Learning Perl Notes --2

Array and lists

A list is an ordered collection of scalars. An array is a variable that contains a list. They’re interchangeable. But, to be accurate, the list is the data, and the array is the variable.

1. There’s no limit on array length which is automatically extended.
2. To refer to an entire array, just use the @ before the name of the array (and no index brackets after it), for example @myArray.
3. Access array element by index: $myArray[0] .
4. the last element index is $#myArray.
5. array copy: when an array is copied to another array, it’s still a list assignment. Ror example:
   @copy = @quarry; # copy a list from one array to another
6. pop
   pop operator takes the last element off of an array and returns it:
   If the array is empty, pop will leave it alone, and it will return undef.
7. push: adds an element (or a list of elements) to the end of an array
8. The shift and unshift Operators : perform the corresponding actions on the “start” of the array

9. The foreach Control Structure
   foreach $rock (qw/ bedrock slate lava /) {
   print "One rock is $rock.\n"; # Prints names of three rocks
   }
   The control variable ($rock in that example) takes on a new value from the list for each iteration. The first time through the loop, it’s "bedrock"; the third time, it’s "lava".
   Notes:

• The control variable is not a copy of the list element—it actually is the list element. That is, if you modify the control variable inside the loop, you’ll be modifying the element itself .
• If you omit the control variable from the beginning of the foreach loop, Perl uses its default variable, $_. For example: foreach (1..10) { # Uses $_ by default print "I can count to $_!\n"; } .

10. reverse returns the reversed list; it doesn’t affect its arguments.
    If the return value isn’t assigned anywhere, it’s useless:
    reverse @fred; # doesn't change @fred
    @fred = reverse @fred; # puts the result back into the original array

Scalar and List Context

1. As Perl is parsing your expressions, it always expects either a scalar value or a list value. What Perl expects is called the context of the expression
   @list = @people; # a list of three people
   $n = @people; # the number 3
2. scalar: Forcing Scalar Context where Perl is expecting a list.
   print "I have ", @rocks, " rocks!\n"; # WRONG, prints all elements in @rocks array
   print "I have ", scalar @rocks, " rocks!\n"; # Correct, gives a number
3. <STDIN> in List Context ,
   <STDIN> returns the next line of input in a scalar context.
   In list context, this operator returns all of the remaining lines up to the end of file. Each line is returned as a separate element of the list. If input is not from a file but the keyboard, then you’ll normally type a Control-D† to indicate to the system that there’s no more input;
   For example:
   @lines = <STDIN>; # read standard input in list context