Perl do-while Statement
Perl do-while statement is another looping construct provided by Perl language. This statement is similar to while statement:
- while statement checks the test condition before executing the block
- do-while statement checks the test condition after executing the block.
In both cases, if the test condition is evaluated as false, the loop will end. This free short tutorial will show you some useful ways to use the Perl do-while statement in Perl scripts.
Let’s look at the syntax form of this statement, first:
Before giving you some examples about how to use it, I want to point out some interesting features of Perl
do-while statement:
- The condition is evaluated after executing the block so the block will be executed at least once, unlike the while statement where if the condition is false initially, the block will be not executed at all
- Don’t think of do as a loop block; here the keyword do means merely a term used in an expression rather than a loop block. And because the do block is not a looping block, you can’t use within the looping controls: next, last and redo
In the following, I’ll show you several examples about how to use the Perl
do-while loop statement.
Example 1.$count = 0;
do {
print "Password: ";
chomp($psw = <STDIN>);
} while ++$count < 3 && $psw ne "1qaz";
if($psw eq "1qaz") {
print "Password OK ";
} else {
print "Wrong password\n";
}
A possible output for this snippet code is:
Password: 1
Password: 2
Password: 3
Wrong password
This code reads a password from standard input. The loop ends either after three iterations or after the input of the correct password.
Example 2.do {
++$var;
print "$var ";
} while $var <= 5;
print "\n";
# it displays 1 2 3 4 5 6
At the first iteration, the $var scalar variable is undefined and ++$var will set its value to 1. The loop ends when $var becomes greater than 5.
If you don’ feel like testing the condition after the block by using Perl do-while statement, you can initialize an iteration variable and use while statement instead, in order to execute your block at least once. You can rewrite the previous example as follows:
$var = 0;
while ($var <= 5){
++$var;
print "$var ";
}
print "\n";
You’ll get the same output as before.
Example 3.As I mentioned before, the do block doesn’t behave like a loop block, therefore you can’t use any of the control looping within the block. Unfortunately you may but the result after the execution of your code will be unpredictable. You know, sometimes Perl let you do a lot of things and if you do not do them in the appropriate way, it will take you a lot of time to debug and correct your script code.
But there is still a way to use the looping controls with the do construct. By instance, you can use the next and redo controls by putting a bare block inside the do block, like in the following snippet code:
my $a = 1;
do
{
# the do block
{
# the bare block
next if $a > 7;
print "$a ";
}
} while $a++ < 10;
print "\na = $a\n";
If you will run this code, you’ll get the output:
The
next looping control will reiterate the looping if $a > 7, skipping the rest of the statements from within the bare block.
Table of Contents:
A Perl Script
Install Perl
Running Perl
Perl Data Types
Perl Variables
Perl Operators
Perl Lists
Perl Arrays
Array Size
Array Length
Perl Hashes
Perl Statements
Perl if
Perl unless
Perl switch
Perl while
Perl do-while (more)
Perl until
Perl do-until
Perl for
Perl foreach
Built-in Perl Functions
Functions by Category
String Functions
Regular Expressions and Pattern Matching
List Functions
Array Functions
Hash Functions
Miscellaneous Functions
Functions in alphabetical order
chomp
chop
chr
crypt
defined
delete
each
exists
grep
hex
index
join
keys
lc
lcfirst
length
map
oct
ord
pack
pop
push
q
qq
qw
reverse
rindex
scalar
shift
sort
splice
split
sprintf
substr
tr
uc
ucfirst
undef
unpack
unshift
values
return to Perl Basics
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