Control Structures

1
Control Structures
2
if-elsif-else

• semantically the same as C/C
• syntactically, slightly different.
• if (a gt 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
do

• Execute all statements in following block, and
  return value of last statement executed
• When modified by while or until, run through
  block once before checking condition
• do
• i
• while (i lt 10)
• Note that Perl does not consider do to be an
  actual loop structure. This is important later
  on

6
for loops

• Perl has 2 styles of for.
• First kind is virtually identical to C/C
• for (INIT TEST INCRMENT)
• for (i 0 i lt 10 i) print \i i\n
• yes, the brackets are required.

7
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 (_at_nums)
• sum value

8
More About for/foreach

• for and foreach are actually synonyms
• Anywhere you see for you can replace it with
  foreach and viceversa
• Without changing ANYTHING ELSE
• they can be used interchangeably.
• usually easier to read if conventions followed
• for (i 0 ilt10 i)
• foreach i (_at_array)
• but this is just as syntactically valid
• foreach (i 0 ilt10 i)
• for i (_at_array)

9
Two More Things (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
• _at_array (1, 2, 3, 4, 5)
• foreach (_at_array)
• _ 2
• _at_array now ? (2, 4, 6, 8, 10)

10
Reading it in English

• Perl has a cute little feature that makes simple
  loop constructs more readable
• If your if, unless, while, until, or foreach
  block contains only a single statement, you can
  put the condition at the end of the statement
• if (a gt 10) print \a is a\n
• print \a is a\n if a gt 10
• Using this modifier method, brackets and
  parentheses are unneeded
• This is syntactic sugar whichever looks and
  feels right to you is the way to go.

11
Loop Control next, last, redo

• last ? equivalent of C break
• exit innermost loop
• next ? (mostly) equivalent of C continue
• begin next iteration of innermost loop
• redo ? no real equivalent in C
• restart the current loop, without evaluating
  conditional
• Recall that do is not a looping block. Hence,
  you cannot use these keywords in a do block (even
  if its modified by while)

12
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 the loop is broken out of via
  next (but not if broken via last or redo)
• foreach i (_at_array)
• next if (i 2 ! 0)
• sum i
• continue
• nums

13
Breaking Out of More Loops

• next, last, redo operate on innermost loop
• Labels are needed to break out of nesting loops
• TOP while (i lt 10)
• MIDDLE while (j gt 20)
• BOTTOM foreach (_at_array)
• if (j 2 ! 0)
• next MIDDLE
• if (i 3 lt 10)
• last TOP

14
goto

• yes, it exists.
• No, dont use it.