Complex Selections and Repetitions

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Chapter 9

• Complex Selections and Repetitions

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• 9.1 INTRODUCTION
• Then we introduce the switch statement, which can
  also be used for multiway selection.
• For statement, which is another pretest
  repetition structure.
• Do-while statement. The do-while statement is the
  C implementation of the posttest repetition
  structure.

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• 9.2 COMPLEX PREDICATES
• Logical Expressions and Logical Operatiors
• Logical and (conjunction)
• Logical or (disjunction)
• Logical not (negation)
• The C Logical Operators
• Logical and is represented by .
• The symbol stands for logical or.
• The C logical not operator is !.
• Example 9.1
• (semester_average gt 80 )
  (semester_average lt 89)

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• Example 9.3
• ! (student_status u)
• Simplifying Complex Predicates
• in some selection problems there are several
  forms for expressing the same condition.
• We should try to get rid of the negation and
  find the simplest form in order to enhance the
  readability of the code.

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• Predicate Equivalent Simple Form
• ! (a b) a ! b
• ! (a lt b) a gt b
• ! (a gt b) a lt b
• ! (Expression_1 Expression_2) (!
  Expression_1) (Expression_2)
• Figure 9.1 Equivalent forms for predicates
  involving negation

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• Precedence of Logical Operators
• Logical not , unary arithmetic operators
• Binary arithmetic operators
• Relational operators
• Logical and
• Logical or
• Example 9.6
• xy gt 13 ! (x-y) xy -16 4
• 4 6 8 2 1 9 3 5 7

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• 9.3 MULTIWAY SELECTION USING THE switch AND
  break STATEMENTS
• switch (ControllingExpression)
• CaseClause-1
• CaseClause-2
• .
• .
• CaseClause-n
• DefaultClause
• / end switch /

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• In the switch statement the controlling
  expression is evaluated first. The controlling
  expression must compute to an integral value and
  must be of type int or char
• If the value of the controlling expression does
  not match any of the constant values in the case
  clauses, the content of the default clause is
  executed.

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• switch (major_code)
• case 1
• printf(student major is computer science.
• break
• case 7
• printf(Student major is computer
  engineering.
• break
• default
• printf(Student major is a noncomputer
  field.)
• / end switch /

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• If the actions of two or more consecutive cases
  are identical, all we have to do is list the
  cases with empty statements and specify the
  action and a break statement in the final case.

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• switch (character)
• case 0 case 1 case 2 case 3
  case 4 case 5 case 6 case 7
• case 8 case 9
• printf(The content is a decimal
  integer.)
• break
• default
• printf(The content is a nondecimal
  character.)
• / end swithc /

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• 9.4 STYLE CONSIDERATIONS FOR MULTIWAY SELECTION
  STRUCTURES
• Keep the use of logical negation to a minimum in
  forming complex predicates.
• Remember that a nested if statement is more
  general than a switch statement.
• Use proper indentations in forming switch
  statements.
• Whenever possible, use a default clause in your
  switch statements.

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• 9.5 THE PRETEST REPETITION STRUCTURE
• The for Statement for Pretest Repetition
• for (InitializationExpression
  LoopControExpression UpdateExpression)
• Loopbady
• / end for /

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• Executes the InitializationExpression.
• Evaluates the LoopControlExpression. If it
  computes to zero, the loop is exited.
• If the LoopControlExpression yields a nonzero
  value, the LoopBody is executed and then the
  UpdateExpression is evaluated.
• Tests the LoopControlExpression again. Thus the
  LoopBody is repeated until the LoopControlExpressi
  on computes to a zero value.

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• int number, sum 0 , counter 1
• .
• .
• While (counter lt number)
• sum sum counter
• counter counter 1
• / end while /

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• int number , sum , counter
• ..
• ..
• Sum 0
• for (counter 1 counter lt number
  counter counter 1)
• sum sum counter
• / end for /

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• Equivalence of for and while Statements
• the same repetition problem can be expressed
  equivalently using either a while or a for
  statement.
• Using the for Statement for Counter-Controlled
  Loops
• Using the for Statement for Sentinel-Controlled
  Loops

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• char answer
• printf(Do you want to continue? (y/n))
• for (scanf(c, answer)
• answer ! y answer ! Y answer !
  n answer ! N
• scanf(c , answer))
• printf(Please type y or n )
• / end for /

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• Checking for Incorrect Data in a Loop and the
  continue Statement
• we may prefer to warn the user and skip the rest
  of the loop body. Causes the program control to
  skip the rest of the loop body and execute the
  loop again.

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• for (scanf(d, test_score) test_score !0
• scanf(d, test_score))
• if (test_score lt0 test_score gt100)
• printf(Incorrect test score ! Enter a
  correct value )
• continue
• / end if /
• sum sum test_score
• number_of_students number_of students 1
• printf(Enter a test score. Enter 0 to stop
  )
• / end for /

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• 9.6 THE POSTTEST REPETITION STRUCTURE
• the loop body is executed before the loop
  control expression is tested
• The do-while Statement
• It executes the LoopBody.
• It evaluates the LoopControlExpression. If the
  value of the LoopControlEcpression is 0, the
  computer exits the loop otherwise, it does the
  LoopBody again.

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• int number, sum 0 , counter 0
• .
• .
• Do
• sum sum counter
• counter counter 1
• while (counter lt number)
• / end do-while /
• Use of do-while for Counter- and
  Sentinel-Controlled Loops

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• 9.7 NESTED LOOPS
• A nested loop is a repetition structure that
  contains one or more loops in its body. A loop
  contained in another loop forms a doubly nested
  loop.
• int control_var1, control_var2
• for (control_var11 control_var1lt8
  control_var1 2)
• for (control_var2 control_var1 control_var2
  lt 10
• control_var2 3 )
• printf(control_var1 d control_var2
  d\n ,
• control_var1, control_var2)
• / end for /
• /end for /

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• control_var1 1 control_var2 1
• control_var1 1 control_var2 4
• control_var1 1 control_var2 7
• control_var1 1 control_var2 10
• control_var1 3 control_var2 3
• control_var1 3 control_var2 6
• control_var1 3 control_var2 9
• control_var1 5 control_var2 5
• control_var1 5 control_var2 8
• control_var1 7 control_var2 7
• control_var1 7 control_var2 10

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• 9.8 STYLE CONSIDERATIONS FOR REPETITION
  STATEMENTS
• Indent the loop body of for statements
• Avoid the following permitted syntax elements of
  the for statement missing initialization,
  update, and/or loop control expressions in the
  header.
• Avoid using while and for statements for the
  implementation of posttest repetition structures.
• Indent the loop body of do-while

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• 9.9 EXAMPLE PROGRAM 1 A C Program that Computes
  Distribution of Letter Grades