1. Control Structures: if Conditional
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2. Review Double quotes vs. Single-quotes
Variables inside double quotes are replaced by their values.
e.g. “Hello, $name” == Hello, Jim (when $name == “Jim”) ‘Hello, $name’ == Hello, $name
Escape character (i.e. \) has special meaning inside double quotes
e.g. “Hello, \Ujim” == Hello, JIM ‘Hello, \Ujim’ == Hello, \Ujim
Interchangeability of Strings and Numeric Values
String can be used as numbers when appropriate, and vice versa
e.g. $a= “25”; $sum= $a+5;
e.g. $number = <STDIN>; chop $number; $sum= $number+5;
String used in integer context inappropriately will produce unexpected results.
$b = “five” + 5; ($b == 5)
$b = “15k50” + 5; ($b == 20)
$b = “k1550” + 5; ($b == 5)
Initial value of Scalar Variable
Perl scalar variables have an initial value of the null string (“”)
Null string is converted to 0 in integer context
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3. Control Structures: Overview
What are Control Structures?
Program statements that control the program flow
i.e. controls which statement to execute next
Perl Control Structures
if conditional
Execution of statements depend on current condition of the expression
if (expression) { statements }
e.g. if ($count<5) { print “count is $count\n”; }
while loop
Statements are executed while expression is true
Syntax: while (expression) { statements }
e.g. while ($i<5) { print “count is $i\n”; $i++; }
for loop
Statements repeats for a given number of times
Syntax: for (expression) { statements }
e.g. for ($i=0; $i<5; $i++) { print “count is $i\n”; }
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4. Conditional Expressions
Conditional Expression is a key component of Control Structures
The “value” of the conditional expression determines the program flow
Syntax: KEYWORD (expression) { statements }
e.g. if ($count<5) { print “count is $count\n”; }
Conditional Expressions can be any Perl expressions
Constant or Variable
e.g. (1), (0), (“true”), ($a)
Using Comparison Operators
==, <, >, eq, lt, gt, etc.
test for single condition, e.g. ($a > $b)
Using Logical Operators
&&, ||, !, etc.
test for multiple conditions, e.g. ($a<0 || $a>9)
Evaluation of Conditional Expression
True if the value of the expression is non-zero.
e.g. (1), (“false”), (“00”), (“ ”)
False if the value of the expression is zero (or null).
e.g. (0), (“”), (“0”)
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5. Comparison Operators
Comparison Operators (CO) are used in Conditional Expressions, which are used in Control Structures.
Control Structure = KEWORD (conditional expression) { statements }
Conditional Expression = (value1 CO value2)
Numeric Comparison Operators
==, !=, >, >=, <, <=
String Comparison Operators
eq, ne, gt, ge, lt, le
Strings are compared from left to right in the alphabetical order
“aa” is less than “ab”
Alphabetical order 
null < blank < !”#$%&’()*+,-./ < numbers < < A-Z < [\]^-`< a-z < {|}~
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6. Comparison Operators: Pitfalls
String vs. Integer comparison
When comparing numbers, comparison operator drives the evaluation of an expression
numeric comparison for numeric comparison operator
TRUE: (“123” > “45”), (123 > 45)
alphabetical comparison for string comparison operator
FALSE: (“123” gt “45”), (123 gt 45)
 Example script
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7. Logical Operators
Logical Operators, also used in Conditional Expressions, can test for multiple conditions
e.g. ( ($a<0) || ($a>9) )
Logical AND
&& (and)
(A && B) : TRUE when both A and B are true
Logical OR
|| (or)
(A || B) : TRUE when either A or B are true
Logical NOT
! (not)
(!A) : TRUE when A is false
*Note: FALSE == 0 or null, TRUE == NOT(0 or null)  Example script
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8. if Conditional Syntax Single-Line Conditional Statements
if (condexpr1) { statements1 } elsif (condexpr2) { statements2 }
else { statements3 }
statements are executed if condexpr is true
Single-Line Conditional Statements
When statement block consists of only one statement,
Syntax: statement KEYWORD (expression)
e.g. print “You cannot buy beer\n” if ($age<21);
 Example script
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