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Control Structures

Control Structures. if-elsif-else. semantically the same as C/C++ syntactically, slightly different. if ($a &gt; 0){ print “$a is positive<br>”; } elsif ($a == 0){ print “$a equals 0<br>”; } else { print “$a is negative<br>”; } brackets are *required*!. unless.

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Control Structures

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  1. Control Structures

  2. if-elsif-else • semantically the same as C/C++ • syntactically, slightly different. if ($a > 0){ print “\$a is positive\n”; } elsif ($a == 0){ print “\$a equals 0\n”; } else { print “\$a is negative\n”; } • brackets are *required*!

  3. unless • another way of writing if (! …) { } • analogous to English meaning of “unless” • unless (EXPR) BLOCK • “do BLOCK unless EXPR is true” • “do BLOCK if EXPR is false” • can use elsif and else with unless as well

  4. while/until loops • while is similar to C/C++ • while (EXPR) BLOCK • “While EXPR is true, do BLOCK” • until (EXPR) BLOCK • “Until EXPR is true, do BLOCK” • “While EXPR is false, do BLOCK” • another way of saying while (! …) { } • again, brackets are *required*

  5. for loops • Perl has 2 styles of for. • First kind is virtually identical to C/C++ • for (INIT; TEST; INCRMENT) { } for ($i = 0; $i < 10; $i++){ print “\$i = $i\n”; } • yes, the brackets are required.

  6. foreach loops • Second kind of for loop in Perl • no equivalent in core C/C++ language • foreach VAR (LIST) {} • each member of LIST is assigned to VAR, and the loop executed $sum = 0; foreach $value (@nums){ $sum += $value; }

  7. More About for/foreach • for and foreach are actually synonyms • they can be used interchangeably. • code *usually* easier to read if conventions followed: • for ($i = 0; $i<10; $i++) {} • foreach $i (@array) {} • list argument of foreach can be any list • foreach $i (@array) {} • foreach $i (1, 2, 3, 4, 5) {} • foreach $i (1 .. 5) {} • foreach $field (split /:/, $data) {}

  8. Two More Thing… (about for) • foreach VAR (LIST) {} • while iterating through list, VAR becomes an *alias* to each member of LIST • Changes within loop to VAR affect LIST • if VAR omitted, $_ used @array = (1, 2, 3, 4, 5) foreach (@array) { $_ *= 2; } • @array now  (2, 4, 6, 8, 10)

  9. Loop Control – next, last, redo • last equivalent of C/C++ break • exit innermost loop • next  (mostly) equivalent of C/C++ continue • begin next iteration of innermost loop • redo  no real equivalent in C/C++ • restart the current loop, without evaluating conditional

  10. continue block • while, until, and foreach loops can have a continue block. • placed after end of loop, executed at end of each iteration • executed even if loop broken out of via next (but not if broken via last or redo) foreach $i (@array) if ($i % 2 != 0){ next; } $sum += $i; } continue { $nums++; }

  11. Breaking Out of More Loops • next, last, redo operate on innermost loop • Labels to break out of nesting loops TOP: while ($i < 10){ MIDDLE: while ($j > 20) { BOTTOM: foreach (@array){ if ($j % 2 != 0){ next MIDDLE; } if ($i * 3 < 10){ last TOP; } } } }

  12. goto • yes, it exists. • No, don’t use it.

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