By Diane Zak
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Objectives
• In this chapter, you will:
• Code the selection structure using the Perl if statement
• Compare data using comparison operators
• Include logical operators in an if statement’s condition
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Objectives
• In this chapter, you will:
• Validate data passed to a script
• Add items to an array using the push function
• Determine the size of an array
• Code a multiple-path selection structure
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Introduction
• All scripts are written using one or more control or logic structures:
– Repetition
– Sequence
– Selection
• Making a decision or comparison and selecting one of two paths based on the result
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The Super Bowl Form and Script
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• It is good programming practice for the script to verify that the data is valid
– Valid data : data that the script is expecting
– The Super Bowl form will check if the data is valid, and if not, ask the user to correct it
• Errors will be stored in the @errors array
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The Super Bowl Form and Script
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Coding the Selection
Structure in Perl
• if statement:
– Used for coding the selection structure
– Syntax: if (condition) { one or more statements to be processed when the condition evaluates to true
} else { one or more statements to be processed when the condition evaluates to false
}
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Coding the Selection
Structure in Perl
• if statement:
– The else clause is optional
– The condition expression must be a Boolean
– An expression that results in true or false.
– Can contain variables, numbers, strings, functions, arithmetic operators, comparison operators , and logical operators
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Comparison Operators
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• Comparison operators are also called relational operators
– One set to compare numeric values
– One set to compare string values
– Each operator has a precedence value
• The order in which Perl performs the comparisons in an expression
• For example, comparisons with a precedence number of 1 will be performed before comparisons with a precedence of 2
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Comparison Operators
6 for Numeric Values
• Test for equality using 2 equal signs ==
– Easy to confuse with the assignment operator (=)
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Comparison Operators
6 for Numeric Values
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Comparison Operators
6 for String Values
• When comparing 2 strings, the ASCII value of each character in the first string is compared to the corresponding character in the second string
– Capital letters have a lower ASCII value than their corresponding lowercase letters
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Comparison Operators
6 for String Values
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Logical Operators
• and and or are the most commonly used logical operators
– Combine multiple compound conditions into one condition
– and :
• All conditions must be true for the compound condition to be true
• If the first condition is false, the second condition will not be evaluated
– or :
• Any condition must be true for the compound condition to be true
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Logical Operators
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Logical Operators
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Validating Script
Input Data
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• Can choose to check for correct or incorrect values
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Adding Items to an Array
• The push function is used to add item(s) to the end of an array
– Syntax:
• push ( array , list );
– Examples:
• push (@states, “Hawaii”);
• push (@nums, 10, 25);
– Can also add items to the end of an array with an assignment statement
• Syntax:
– array = ( array , list );
• Example:
– @nums = (@nums, 10, 25);
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Modifying the Super
Bowl Script
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– Two if statements are used to validate data
– push is used to append an error message to the @errors array.
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Determining the
Size of an Array
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• There are 2 ways to determine the size of an array:
– Assign the name of an array to a scalar variable
• Example:
– $size = @errors;
– Now, $size will be equal to the number of scalar variables in the array
– The scalar function
• Syntax:
– scalar ( array );
• Example:
– print scalar (@errors);
– Will print the number of scalar variables in the
@errors array
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Completing the Super Bowl Script
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Completing the Super Bowl Script
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Completing the
Super Bowl Script
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Using the if / elsif / else
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Form of the if statement
• if / elsif / else structure can be used if there are multiple alternatives
– Can choose to use nested if statement(s) or use elsif for every alternative
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Using if / elsif / else
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Summary
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• The selection structure allows a script to make a decision or comparison and based on the result, select one of two paths.
• Scripts should validate data.
– If the data is not valid, an error message should be displayed and the user should be aware of the errors and how to correct them.
• The if statement is used to code the selection structure.
– There are 2 clauses:
• if
• else
(optional)
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Summary
• The condition in an if statement can contain variables, numbers, strings, functions, arithmetic operators, comparison/relational operators, and logical operators
• Comparison operators:
– Numeric: <, <=, >, >=, ==, !=
– String: lt, le, gt, ge, eq, ne
– String comparison operators should be used to compare strings and not numbers, and vice versa
• Logical operators:
– and: compound condition is true only if both conditions are true
– or: compound condition is true if any of the conditions are true
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Summary
• The push function is used to add one or more items to the end of an array.
– Syntax: push ( array , list );
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• The size of an array can be determined by using the scalar function or by assigning an array to a scalar variable.
• The if / elsif / else form of the if statement can be used to code a multiple-path or extended selection structure
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