PERL Unit 4 Control Flow

8/8/2019 PERL Unit 4 Control Flow

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Unit 4Flow Control

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IBM Learning Services

Worldwide Certified Material


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Objectives After completing this unit, students

should be able to use: Statement blocks

if/else statement

elsifbranch

unless variation

while and until statements

for statement

foreach statement


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Statement Block It is a list of statements surrounded

by curlybraces

Statements are executed sequentially Syntax

{ first_stmt;

second_stmt;

. . .

nth statement ; }


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Statement Block Statement block acts like a single

statement and can replace a single

statement in most places. As a special concession to writers of

short programs, the last ; before theclosing } is optional.


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if/else Construct The basic decision-making control

structure

Syntaxif (test_expr) { stmt/s; }

else { stmt/s ; }

The body of the if/else are defined as

a statement block, the braces arerequired even if there is only onestatement.


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elsif Construct Provides for multiple test conditions

Syntax

if (test_expr) { stmt/s; }elsif (test_expr) { stmt/s; }

else { stmt/s; }

You can have as many

elsif asrequired

Else clause is still optional


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unless Construct When you want to execute only the

else part of if/else

Execute the statement block is thetest is false

Syntax

unless (test_expr) { stmt/s; }

Example

if ($month 12} { . . . }


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unless Construct

if (! ($month > 0 && $month < 13} { . . . }

Use unless() to execute statementswhen a conditions is false

This saves having to invert thecondition


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while Construct Execute a statement block while a

condition is true

Syntaxwhile (test_expr) { stmt/s;}

the loop may execute endlessly if the

condition does not change


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do/while Construct Test the condition after the first

execution of the block

Syntaxdo { stmt/s ; } while (test_expr);


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Default Variable : $_

Special global variable

Used by many functions if no other

variable specified: print;

chop;

chomp;

$_ is used if no parameter isprovided.


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Default Variable : $_

$_ corresponds to the word it, thecurrent subject, except that when $_

is used, it is often not visible.


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while() shourtcut using $_ When the input operator is the

only condition in a while()

The input value is assigned to$_ Example

while () {

chomp;print; }


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while() shourtcut using $_ Is the same as

while (defined($_=))

{ chomp $_;print $_; }


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until Construct Variation of the while statement

Execute a statement block until a

condition is true Syntx

until (test_expr) { stmt/s; }

do { stmt/s; } until (test_expr);


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until Construct The statement block is executed while

the test condition is false

The first form may not execute at allif the condition is initially true

until is less common than while(), butuseful when the condition is reversed.

The do/until version corresponds todo/while


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for Construct A loop with initialization, test and

increment expressions

Syntaxfor (init_expr; test_expr; incr_expr)

{ stmt/s; }


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foreach Construct Executes a block for each element of

a list of values

Sets a loop variable to each value inturn

Syntax

foreach $item (list) {stmt/s;}


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foreach Construct Example

foreach $flag(red, white, blue)

{ print $flag; } If omitted, the loop variable default

to $_

foreach (red, white, blue){ print; }

PERL Unit 4 Control Flow

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