PERL TUTORIAL PART 3 – ELECDUDE
In this, lets go through the different
control structures available in Perl and its examples.
Click here to goto Tutorial Part 1.
Click here to goto Tutorial Part 2.
CONTROL STRUCTURES IN PERL:
Perl has
different control structures, which are as follows:
·
The if/unless
Statement
·
The while/until
Statement
·
The for
Statement
·
The foreach
Statement
if Statement:
This construct takes a control expression (evaluated for
its truth) and a block. It may optionally have an else followed by a block as
well. It is similar to C language.
Syntax:
if (<expression>)
{
true_statement; #executes if
expression is true
.....
true_statementn;
} else {
false_statement; #executes if
expression is FALSE
.....
false_statementn;
}
Another form of IF is the if...elsif...else statement , which has multiple expressions &
statements.
Syntax:
if (<expression1>)
{
true_statement1;
#executes
if expression1 is true.
.....
true_statementn1-n;
} elsif (<expression2>)
{
true_statement2;
#executes
if expression1 is false & expression2 is true.
.....
true_statement2-n;
}else {
false_statement1;
#executes
is both the above expressions are false
.....
false_statementn;
}
unless Statement:
This construct takes a control expression (evaluated for
its falseness) and a block. It may optionally have an else followed by a block
as well. It is same as if statement
executing a false condition.
Syntax:
unless (<expression>)
{
false_statement; #executes if
expression is FALSE
.....
false_statementn;
} else {
true_statement; #executes if
expression is true
.....
true_statementn;
}
Another form of IF is the unless...elsif...else statement , which has multiple expressions &
statements.
Syntax:
unlsess (<expression1>)
{
false_statement1;
#executes
if expression1 is false.
.....
flase_statementn1-n;
} elsif (<expression2>)
{
true_statement2;
#executes
if expression1 is ture & expression2 is true.
.....
true_statement2-n;
}else {
false_statement1;
#executes
is both the above expression1 is true & 2 is false
.....
false_statementn;
}
Examples:
$a=5;
if($a==20) {
printf "\ta is equal 20.\n";
} elsif ( $a < 20 ){
printf "\ta is less than 20.\n";
}
else {
printf "\ta is not less than nor equal to 20.\n";
}
unless($a==20) {
printf "\ta is equal 20.\n";
} elsif ( $a < 20 ){
printf "\ta is not less than 20.\n";
} else {
printf "\ta is less than 20.\n";
}
Loop statements:
Perl can iterate using the while statement The FOR, FOREACH and WHILE are also known as loop statements, as these repeatedly
execute a block of statements for a specific condition.
WHILE statement executes the statement block till the expression
returns TRUE.
while (expression) {
statement_1;
statement_2;
statement_n;
}
DO.. WHILE is
similar to that of WHILE, but the condition is evaluated at the end of
execution of block statements. This is similar to C language. The difference
between WHILE & DO WHILE is that, in DO WHILE loop the block of statements is
executed once regardless the result of expression.
do{
statement_1;
statement_2;
statement_n;
} while (expression;
The UNTIL executes the block of statements till the expression returns
FALSE. Replacing the while with until yields the desired effect.
until (expression) {
false_statement_1;
false_statement_2;
false_statement_n;
}
Examples:
$a = 10;
### while
while( $a < 20 ){
printf "Value of a:
$a\n";
$a = $a + 1;
}
### do..while
$a = 13;
do{
printf "Value of a:
$a\n";
$a = $a + 1;
}while( $a < 13 );
printf "Now Value of a: $a\n";
### until
$a = 5;
until( $a > 10 ){
printf "Value of a:
$a\n";
$a = $a + 1;
}
for Statement is another Perl iteration construct, which looks
like C or Java's for statement.
Syntax is:
for ( initialise; expression; update ) {
statement1;
statement2;
...
statementn;
}
Example:
for( $a = 10; $a < 20; $a=$a+1 ){
print "value of
a: $a\n";
}
The foreach loop iterates over a normal
list value and sets the variable VAR to be each element of the list in turn. If
the variable is preceded with the keyword my, then it visible only within the
loop. Otherwise, the variable is implicitly local to the loop and regains its
former value upon exiting the loop. The foreach
keyword is actually a synonym for the for
keyword, so you can use either. If VAR is omitted, $_ is set to each value.
Syntax:
foreach VAR (VAR) {
statement1;
...
statementn;
}
##
foreach $local (@array1) {
$local has the elements of @array1 each for every iteration
}
##
foreach (@array1) {
$_ has the elements of @array1 each for every iteration
}
##
for $local (@array1) {
$local has the elements of @array1 each for every iteration
}
Example:
@list = (2, 20, 30, 40, 50);
foreach $a (@list){ #$a is local to
the loop only
print "value is:
$a\n";
}
my $b=123;
for $b (@list){
#$b
is local to the loop with @list contents only, & the former global value of
$b not accessible within the loop.
print "value is:
$b\n";
}
#$b
regains the value 123.
@list = (2, 20, 30, 40, 50);
foreach my $a (@list){ #$a is local to the loop only
print "value is:
$a\n";
}
print "\na= $a";
my $b=12345;
print "\nb= $b\n\n";
for $b (@list){ #$b is local to the loop & becomes global on
loop exit
print "value is:
$b\n";
}
print "\nb= $b";
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