Perl CheatSheet 5 - Alex Jiang

init. one element in list. $fred[0] = "do";
maximum element index: $index = $#fred
last value of list: $last = $fred[$#fred] or $last = $fred[-1]
out-of-bound subscripts return undef, print nothing
assigning beyond $#list, just stretches the list, filling the middle element with undefined. e.g. $fred[20] = hello, then $fred[1…19] is there, but undefined

list e.g. : (1,2,3) or ("fred", 4.5) or (1..10)
e.g. @ar = (1,2,3,4) @ar[1..3] == 4 is true, because @ar[1..3] in this context, only return 4 which is 4 == 4
tips: qw|i am a idoit| quote each element with " ", so you don’t need to manually type all quotes.
$" list separator, e.g. $" = "++", print @ar; will print something like aa++bb++cc++dd

assignment of list: ($a, $b) = ("a", 2)
- list built up before assignment, swap made easy: ($a, $b) = ($b, $a)
@: all of the. @rock = qw(rock1 rock2)
@ can be nested. @rock_more = ("diamond", @rock, "go");
remove end of array [has side effect]: $final = pop @num;
append to end of array [has side effect]: push @num, 10..15
remove from beginning [has side effect]: $first = shift @num;
reverse and sort [create copy]

designer’s choice, programmers don’t need to think about var name and type.
omit control var from beginning of loop, perl uses $_ as default control var.
for (1..10) { $_ += 10; }
$_ = "perl ruby python"; print, will print perl ruby python
$_ is by default a global variable. However, as of perl v5.10.0, you can use a lexical version of $_ by declaring it in a file or in a block with my

designer’s choice: you can’t identify the meaning of expression until you know the context
e.g. @people = qw(me mum); @list = @people list here is list, #num = @people the number of people, 2
e.g. 123 + sth is scalar context, because + is for scalar !
e.g. print @ar."\n": is scalar context, print size of array
e.g. scalar (@ar) is scalar context, print size of array
in a nutshell, if you see some operators belonging to list, then its list context, otherwise, its scalar context.

for $item (@rock = qw(java ruby python)) { $item .= " lang" }
- $item automatically become my var. any outer $item is unaffected
- $item as pointer, assigning to $item affects the @rock
- for (@rock) { $_ = "hard $_" }
- " " evaluates all symbols in it, ' ' just treats as literal string