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

APPENDIX ■ PERL BASICS 301We’ll see this operator used when we look at sorting things, where we have to knowwhether something goes before, after, or in the same place as something else.Boolean OperatorsAs well as being able to evaluate the truth and falsehood of some statements, we can alsocombine such statements. For example, we may want to do something if one number is biggerthan another and another two numbers are the same. The combining is done in a very similarmanner to the bitwise operators we saw earlier. We can ask if one value and another value areboth true, or if one value or another value are true, and so on.The operators even resemble the bitwise operators. To ask if both truth values are true, wewould use && instead of &.So, to test whether 6 is more than 3 and 12 is more than 4, we can write6 > 3 && 12 > 4To test if 9 is more than 7 or 8 is less than 6, we use the doubled form of the | operator, ||:9 > 7 || 6 > 8To negate the sense of a test, however, use the slightly different operator !. This hasa higher precedence than the comparison operators, so use parentheses. For example, thistests whether 2 is not more than 3:!(2>3)while this one tests whether !2 is more than 3:!2>32 is a true value. !2 is therefore a false value, which gets converted to 0 when we do a numericcomparison. We’re actually testing if 0 is more than 3, which has the opposite effect to what wewanted.Instead of those forms, &&, ||, and !, we can also use the slightly easier-to-read versions,AND, OR, and NOT. There’s also XOR, for exclusive or (one or the other but not both are true),which doesn’t have a symbolic form. However, you need to be careful about precedence again:#!/usr/bin/perl -w# bool6.plprint "Test one: ", 6 > 3 && 3 > 4, "\ n";print "Test two: ", 6 > 3 and 3 > 4, "\ n";This prints, somewhat surprisingly, the following:$ perl bool6.plUseless use of a constant in void context at bool6.pl line 5.Test one:Test two: 1$We can tell from the presence of the warning about line 5 and from the position of theprompt that something is amiss (or least Unix users can—Windows users need to be a bitmore alert since Windows automatically adds a newline character at the end of the program so