Perl Cd Bookshelf [Electronic resources] نسخه متنی

2.4.1. Problem

Use a for loop, or .. in
conjunction with a foreach loop:

foreach ($X .. $Y) {
# $_ is set to every integer from X to Y, inclusive
}
foreach $i ($X .. $Y) {
# $i is set to every integer from X to Y, inclusive
}
for ($i = $X; $i <= $Y; $i++) {
# $i is set to every integer from X to Y, inclusive
}
for ($i = $X; $i <= $Y; $i += 7) {
# $i is set to every integer from X to Y, stepsize = 7
}

The first two approaches use a foreach loop in
conjunction with the $X .. $Y construct, which
creates a list of integers between $X and
$Y. Now, if you were just assigning that range to
an array, this would use up a lot of memory whenever
$X and $Y were far apart. But
in a foreach loop, Perl notices this and doesn't
waste time or memory allocating a temporary list. When iterating over
consecutive integers, the foreach loop will run
faster than the equivalent for loop.

Another difference between the two constructs is that the
foreach loop implicitly localizes the loop
variable to the body of the loop, but the for loop
does not. That means that after the for loop
finishes, the loop variable will contain the value it held upon the
final iteration. But in the case of the foreach
loop, that value will be inaccessible, and the variable will hold
whatever it held—if anything—prior to entering the loop.
You can, however, use a lexically scoped variable as the loop
variable:

foreach my $i ($X .. $Y) { ... }
for (my $i=$X; $i <= $Y; $i++) { ... }

The following code shows each technique. Here we just print the
numbers we generate:

print "Infancy is: ";
foreach (0 .. 2) {
print "$_ ";
}
print "\n";
print "Toddling is: ";
foreach $i (3 .. 4) {
print "$i ";
}
print "\n";
print "Childhood is: ";
for ($i = 5; $i <= 12; $i++) {
print "$i ";
}
print "\n";
Infancy is: 0 1 2
Toddling is: 3 4
Childhood is: 5 6 7 8 9 10 11 12

The for and foreach operators
in perlsyn(1) and the "For Loops" and "Foreach
Loops" sections of Chapter 4 of Programming
Perl

2.3. Comparing Floating-Point Numbers		2.5. Working with Roman Numerals