C Programming: Program Design Including Data Structures, Second Edition

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C Programming Program Design Including Data
Structures, Second Edition

• Chapter 4 Control Structures I (Selection)

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Objectives

• In this chapter you will
• Learn about control structures
• Examine relational and logical operators
• Explore how to form and evaluate logical
  (Boolean) expressions
• Discover how to use the selection control
  structures if, if...else, and switch in a program
• Learn to use the assert function to terminate a
  program

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Control Structures

• A computer can proceed
• In sequence
• Selectively (branch) - making a choice
• Repetitively (iteratively) - looping
• Some statements are executed only if certain
  conditions are met
• A condition is represented by a logical (Boolean)
  expression that can be true or false
• A condition is met if it evaluates to true

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C control structures

• Selection
• if
• if . . . else
• switch
• Repetition
• for loop
• while loop
• do . . . while loop

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Relational Operators

• Relational operators
• Allow comparisons
• Require two operands (binary)
• Return 1 if expression is true, 0 otherwise
• Comparing values of different data types may
  produce unpredictable results
• For example, 8 lt '5' should not be done
• Any nonzero value is treated as true

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Relational Operators

• are used in expressions of form
• ExpressionA Operator ExpressionB
• temperature gt humidity
• B B - 4.0 A C gt 0.0
• abs (number ) 35
• initial ! Q

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• int x, y
• x 4
• y 6
• EXPRESSION VALUE
• x lt y true
• x 2 lt y false
• x ! y true
• x 3 gt y true
• y x false
• y x2 true
• y x 3 7 (true)

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In C

• the value 0 represents false
• ANY non-zero value represents true
• C lt G is true
• c lt g is true
• c lt G is false,
• because G comes before in the sequence of the
  ASCII character set
• Compare ints with ints, floats with floats, chars
  with chars otherwise implicit type coercion takes
  place.
• Use explicit type casting
• someFloat lt static_castltfloatgt (someInt)
• boolVar lt 4 is true, why? boolVar yields

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Comparing string Types

• Relational operators can be applied to strings
• Strings are compared character by character,
  starting with the first character
• Comparison continues
• until either a mismatch is found or
• all characters are found equal
• If two strings of different lengths are compared
  and the comparison is equal to the last character
  of the shorter string
• The shorter string is less than the larger string

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string Comparison Example

• Suppose we have the following declarations
• string str1 "Hello"
• string str2 "Hi"
• string str3 "Air"
• string str4 "Bill"

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Logical (Boolean) Operators

• Logical (Boolean) operators enable you to combine
  logical expressions
• Three logical (Boolean) operators
• ! - not
• and
• - or
• Logical operators
• take logical values as operands and yield logical
  values as results
• ! is unary and are binary operators
• Putting ! in front of a logical expression
  reverses its value

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Precedence of Operators

• Relational and logical operators are evaluated
  from left to right
• so the associativity is left to right
• Parentheses can override precedence

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• //Chapter 4 Logical operators p.157
• include ltiostreamgt
• using namespace std
• int main()
• bool found true
• bool flag false
• int num 1
• double x 5.2
• double y 3.4
• int a 5, b 8
• int n 20
• char ch 'B'
• cout ltlt "Line 1 !found evaluates to "
• ltlt !found ltlt endl //Line 1
• cout ltlt "Line 2 x gt 4.0 evaluates to "
• ltlt (x gt 4.0) ltlt endl //Line 2
• cout ltlt "Line 3 !num evaluates to "

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Short-Circuit Evaluation

• Short-circuit evaluation evaluation of a logical
  expression are evaluated left to right and stops
  as soon as the value of the expression is known
• Example
• Assume age 25 grade B
• (age gt 21) ( x 5) //Line 1
• (grade A) (x gt 7) //Line 2

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Short-Circuit Example

• int age, height
• age 25
• height 70
• EXPRESSION
• (age gt 50) (height gt 60)
• false
• Evaluation can stop now because result of is
  only true when both sides are true. It is
  already determined that the entire expression
  will be false.

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The bool Data Type and Logical (Boolean)
Expressions

• The data type bool has logical (Boolean) values
  true and false
• bool, true, and false are reserved words
• The identifier true has the value 1
• The identifier false has the value 0

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Logical (Boolean) Expressions (continued)

• Logical expressions can be unpredictable
• The following expression appears to represent a
  comparison of 0, num, and 10
• 0 lt num lt 10
• It always evaluates true because 0 lt num
  evaluates to either 0 or 1, and 0 lt 10 is true
  and 1 lt 10 is true
• The correct way to write this expression is
• 0 lt num num lt 10

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One-Way (if) Selection

• The syntax of one-way selection is
• if(expression)
• statement
• Statement is executed if the value of the
  expression is true
• Statement is bypassed if the value is false
  program goes to the next statement

