Perl Cheatsheet 2 - Alex Jiang

SLP week1: A review

Perl, is eager to give you the answer, and try its best to guess an answer for you!
No reserved word, prefix with var distinguish data from build-ins (so developers can add more build-ins later on without conflicts)
in Perl, values are all scalars, we don’t have data type explicitly stating numerical value and string, perl will take longest path to legal values, evaluate it, then quit
- e.g. scalar conversion (13+4)."love" = "17love", "12god" / 3 = 4
- e.g. "recursion" x 3.3 = recursionrecursionrecursion

=~ stands for searching for pattern e.g. =~ /a*/
use warning; will warn you dangerous thing, but will still execute all code without stopping
Syntax
- every statement ends with a semi-colon
- make sure block statement wrapped with { and }
- case sensitive
- reference: $ref = \$whence; the \ is to get the address of $var

Variable Define
- check if defined: ` defined($var)` the var is defined but has empty value is different from var isn’t defined (like C)
- e.g. $line = <STDIN>; if (defined($line)) {…} good practice
- e.g. chomp( $line = <STDIN> ) to get rid of new line symbol good practice
- to release a var, garbage collect a var, use ` undef($var)`
Scalar var
- _ in numerical var is ignored e.g. $money = 2020_22_11
- V string, list ASCII value e.g. $lang = [v]80.101.22.11 [v]:optional
Scope of var
- my: the inner block, within the { and }
- local: a copy of var within the { and }
- our: global
- once var get out of scope, garbage collected, totally garbage collected var evaluated to 0 !

True and False
- False: "0", empty string, undefined var, and 0
Operators in Conditions
- operator set the context, again, guess an answer for you
- <=> and cpm are comparison, with signed result
Condition
- if/unless (x > 50) { print; }
  - even only one line statement needs braces, to avoid C programming dangle else trap
- print if/unless (x > 50);
  - e.g. for printing print $a, " is $b", $c;
Loop
- for (my $x = 0; $x <= 0; $sp++) {
- for/foreach $x (@array) {
- switch: SWITCH: { if (/perl/) { $perl = 1; last SWITCH;