Handout 3

Overview

• Questions from last time
• Control structures continued
• Input-Output

Control structures

• if.
• while.
• for. A last point: incrementing and decrementing are so common that there is a common abbreviation for it: $a++ is the same as $a = $a + 1 and $a-- is the same as $a = $a - 1.
• foreach. The foreach structure is quite useful; it allows you to iterate over some list. Here's how it looks:

  		foreach $variable (list of items) {
  			list of statements
  		}

  The list of items can either be a comma-separated list (example) or an array (example).

  Numbers and letters are used in list context so often that there is a special abbreviation that you can use for them, e.g. ("a".."e") is the same thing as ("a", "b", "c", "d", "e").

Input-Output

• For programs to do anything of use, they must be able to respond to their environment: you must be able to get data into and out of them. We will treat three input-output types: command line, prompt, and file.
• Command line. Command-line input is where things happen, as you type in the word perl followed by the name of the program at the DOS prompt. Anything that follows the name of the program is automatically put in the special array variable @ARGV. This can then be referred to in programs (example).