Selection Control Structures

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Selection Control Structures
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Objectives

• In this chapter you will be able to
• Elaborate on the uses of simple selection,
  multiple selection, and nested selection in
  algorithms
• Introduce the case construct in pseudocode
• Develop algorithms using variations of the
  selection control structure

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The Selection Control Structure

• You can use the selection control structure in
  pseudocode to illustrate a choice between two or
  more actions, depending on whether a condition is
  true or false
• The condition in the IF statement is based on a
  comparison of two items, and is usually expressed
  with one of the following relational operators
• lt less than gt greater than
• equal to lt less than or equal to
• gt greater than or equal to lt gt not equal to

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1 Simple Selection (Simple IF Statement)

• Simple selection occurs when a choice is made
  between two alternate paths, depending on the
  result of a condition being true or false
• The structure is represented in pseudocode using
  the keywords IF, THEN, ELSE, and ENDIF
• Only one of the THEN or ELSE paths will be
  followed, depending on the result of the
  condition in the IF clause

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2 Simple Selection with Null False Branch (Null
ELSE Statement)

• The null ELSE structure is a variation of the
  simple IF structure
• It is used when a task is performed only when a
  particular condition is true
• If the condition is false, then no processing
  will take place and the IF statement will be
  bypassed

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3 Combined Selection (Combined IF Statement)

• A combined IF statement is one that contains
  multiple conditions, each connected with the
  logical operators AND or OR
• If the connector AND is used to combine the
  conditions, then both conditions must be true for
  the combined condition to be true
• If the connector OR is used to combine any two
  conditions, then only one of the conditions needs
  to be true for the combined condition to be
  considered true

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The NOT Operator

• The NOT operator can be used for the logical
  negation of a condition, as follows
• IF NOT (record_code 23) THEN
• update customer record
• ENDIF
• Note that the AND and OR operators can also be
  used with the NOT operator, but great care must
  be taken and parentheses should be used to avoid
  ambiguity as illustrated on page 38 of the
  textbook

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4 Nested Selection (Nested IF Statement)

• Nested selection occurs when the word IF appears
  more than once within an IF statement
• Nested IF statements can be classified as linear
  or non-linear

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Linear Nested IF Statements

• The linear nested IF statement is used when a
  field is being tested for various values and a
  different action is to be taken for each value
• This form of nested IF is called linear, because
  each ELSE immediately follows the IF condition to
  which it corresponds

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Non-Linear Nested IF Statements

• A non-linear nested IF occurs when a number of
  different conditions need to be satisfied before
  a particular action can occur
• It is termed non-linear because the ELSE
  statement may be separated from the IF statement
  with which it is paired

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Algorithms Using Selection

• Let us look at some programming examples that use
  the selection control structure
• In each example, the problem will be defined, a
  solution algorithm will be developed, and the
  algorithm will be manually tested

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Example 4.1 Read Three Characters

• Design an algorithm that will prompt a terminal
  operator for three characters, accept those
  characters as input, sort them into ascending
  sequence, and output them to the screen
• A Defining diagram (illustrated on page 40)
• B Solution algorithm
• The solution algorithm requires a series of IF
  statements to sort the three characters into
  ascending sequence as the code shows on page 41
  of the textbook
• In this solution, most of the logic of the
  algorithm is concerned with the sorting of the
  three characters into alphabetic sequence

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Example 4.1 Read Three Characters

• C Desk checking
• Two sets of valid characters will be used to
  check the algorithm the characters k, b and g as
  the first set, and z, s and a as the second
• Examine the input data, expected results, and
  desk check table illustrated on page 42 of the
  textbook
• Line numbers have been used to identify each
  statement within the program
• Note that when desk checking the logic, each IF
  statement is treated as a single statement

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Example 4.2 Process Customer Record

• A program is required to read a customers name,
  a purchase amount, and a fax code. The tax code
  has been validated and will be one of those
  listed on page 43 of the textbook. The program
  must then compute the sales tax and the total
  amount due, and print the customers name,
  purchase amount, sales tax, and total amount due
• A Defining diagram (shown on page 43)
• B Solution algorithm
• The solution algorithm requires a linear nested
  IF statement to calculate the sales tax
  illustrated in the code on page 43 of the textbook

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Example 4.2 Process Customer Record

• C Desk checking
• Two sets of valid input data for purchase amount
  and tax code will be used to check the algorithm
• Examine the input data, expected results, and
  desk check tables shown on page 44 of the
  textbook
• As the expected result for the two test cases
  matches the calculated result the algorithm is
  correct

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Example 4.3 Calculate Employees Pay

• A program is required by a company to read an
  employees number, pay rate, and the number of
  hours worked in a week. The program is then to
  validate the pay rate and the hours worked fields
  and, if valid, compute the employees weekly pay
  and print it along with the input data
• A Defining diagram (shown on page 45)
• B Solution algorithm
• The solution to this problem will require a
  series of simple IF and nested IF statements
• First, the variables pay_rate and hrs_worked
  must be validated, and if either is found to be
  out of range, an appropriate message is to be
  placed into a variable called error_message

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Example 4. 3 Calculate Employees Pay

• Boolean variables
• The variable valid_input_fields is a Boolean
  variable, it may contain only one of two possible
  values (true or false)
• When using the IF statement with a Boolean
  variable, the IF statement can be simplified in
  pseudocode
• C Desk checking
• Two sets of valid input data for pay rate and
  hours worked will be used to check this algorithm
  as illustrated in the three tables on pages 46
  and 47 of the textbook

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The Case Structure

• The case control structure in pseudocode is
  another way of expressing a linear nested IF
  statement
• It is used in pseudocode for two reasons it can
  be directly translated into many high-level
  languages, and it makes the pseudocode easier to
  write and understand
• Nested IFs often look cumbersome in pseudocode
  and depend on correct structure and indentation
  for readability

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Example 4.4 Process Customer Record

• A program is required to read a customers name,
  a purchase amount, and a tax code. The tax code
  has been validated and will be one of the items
  listed on page 49 of the textbook. The program
  must then compute the sales tax and the total
  amount due, and print the customers name,
  purchase amount, sales tax, and total amount due
• A Defining diagram (shown on page 49)
• B Solution algorithm (shown on page 49)
• The solution algorithm will be expressed using a
  CASE statement

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Example 4.4 Process Customer Record

• C Desk checking
• Two sets of valid input data for purchase amount
  and tax code will be used to check the algorithm
• Note that the case structure serves as a single
  pseudocode statement
• Examine the input data, expected results, and
  desk check tables shown on page 50 of the textbook

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Summary

• This chapter covered the selection control
  structure in detail
• Descriptions and pseudocode examples were given
  for simple selection, null ELSE, combined IF, and
  nested IF statements
• The case structure was introduced as a means of
  expressing a linear nested IF statement in a
  simpler and more concise form