while Loops and Loop Control Statements

while Loops

while loops evaluate a logical expression and execute a statement block repeatedly as long as the logical expression evaluates to true. The expression is evaluated first at the beginning of the of the loop, and again after each execution of the statement block.

Copy the text below and run it as a Perl script.

# the following 'while' loop works just like a 'for' loop;
# $i is a control variable and is initialized as 0;
# the expression after the reserved word 'while' is 
# tested before the first iteration, and the following 
# code block is executed if it evaluates to 'true';
# at the end of the following statement block the expression 
# is re-evaluated and the block is executed again if the 
# condition is still 'true'

$i = 0;
while ($i <= 100) # loop control condition
{ 
  print "iteration $i\n"; 
  $i = $i + 1;
}

Loop Control Statements

The behavior of all loops, including for and foreach, can be manipulated by the following reserved words: next and last.

next skips all subsequent statements in the statement block and re-evaluates the loop condition. If this condition still evaluates to true, the next iteration will start.

last will terminate a loop; the program will resume execution after the statement block in which last occurs.

$i = 0;
while ($i++ <= 100) # loop control condition
{
  next if ($i % 5 != 0); # The modulus operator '%' divides
                         # the integer to the left by the 
                         # integer on the right and returns
                         # the remainder.
                         # If the number is not divisible by 5
                         # (the remainder is not 0), the loop 
                         # will immediately move on to the next 
                         # iteration without executing any
                         # statements that follow it
  
  print "iteration $i\n";
  
  last if ($i == 90);    # the 'last' keyword ends the loop
                         # even if the initial condition is
                         # still true
}

The following example does essentially the same as the preceding one, but the next and last statements have been built into an if/elsif/else construct.

$i = 0;
while ($i++ <= 100)
{ 
  if ($i == 90)
  {
    print "\'last\' called at $i\n";
    last;
  }
  elsif ($i % 5 == 0)
  {
    print "iteration $i\n";
  }
  else
  {
    next;
  }
}