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================================================================================
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If, Else, Elsif
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In perl, we can control how a program executes by using if, elsif, and else:
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if (CONDITION) {
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STATEMENT;
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} elsif (CONDITION) {
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STATEMENT;
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} else {
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STATEMENT;
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If the first CONDITION is true, we execute the statements in the first block;
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else (otherwise) if the second CONDITION is true, we execute the statements
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in the second block; else we execute the statements in the last block.
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NOTE: In many languages, the keyword is else if. In perl, however, it's elsif
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(there is only one letter 'e').
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my @feeds;
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if (scalar(@feeds) == 0) {
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print "You have no feeds\n";
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} elsif (scalar(@feeds) < 10) {
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print "You have ".scalar(@feeds)." feeds\n";
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} else {
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print "You have too many feeds!\n";
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The code above first checks if scalar(@feeds) (the length of the array
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@feeds) is equal to zero. If so, it says you have no feeds. Otherwise, it
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checks if the length of the feeds is less than 10. If so, it reports the
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number of feeds. Otherwise, it says you have too many feeds.
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================================================================================
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Booleans
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Perl is flexible about what it considers to be true or false. The empty
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string "" and "0" are considered false. All other strings are true. The
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number 0 is considered false; all other numbers are true. All references
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are true; undef (for when a variable is undefined) is false.
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This can help save some typing. For example:
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if (scalar(@feeds) == 0) {
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print "You have no feeds\n";
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can now be rewritten as:
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if (scalar(@feeds)) {
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print "You have no feeds\n";
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================================================================================
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Comparisons
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Perl has different operators for comparing strings and numbers:
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Comparison | String | Numeric
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--------------+---------------+------------------
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equal | eq | ==
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--------------+---------------+------------------
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not equal | ne | !=
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--------------+---------------+------------------
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less than | lt | <
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--------------+---------------+------------------
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greater than | gt | >
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--------------+---------------+------------------
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less than or | le | <=
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equal to | |
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--------------+---------------+------------------
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greater than | ge | >=
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or equal to | |
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--------------+---------------+------------------
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comparison | cmp | <=>
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For example, if you want to compare if two strings are equal:
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if ($arguments->{who} eq "nickname") {
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This checks if the sender's nick is the same as "nickname". If you want
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to compare if two numbers are equal:
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if ($arguments->{who} eq "nickname") {
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The spaceship operator <=> and cmp compares the first scalar to the second.
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-1 is returned if the first is less than the second, 0 if they are equal,
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and +1 is the first is greater than the second:
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print $x <=> $y;
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This prints -1 if $x < $y, 0 if $x == $y, and 1 if $x > $y.
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WARNING: Make sure to use the correct operator for strings and numbers.
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"6" < "1337" is true because the number 6 is less than the number 1337,
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but "6" lt "1337" is false because the string "1337" comes before "6"
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when sorted alphabetically. (It's the same reason why the word "bot" comes
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before the word "computer" in the dictionary.)
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================================================================================
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Logical Operators
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Operator | Operator | Meaning
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--------------+--------------+-------------------------------------------
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&& | and | True if both operands are true,
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| | false otherwise
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--------------+--------------+-------------------------------------------
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|| | or | True if either operand is true,
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| | false otherwise
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--------------+--------------+-------------------------------------------
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! | not | False if operand is true,
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| | true if operand is false
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--------------+--------------+-------------------------------------------
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| xor | True if the first or second operand is
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| | true, false otherwise
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We can use the || or operator for providing default values:
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$var ||= 1;
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This means the same as:
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$var = $var || 1;
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If $var is not defined, then $var is false, and undef || 1 will
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return the second value, 1. So $var gets the default value of 1
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if it is undefined.
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================================================================================
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While, Do
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In a while loop, perl checks if CONDITION is true; if so, it executes
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STATEMENT, then repeats the loop again. If CONDITION is false, it leaves
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the loop.
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while (CONDITION) {
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STATEMENT;
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Here's a sample while loop:
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while (scalar(@feedURLs)) {
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my $url = pop(@feedURLs);
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process($url);
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While there are still urls remaining, we pop the last url from
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@feedURLs, remove it from the array, and process it.
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================================================================================
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For Loop
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for (INITIALIZE; CONDITION; STEP) {
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STATEMENT;
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INITIALIZE is executed only once at the beginning.
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Next, CONDITION is checked. If CONDITION is false, the loop is finished.
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If CONDITION is true, then STATEMENT is executed, and then STEP.
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Then the loop repeats itself.
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for (my $i = 0; $i < scalar(@nicks); $i++) {
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$self->say(
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channel => "#perl103"
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body => "Hi, $nick[$i]";
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In this code, we first run INITIALIZE: set $i to zero. Then, we check
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CONDITION: is $i less than the length of the array @nicks? If true,
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we execute STATEMENT. If not, the loop is finished.
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For STATEMENT, we send a message to the channel #perl103 and say hi to
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$nick[$i], the nick at index $i in the array. If $i = 0, then we say hi
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to $nick[0], the first nick. If $i = 1, then we say hi to $nick[1],
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the second nick.
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In other words, this loop sends a hello message to channel #perl103 to
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every nick in the array @nicks.
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================================================================================
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Challenge
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View the file ~/challenge to finish the lesson.
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================================================================================