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• The syntax of one-way selection is
• if(expression)
• statement
• Statement is executed if the value of the
  expression is true
• Statement is bypassed if the value is false
  program goes to the next statement

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One-Way (if) Selection (continued)

• The expression is sometimes called a decision
  maker
• because it decides whether to execute the
  statement that follows it
• The statement following the expression is
  sometimes called the action statement
• The expression is usually a logical expression
• The statement is any C statement
• if is a reserved word

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Comparing float Values

• do not compare float values for equality, compare
  them for near-equality.

float myNumber float yourNumber cin
gtgt myNumber cin gtgt yourNumber if ( fabs
(myNumber - yourNumber) lt 0.00001 ) cout ltlt
They are close enough! ltlt endl
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• Two-way selection takes the form
• if(expression)
• statement1
• else
• statement2
• If expression is true, statement1 is executed
  otherwise statement2 is executed
• statement1 and statement2 are any C statements
• else is a reserved word

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• //Program Weekly wages
• include ltiostreamgt
• include ltiomanipgt
• using namespace std
• int main ()
• double wages, rate, hours
• cout ltlt fixed ltlt showpoint ltlt setprecision(2) //
  Line 1
• cout ltlt "Line 2 Enter working hours and rate
  " //Line 2
• cin gtgt hours gtgt rate //Line 3
• if (hours gt 40.0) //Line 4
• wages 40.0 rate 1.5
  rate (hours - 40.0) //Line 5
• else //Line 6
• wages hours rate //Line 7

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Compound (Block of) Statement

• Compound statement (block of statements)
• statement1
• statement2
• .
• .
• .
• statementn
• A compound statement is a single statement

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Compound Statement Example

• if(age gt 18)
• coutltlt" Eligible to vote."ltltendl
• coutltlt" No longer a minor."ltltendl
• else
• coutltlt"Not eligible to vote."ltltendl
• coutltlt"Still a minor."ltltendl

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Use of blocks recommended

• if ( Expression )
• else

if clause
else clause
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• What happens if you omit braces?
• if ( (carDoors 4 ) (driverAge gt 24) )
• premium 650.00
• cout ltlt LOW RISK
• else
• premium 1200.00
• cout ltlt HIGH RISK
• monthlyPayment premium / 12.0 5.00
• COMPILE ERROR OCCURS. The if clause is the
  single statement following the if.

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Braces can only be omitted when each clause is a
single statement

• if ( lastInitial lt K )
• volume 1
• else
• volume 2
• cout ltlt Look it up in volume
• ltlt volume ltlt of NYC phone book

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Nested if Statements

• if ( Expression1 )
• Statement1
• else if ( Expression2 )
• Statement2
• .
• .
• .
• else if ( ExpressionN )
• StatementN
• else
• Statement N1
• EXACTLY 1 of these statements will be executed.

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Multi-way Branching

• if ( creditsEarned gt 90 )
• cout ltlt SENIOR STATUS
• else if ( creditsEarned gt 60 )
• cout ltlt JUNIOR STATUS
• else if ( creditsEarned gt 30 )
• cout ltlt SOPHOMORE STATUS
• else
• cout ltlt FRESHMAN STATUS

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Nested if

• Nesting one control statement in another
• An else is associated with the most recent if
  (that has not been paired with an else)
• For example
• if(score gt 90)
• coutltlt"The grade is A"ltltendl
• else if (score gt 80)
• coutltlt"The grade is B"ltltendl
• else
• coutltlt"The grade is F"ltltendl

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Bad Example has output FAIL

• float average
• average 100.0
• if ( average gt 60.0 )
  // line 3
• if ( average lt 70.0 ) // line 4
• cout ltlt Marginal PASS // line 5
• else
  // line 6
• cout ltlt FAIL //
  line 7
• WHY? The compiler ignores indentation and pairs
  the else with the second if.
• An else is always paired with the closest
  precedence if (that doesnt have an else paired
  with it.)

100.0
average
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To correct the problem, use braces

• float average
• average 100.0
• if ( average gt 60.0 )
• if ( average lt 70.0 )
• cout ltlt Marginal PASS
• else
• cout ltlt FAIL

100.0
average
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Input Failure and the if Statement

• If input stream enters a fail state
• All subsequent input statements associated with
  that stream are ignored
• Program continues to execute
• May produce erroneous results

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Each I/O stream has a state (condition)

• An input stream enters fail state when you
• try to read invalid input data
• try to open a file which does not exist
• try to read beyond the end of the file

• An output stream enters fail state when you
• try to create a file with an invalid name
• try to create a file on a write-protected disk
• try to create a file on a full disk

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How can you tell the state?

