CSC 4630

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CSC 4630

• Perl 2
• adapted from R. E. Beck

2
I/O and Arithmetic

• Form pairs of programmer/investigator/discover
• Exercise 1. Write a program that prompts the
  user for a number, representing the radius of a
  sphere. The program prints out the volume of the
  sphere. (Note B to 10 significant figures is
  3.141592654)

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Operator x

• Exercise 2. Write a program that prompts the
  user for a (short) string and a positive integer
  and outputs the string repeated the number of
  times indicated. For example, with input ha 4
  the program gives hahahaha
• Exercise 3. Modify the previous program to print
  the output on separate lines, one for each
  instance of the input string.

4
Arrays in PERL

• Look much like lists from LISP except elements
  must be scalars.
• Size does not need to be specified initially.
• Use length as a descriptive measure.
• Values can be specified by list literals.

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List Literals

• Several forms
• Values separated by commas enclosed in
  parentheses.
• (3.4, 5.6, 8.1, 0)
• (fred, george, 1, 3)
• Values can be determined by expressions, which
  are evaluated when the literal is used.
• (a, b, ab, a-b)

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List Literals (2)

• The empty list, denoted by ()
• Values generated by the list constructor operator
  denoted by ..
• (1 .. 5) generates the list (1, 2, 3, 4, 5)
• (2.2 .. 5.7) generates the list (2.2, 3.2, 4.2,
  5.2)
• (0 .. 4, 8, 9) generates the list (0, 1, 2, 3, 4,
  8, 9)
• Values generated by the quote word function
• qw(alice bob chuck) generates the list
• (alice, bob, chuck)

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Array Variable

• Holds a single list value, meaning zero or more
  scalar values.
• Name starts with _at_ (rather than ).
• Can be treated as a whole by certain array
  operators and array functions. (Similar to LISP
  or APL)

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Array Operators

• Assignment
• _at_a (8, _at_a) Appends 8 as the first element of
  _at_a.
• _at_fred qw(one two)
• _at_barney (4, 5, _at_fred, 8, 9) Makes _at_barney
  have the value (4, 5, one, two, 8, 9)
• (a, b, c) (1, 2, 3) Simultaneous scalar
  assignment
• (a, b) (b, a) Swap values (temporary
  location is hidden)

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Array Operators (2)

• Fancier assignment statements
• Embedded array
• (d, _at_fred) (a, b, c)
• d becomes the value of a
• _at_fred becomes (b, c)
• Array sharing, array decomposition
• (e, _at_fred) _at_fred
• e becomes the first element of _at_fred
• _at_fred becomes tail(_at_fred)

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Array Operators (3)

• Length mismatches
• Too many on right excess discarded
• Too many on left excess set to undef
• Length function implicit
• a _at_fred sets a to length(_at_fred)
• Compare with
• (a) _at_fred
• _at_a (a)

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Array Element Access

• Index origin is 0
• Example
• _at_fred (1..5)
• fred2 is 3
• Slice a subarray chosen by an index list
• Example
• _at_fred (1..5)
• _at_fred2,3,4 is (3,4,5)

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Array Element List (2)

• More slices
• _at_fred0,1,2 _at_fred1,1,1 makes the first
  three elements of _at_fred equal to fred1
• _at_fred (7,8,9)
• _at_barney (2,1,0)
• _at_bf _at_fred_at_barney so _at_bf is (9,8,7)

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Array Element List (3)

• fred is the index value of the last element of
  _at_fred
• So, fredfred is the value of the last
  element of _at_fred
• And so is fred-1 since negative indices count
  backwards from the end, starting at -1.

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Arrays as Stacks

• push (_at_fred, new) is equivalent to
• _at_fred (_at_fred, new)
• a pop(_at_fred) removes the last value of _at_fred
  and assigns it to a.
• Right is no better than left
• unshift (_at_fred, new) is equivalent to
• _at_fred (new, _at_fred)

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Arrays as Stacks (2)

• a shift(_at_fred) assigns the first element of
  _at_fred to a and removes it from _at_fred

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More Array Functions

• reverse
• sort
• chomp