Lecture 10:Control Structures II (Repetition)

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Lecture 10Control Structures II (Repetition)

• Introduction to Computer Science
• Spring 2006

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The while Loop

• The general form of the while statement is
• while(expression)
• statement1
• statement2
• while is a reserved word
• While Loop works in the following procedure
• Expression provides an entry condition
• Statement executes if the expression initially
  evaluates to true
• Loop condition is then reevaluated
• Statement continues to execute until the
  expression is no longer true
• Infinite loop continues to execute endlessly

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Counter-Controlled while Loops

• If you know exactly how many pieces of data need
  to be read, the while loop becomes a
  counter-controlled loop
• The syntax is
• counter 0
• while(counter lt N)
• counter

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Counter-Controlled while Loops
include ltiostreamgt using namespace std int
main() int i i0 while (ilt20) coutltlti
ltlt" " ii5 coutltltendl return(0)
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Sentinel-Controlled while Loops

• Sentinel variable is tested in the condition and
  loop ends when sentinel is encountered
• The syntax is
• cingtgtvariable
• while(variable ! sentinel)
• .
• cingtgt variable
• .

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Sentinel-Controlled while Loops
include ltiostreamgt using namespace std const
int SENTINEL-999 int main() int
number cout ltlt "If you want to terminate the
program, please enter -999."ltltendl cout ltlt
"Enter a number " cingtgtnumber while (number
! SENTINEL) coutltltnumberltlt" "ltltendl cout
ltlt "Enter a number " cingtgtnumber coutltlten
dl return(0)
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Flag-Controlled while Loops

• A flag-controlled while loop uses a Boolean
  variable to control the loop
• The flag-controlled while loop takes the form
• found false
• while(!found)
• if(expression)
• found true

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Flag-Controlled while Loops
include ltiostreamgt using namespace std int
main() bool found int number foundfalse
while (!found) cout ltlt "Enter a number
" cingtgtnumber cout ltlt "Your input is "
ltltnumberltltendl if (number gt 10) foundtrue
coutltltendl return(0)
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EOF-Controlled while Loops

• Use an EOF (End Of File)-controlled while loop
• The logical value returned by cin can determine
  if the program has ended input
• The syntax is
• cin gtgt variable
• while (cin)
• .
• cin gtgt variable
• .

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EOF-Controlled while Loops
include ltiostreamgt include ltstringgt using
namespace std int main() int
x cingtgtx while(cin) coutltltx cingtgtx
return 0
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The eof Function

• The function eof can determine the end of file
  status
• Like other I/O functions (get, ignore, peek), eof
  is a member of data type istream
• The syntax for the function eof is
• istreamVar.eof()
• where istreamVar is an input stream variable,
  such as cin

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The eof Function
include ltfstreamgt include ltiostreamgt using
namespace std int main() ifstream
infile char ch infile.open("c\\tmp\\input.da
t") infile.get(ch) while (!infile.eof())
cout ltlt ch infile.get(ch) coutltltendl re
turn(0)
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End of lecture 10

• Thank you!