• Can use if statements to check status of input
  stream?
• If stream enters the fail state, include
  instructions that stop program execution
• The stream identifier can be used as if it were a
  Boolean variable. It has value false (meaning
  the last I/O operation on that stream failed)
  when the stream is in fail state.
• When you use a file stream, you should check on
  its state.

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Checking on the State

• ofstream myOutfile
• myOutfile.open (A\\myOut.dat)
• if ( ! myOutfile )
• cout ltlt File opening error.
• ltlt Program terminated. ltlt endl
• return 1
• // otherwise send output to myOutfile

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Conditional Operator (?)

• Conditional operator (?) takes three arguments
  (ternary)
• Syntax for using the conditional operator
• expression1 ? expression2 expression3
• If expression1 is true, the result of the
  conditional expression is expression2.
• Otherwise, the result is expression3
• max (a gtb) ? a b

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• expressions value determines which statement is
  selected for execution

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switch Structures

• Switch structure alternate to if-else
• Switch expression is evaluated first
• Value of the expression determines which
  corresponding action is taken
• Expression is sometimes called the selector

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switch Structures (continued)

• Expression value can be only integral
• Its value determines which statement is selected
  for execution
• A particular case value should appear only once

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C Built-In Data Types
Simple
Composite
Integral
Floating
array struct union class
char short int long enum
float double long double
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switch Structures (continued)

• One or more statements may follow a case label
• Braces are not needed
• to turn multiple statements into a single
  compound statement
• The break statement may or may not appear after
  each statement
• switch, case, break, and default are reserved
  words

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switch Statement Rules

• When value of the expression is matched against a
  case value,
• Statements execute until break statement is found
  or the end of switch structure is reached
• If value of the expression does not match any of
  the case values
• Statements following the default label execute If
  no default label and no match the entire switch
  statement is skipped
• A break statement causes an immediate exit from
  the switch structure

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switch (a) //Line 4 case
0 //Line 5 case 1 cout ltlt
"Hello " //Line 6 case 2 cout ltlt
"there. " //Line 7 case 3 cout ltlt "I
am " //Line 8 case 4 cout ltlt
"Mickey." ltlt endl //Line 9
break //Line 10 case 5 cout ltlt
"How " //Line 11 case
6 //Line 12 case
7 //Line 13 case 8 cout ltlt "are
you?" ltlt endl //Line 14
break //Line 15 case 9
break //Line 16 case 10 cout ltlt
"Have a nice day." ltlt endl //Line 17
break //Line 18 default cout ltlt
"Sorry, the number is out of " ltlt "range."
ltlt endl //Line 19 cout ltlt "Out
of switch structure." ltlt endl //Line 20
return 0

• //Program Effect of break statements in a switch
  structure
• include ltiostreamgt
• using namespace std
• int main ()
• int a
• cout ltlt "Enter an integer between 0 and 10
  " //Line 1
• cin gtgt a //Line 2
• cout ltlt "\nThe number you entered is " ltlt a ltlt
  endl //Line 3

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The assert Function

• Certain types of errors that are very difficult
  to catch can occur in a program
• For example, division by zero can be difficult to
  catch using any of the programming techniques
  examined so far
• The predefined function, assert,
• is useful in stopping program execution when
  certain elusive errors occur

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The assert Function (continued)

• The syntax for assert
• assert(expression)
• expression is any logical expression
• If expression evaluates to true,
• the next statement executes
• If the expression evaluates to false,
• the program terminates and indicates where in the
  program the error occurred
• assert (denominator)
• quotient numerator / denominator
• if denominator 0, an error msg displays similar
  to that
• Assertion failed denominator, file
  c\temp\assertfunction\assertfunction.cpp, line
  20

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The assert Function (continued)

• To use assert, include cassert header file
• Assert is useful for enforcing programming
  constraints during program development
• After developing and testing a program, remove or
  disable assert statements
• The preprocessor directive
• define NDEBUG //must be placed before the
  directive include ltcassertgt //to disable the
  assert statement

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Programming Example

• This programming example calculates a customers
  bill for a local cable company (the solution with
  functions in ch6 p.289)
• There are two types of customers
• Residential
• Business
• Two rates for calculating a cable bill
• One for residential customers
• One for business customers

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Rates

• For residential customer
• Bill processing fee 4.50
• Basic service fee 20.50
• Premium channel 7.50 per channel
• For business customer
• Bill processing fee 15.00
• Basic service fee 75.00 for first 10
  connections and 5.00 for each additional
  connection
• Premium channel cost 50.00 per channel for any
  number of connections

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Requirements

• Ask user for account number and customer code
• Assume R or r stands for residential customer and
  B or b stands for business customer

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Input and Output

• Input
• Customer account number
• Customer code
• Number of premium channels
• For business customers extra, number of basic
  service connections
• Output
• Customers account number
• Billing amount

