Control structure

1
chapter2

• Control structure

2
objectives

• To understand basic problem-solving techniques.
• To be able to use the if, if/else and switch
  selection structures to choose among alternative
  actions.
• To be able to use the while, do/while and for
  repetition structures to execute statement in a
  program repeatedly.
• To understand counter-controlled repetition and
  sentinel-controlled repetition.
• To be able to use the increment and logical
  operators.
• To be able to use the break and continue program
  control statements.

3
If/else selection structure

• The if selection structure performs an indicated
  action only when the condition is true, otherwise
  the action is skipped.
• If (BOOLEAN STATEMENT)
• IF_STATEMENT
• If (grade gt 60) cout ltlt passed
• If (BOOLEAN STATEMENT)
• IF_STATEMENT
• else
• ELSE_STATEMENT

4
If/else selection structure

• If (grade gt 60) cout ltlt passed
• else cout ltlt failed
• cout ltlt (grade gt 60 ? passed failed)
• if (grade gt 90) cout ltlt A
• else if (grade gt 80) cout ltlt B
• else if (grade gt 70) cout ltlt C
• else if (grade gt 60) cout ltlt D
• else cout ltlt F

5
while repetition structure

• A repetition structure allows the programmer to
  specify that a program should repeat an action
  while some condition remains true.
• While there are more items on my shopping list
• purchase next item and cross it off my list.
• int product 2
• while (product lt 1000)
• product 2 product //product 2

6
Counter control repetition

• include ltiostream.hgt
• int main()
• int total 0, gradeCounter 1, grade, average
• while( gradeCounter lt 10)
• cout ltlt Enter grade
• cin gtgt grade
• total grade
• gradeCounter
• average total / 10
• cout ltlt class average is ltlt average ltlt endl
• return 0

7
Analyze the program

• Line 3 is declaration and initialization.
• Line 4 indicate that the while structure should
  continue as long as gradeCounters value is less
  than or equal to 10.
• Line 5, 6 means input the next grade
• Line 7 adds grades to the previous value of
  total and assign the result to total.
• Line 8 adds 1 to gradeCounter.
• Line 10 assign the result of the average
  calculation to variable average.

8
Analyze the program

• Line 11 prints class average is followed by
  the value of variable average

9
Formulating Algorithms with top-down, stepwise
refinement

• Case study (Sentinel-controlled Repetition)
• Consider the following problem
• Develop a class-averaging program that will
  process an arbitrary number of grades each time
  the program is run.

10
Case study

• In this example, no indication is given of how
  many grades the user will enter during the
  programs execution.
• How can the program determine when to stop the
  input of grades?
• How will it know when to calculate and print the
  class average?
• One way is sentinel value (flag value) to
  indicate end of data entry .

11
Case study

• We approach the class-average program with a
  technique called top-down , stepwise refinement.
• We begin with a pseudocode representation of the
  top
• Determine the class average for the quiz.
• We divide the top into series of smaller tasks
  and list these in the order in which they need to
  be performed.

12
Case study

• Initialize variables
• Input , sum and count the quiz grades.
• Calculate and print the class average
• This refinement uses only the sequence
  structure--the steps listed should execute in
  order, one after the other.
• In this example, we need a running total of the
  numbers, a count of how many numbers have been
  processed, a variable to receive the value of
  each grade as it is input by the user and a
  variable to hold the calculated average.

13
Case study

• The pseudocode statement
• Initialize variables
• Can be refined as follows
• Initialize total to zero
• Initialize counter to zero
• Dont need to initialize the average and X
  variables.

14
Case study

• The pseudocode statement
• Input , sum and count the quiz grades
• requires a repetition structure that successively
  inputs each grades.
• The second refinement of the preceding pseudocode
  statement is then
• Input the first grade (possibly the sentinel)
• While the user has not yet entered the sentinel
• Add this grade into the running total
• Add one to the grade counter
• Input the next grade (possibly the sentinel)

15
Case study

• The pseudocode statement
• Calculate and print the class average
• Can be refined as follows
• If the counter is not equal to zero
• Set the average to the total divided by the
  counter
• Print the average
• else
• print No grade was entered

16
Case study

• Note that we are being careful here to test for
  the possibility of division by zero-normally a
  fatal error that, if undetected, would cause the
  program to fail (often called bombing or
  crashing).

17
Case study

• Initialize total to zero
• Initialize counter to zero
• Input the first grade (possibly the sentinel)
• While the user has not yet entered the sentinel
• Add this grade to the running total
• Add one to the grade counter
• Input the next grade (possibly the
  sentinel)

18
Case study

• If the counter is not equal to zero
• set the average to the total divided by the
  counter
• Print the average
• Else
• Print No grades were entered
• This algorithm was developed after only two
  levels of refinement. Sometimes more levels are
  necessary.

19
Write program

• // class average program with sentinel-controlled
  repetition.
• include ltiostreamgt
• include ltiomanipgt
• using namespace std
• int main()
• int total 0
• int gradeCouner 0
• int grade
• double average

20
Write program

• cout ltlt Enter grade, -1 to end
• cin gtgt grade
• while ( grade ! -1)
• total total grade
• gradeCounter gradeCounter 1
• cout ltlt Enter grade, -1 to end
• cin gtgt grade

21
Write program

• if ( gradeCounter ! 0)
• averagestatic_castltdoublegt(total)/
  gradeCounter
• coutltltClass average is ltltsetprecision(2)
  ltlt fixed ltlt average ltlt endl
• else
• cout ltlt No grades were entered ltlt endl
• return 0

22
A sample execution

• Enter grade, -1 to end 23
• Enter grade, -1 to end 43
• Enter grade, -1 to end 64
• Enter grade, -1 to end 56
• Enter grade, -1 to end 78
• Enter grade, -1 to end 89
• Enter grade, -1 to end -1
• Class average is 58.83