Introduction to Perl

1. Introductionto Perl Nicole Vecere

2. Background • General Purpose Language • Procedural, Functional, and Object-oriented • Developed for text manipulation • Currently used for web development, administrative systems, etc. • Recommended program setup: #!/usr/bin/perl use strict; use warnings;

3. Scalar Data • Scalar (or basic) data types hold a single data item • This data item may be a number, a string, or a reference • A scalar may not hold multiple values, but it may contain a reference to an array, list, hash, etc. • Initially, the value undef is assigned to all variables • This acts as 0 when applied to a number • Or as an empty string (“”) when applied to a string

4. Variables and Operators • Variables are defined using $ $number = 4; $string = “Hello, world”; • Assignment operator: = • Arithmetic Operators: +, -, *, / • String Operators: . –string concatenation (a dot) • x –string repetition • Numeric Comparison Operators: ==, !=, <, >, <=, >= • String Comparison Operators: eq, ne, lt, gt, le, ge

5. Special Features • Interpolation of scalars into strings • Any scalar variable inside a double-quoted string is replaced by its value $name = “Nicole”; print “My name is $name”; # Prints My name is Nicole • Automatic conversion between numbers and strings $cat = 2; $dog = 4; print $cat * $dog; # Prints 8 print $cat x $dog; # Prints 2222

6. If Else Control Structure • Similar to C++, but curly braces are required. • Example: if (…) { … } else { … }

7. While Control Structure • Similar to C++, but curly braces are required. • Example: while (…) { … }

8. Standard Input • Perl allows for user input from the standard input device • This is generally the user’s keyboard • <STDIN> can be used to read a full line from standard input $line = <STDIN>; # Reads line and stores it in a variable. while(defined($line=<STDIN>)){ print “This line was: $line \n”; } # Prints lines of input right after inputting.

9. Standard Input (Continued) while(<STDIN>) { print “This line was: $_ \n”; } # Each line of input is automatically stored inside # Perl’s default variable $_. while(<>) { print “This line was: $_ \n”; } # The diamond operator has the advantage of allowing command # line arguments.

10. Lists and Arrays • List Representations • (1,2,3,4,5,6,7,8,9,10) # A list consisting of numbers 1,2,…,10. • (1..10) # Same list as above, created by the range operator. • ($a ..$b) # Range determined by current values of $a and $b. • List Assignment • ($cat, $dog) = ($dog, $cat); # Values are swapped. • @numbers = (1..10); # List (1..10) is stored inside an array.

11. Accessing List Elements • Examples: • @values = (1..100); • print $values[0]; # Prints 1 • print $values[35]; # Prints 36 • print $values[100]; • # Nothing is printed. Value of $values[100] is undef. • print $values[$#values]; • # $#values is the largest index. • # Value 99 is printed since it’s the last list element.

12. Reading into a List • Example: while(<>){ $lines[$index] = $_; $index = $index + 1; } print $#lines+1; print @lines; • # Lines of input become elements of a list stored inside • # array @lines. After reading we print out the number of • # input lines, then the whole list.