Execution of -e aborted due to compilation errors.` [download] `use diagnostics` is even better. `$ perl -Mdiagnostics -we "print (localtime(time))[2]" print (...) interpreted as function at -e line 1 (#1) (W syntax) You've run afoul of the rule that says that any list op +erator followed by parentheses turns into a function, with all the list operators arguments found inside the parentheses. See perlop/Terms and List Operators (Leftward).` [download] So stop the parentheses looking like a function. `$ perl -e "print +(localtime(time))[2]" 16` [download] -- <http://dave.org.uk> "The first rule of Perl club is you do not talk about Perl club." -- Chip Salzenberg	[reply] [d/l] [select]
Re^3: array element return syntax by ikegami (Patriarch) on Mar 29, 2006 at 15:46 UTC
`print` is claiming your parens. In other words, Perl sees your code as `print( ... )[2];` To fix the problem, use `print( (localtime(time))[2] );` or `print +(localtime(time))[2];`	[reply] [d/l] [select]
Re: array element return syntax by Rice (Acolyte) on Mar 29, 2006 at 15:40 UTC
Subscripts do operate on lists, as you guessed. `print` also takes a list as its argument, so you need to delimit your lists with parentheses. Try: `print ( (localtime(time))[2] ) ; print ( ($db->Error)[ 1 ] ) ;` [download] or even `print ( (localtime)[2] ) ;` [download]	[reply] [d/l] [select]
Re: array element return syntax by kwaping (Priest) on Mar 29, 2006 at 15:42 UTC
This also works. Update: Not the ideal solution, but at the very least a TIMTOWTDI example. `print [localtime(time)]->[5];` [download] You can also use parentheses, as posted previously. However, when dealing with `print`, for example, you need to let the compiler know that your parens are for wrapping the function and not part of the `print` statement. This will work for that: `print +(localtime(time))[5];` [download] --- It's all fine and dandy until someone has to look at the code.	[reply] [d/l] [select]
Re^2: array element return syntax by merlyn (Sage) on Mar 29, 2006 at 15:56 UTC
This also works. For some meaning of "works", in the same sense that you can add "sleep 1" in various places in your program and it still "works", albeit slower. There's no point in constructing an entire full-fledged array, then take a reference to it, then dereference that reference to get a particular element, then garbage collect the full array, just to get one element of it. It's a lot of extra work. -- Randal L. Schwartz, Perl hacker Be sure to read my standard disclaimer if this is a reply.	[reply]
Re^3: array element return syntax by kwaping (Priest) on Mar 29, 2006 at 15:57 UTC
Thanks! That makes sense. It was just the first thing that sprang to my mind, but now I know better. --- It's all fine and dandy until someone has to look at the code.	[reply]
Re^2: array element return syntax by ikegami (Patriarch) on Mar 29, 2006 at 15:44 UTC
And the slice "equivalent" would be `my ($y, $m, $d) = @{[localtime(time)]}[ 5, 4, 3 ];` [download] These construct an array, which makes them slower.	[reply] [d/l]
Re^2: array element return syntax by Lyndley (Novice) on Mar 29, 2006 at 15:53 UTC
`H:\scripts\7d>perl -e "print ((localtime(time))[2])" 16` [download] =D Excellent thankyou. Could I be cheeky and ask just how `print ((localtime(time))[2])` works at each step as regards each context From : `time` To : `print ((localtime(time))[2])`?	[reply] [d/l] [select]
Re^3: array element return syntax by duff (Parson) on Mar 29, 2006 at 16:16 UTC
`time` returns a number that represents the number of seconds since the start of the epoch. `localtime(time)` returns a list of values that represent the seconds, minutes, etc. in the local time zone `(localtime(time))[2]` indexes into the list (a convenience that perl allows) and returns the 3rd item in the list `print((localtime(time))[2])` outputs the 3rd item in the list returned by `localtime` The only "weird" part might be why there are extra parens on the print. I.e., why not `print (localtime(time))[2];` [download] And the answer is precedence. To perl, that looks like a function call (to the `print` function) with a parameter of `localtime(time)`. So it's really `print(...)[2];` [download] Which doesn't make any sense, so perl tells you so. duff	[reply] [d/l] [select]