Beginning Web Development With Perl : From Novice to ... - Nabo

APPENDIX ■ PERL BASICS 285In the last example in the table, 1F18 is a hexadecimal number referring to a character inthe Unicode character set, which runs from 0000-FFFF. As another example, \ x{ 2620} is theUnicode character for a skull-and-crossbones!White SpaceAs mentioned previously, white space is the name we give to tabs, spaces, and newlines. Perl isvery flexible about where you put white space in your program. You’ve already seen that you’refree to use indentation to help show the structure of blocks. You don’t need to use any whitespace at all, if you don’t want to. If you’d prefer, your programs can all look like this:print"Top level\ n";{ print"2nd level\ n";{ print"3rd level\ n";}print"Where are we?";}This is considered a bad idea. White space is another tool we have to make our programsmore understandable; let’s use it as such.Types of DataA lot of programming jargon is about familiar words in an unfamiliar context. You’ve alreadyseen a string, which was a series of characters. You could also describe that string as a scalarliteral constant. What does that mean?By calling a value a scalar, you’re describing the type of data it contains. If you rememberyour math (and even if you don’t), a scalar is a plain, simple, one-dimensional value. In math,the word is used to distinguish it from a vector, which is expressed as several numbers. Velocity,for example, has a pair of coordinates (speed and direction), and so must be a vector. In Perl,a scalar is the fundamental, basic unit of data of which there are two kinds: numbers andstrings.A literal is value that never changes. The value 5 is a scalar literal—and is literally 5; it cannever be 4. Perl has three types of scalar literals: integers (such as 5), floating-point numbers(like 3.14159), and strings (for example, “hello, world”). To put it another way, a literal is a constant—itnever changes, as opposed to a variable, which is a piece of memory that can holda scalar value. Variables are so named because the value stored within them can vary. Forinstance, $number can be assigned 5, and then later can be changed to the value 6. We will talkmore about variables later in this appendix.NumbersThere are two types of numbers that we’re interested in as Perl programmers: integers andfloating-point numbers. The latter we’ll come to in a minute, but let’s work a bit with integersright now. Integers are whole numbers with no numbers after the decimal point, such as 42,–1, or 10. The following program prints a couple of integer literals in Perl:#!/usr/bin/perl -w# number1.plprint 25, -4;