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Program Analysis

• The purpose of the program is to
• calculate and
• print billing amount
• Calculating the billing amount requires
• Customer for whom the billing amount is
  calculated (residential or business)
• Number of premium channels to which the customer
  subscribes
• For a business customers, you need extra
• Number of basic service connections

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Program Analysis (continued)

• Data needed to calculate the bill, such as bill
  processing fees and the cost of a premium
  channel, are known quantities
• The program should print the billing amount to
  two decimal places

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Algorithm Design

• Set precision to two decimal places
• Prompt user for account number and customer type
• If customer type is R or r
• Prompt user for number of premium channels
• Compute and print the bill
• If customer type is B or b
• Prompt user for number of basic service
  connections and number of premium channels
• Compute and print the bill

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Variables

• int accountNumber //customer acct. num
• char customerType //customer code
• int numberOfPremiumChannels // number of
• //premium channels
• int numberOfBasicServiceConnections //
  // number of basic service connections
  
• double amountDue //billing amount

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Named Constants

• //Named constants residential customers
• const double rBillProcessingFee 4.50
• const double rBasicServiceCost 20.50
• const double rCostOfaPremiumChannel 7.50
• //Named constants business customers
• const double bBillProcessingFee 15.00
• const double bBasicServiceCost 75.00
• const double bBasicConnectionCost 5.00
• const double bCostOfaPremiumChannel 50.00

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Formulas

• Billing for residential customers
• amountDue
• rBillProcessingFee rBasicServiceCost
• numberOfPremiumChannels CostOfaPremiumChannel

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Formulas (continued)

• Billing for business customers
• if(numberOfBasicServiceConnections lt 10)
• amountDue bBillProcessingFee
  bBasicServiceCost
• numberOfPremiumChannels bCostOfaPremiumChannel
• else
• amountDue bBillProcessingFee
• bBasicServiceCost
• (numberOfBasicServiceConnections 10)
  bBasicConnectionCost
• numberOfPremiumChannels bCostOfaPremiumChannel
  

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Main Algorithm

• Output floating-point numbers in fixed decimal
  with decimal point and trailing zeros
• Output floating-point numbers with two decimal
  places, set the precision to two decimal places
• Prompt user to enter account number
• Get customer account number
• Prompt user to enter customer code
• Get customer code

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Main Algorithm (continued)

• If the customer code is r or R,
• Prompt user to enter number of premium channels
• Get the number of premium channels
• Calculate the billing amount
• Print account number and billing amount

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Main Algorithm (continued)

• If customer code is b or B,
• Prompt user to enter number of basic service
  connections
• Get number of basic service connections
• Prompt user to enter number of premium channels
• Get number of premium channels
• Calculate billing amount
• Print account number and billing amount
• If customer code is other than r, R, b, or B,
  output an error message

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• include ltiostreamgt
• include ltiomanipgt
• using namespace std
• //Named constants - residential customers
• const double rBillProcessingFee 4.50
• const double rBasicServiceCost 20.50
• const double rCostOfaPremiumChannel 7.50
• //Named constants - business customers
• const double bBillProcessingFee 15.00
• const double bBasicServiceCost 75.00
• const double bBasicConnectionCost 5.00
• const double bCostOfaPremiumChannel 50.00
• int main()
• //Variable declaration

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• switch (customerType)
• case 'r' //Step 6
• case 'R' cout ltlt "Enter the number"
• ltlt " of premium channels " //Step 6a
• cin gtgt numberOfPremiumChannels //Step 6b
• cout ltlt endl
• amountDue rBillProcessingFee //Step
  6c
• rBasicServiceCost numberOfPremiumChannels
  rCostOfaPremiumChannel
• cout ltlt "Account number " ltlt accountNumber
  ltlt endl //Step 6d
• cout ltlt "Amount due " ltlt amountDue ltlt
  endl // Step 6d
• break
• case 'b'
  //Step 7
• case 'B' cout ltlt "Enter the number of basic
  "
• ltlt "service connections "
  //Step 7a
• cin gtgt noOfBasicServiceConnections
  //Step 7b
• cout ltlt endl

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Summary

• Control structures alter normal control flow
• Most common control structures are selection and
  repetition
• Relational operators , lt, lt, gt, gt, !
• Logical expressions evaluate to 1 (true) or 0
  (false)
• Logical operators ! (not), (and), (or)

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Summary

• Two selection structures one-way selection and
  two-way selection
• The expression in an if or if...else structure is
  usually a logical expression
• No else statement in C. Every else has a
  related if
• A sequence of statements enclosed between braces,
  and , is called a compound statement or block
  of statements

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Summary

• Using assignment in place of the equality
  operator creates a semantic error
• Switch structure handles multiway selection
• Break statement ends switch statement
• Use assert to terminate a program if certain
  conditions are not met