CSCI6370: Topics in Computer Science Advanced Topics in Algorithms and Applications Fall Semester, 2002

1
Lecture 10 Reviews
2
Control Structures

• All C programs written in term of 3 control
  structures
• Sequence structures
• Programs executed sequentially by default
• Selection structures
• Used to choose among alternative courses of
  action
• Three types if, if else, and switch
• Repetition structures
• Used to repeat an action when some condition
  remains true.
• Three types while, do while and for

3
Sequence Structures

• Normally, statements are executed one after the
  other in the order written

4
Selection Structures - if

• if statement
• if (condition)
• statement(s)
• Only performs an action if the condition is true.
• Example
• Flowchart

if ( grade gt 60 ) printf( Passed\n )
if ( grade gt 60 ) printf( Passed\n )
printf( Congratulations!\n )
if ( grade gt 60 ) printf( Passed\n )
printf( Congratulations!\n )
5
Selection Structures - ifelse

• Ifelse statement
• if (condition)
• statement(s)
• else
• statement(s)
• Specifies an action to be performed both when the
  condition is true and when it is false.
• Flowchart

6
Selection Structures - ifelse

• Nested ifelse statements
• Test for multiple cases by placing ifelse
  selection statements inside ifelse selection
  statement
• Once condition is met, rest of statements skipped

if ( score gt 90 ) printf( "The grade is
'A'.\n" ) else if ( score gt 80 )
printf( "The grade is 'B'.\n" )
else if ( score gt 70 )
printf( "The grade is 'C'.\n" )
else if ( score gt 60 )
printf( "The grade is
'D'.\n" )
else
printf( "The grade is 'F'.\n" )

gt 90 A 80 89 B 70 79 C 60 69 D lt 60 F
7
Selection Structures - ifelse

• ifelse statements in sequence

if ( score gt 90 ) printf( "The grade is
'A'.\n" ) else if ( score gt 80 )
printf( "The grade is 'B'.\n" )
if ( score gt 70 ) printf( "The grade
is 'C'.\n" ) else if ( score gt 60 )
printf( "The grade is 'D'.\n" )
else printf( "The grade is
'F'.\n" )
8
Selection Structures - switch

• switch statement
• Useful when a variable or expression is tested
  for all the values it can assume and different
  actions are taken.
• Format
• Series of case labels and an optional default
  case
• switch ( controlling expression )
• case value1
• actions
• case value2
• actions
• default
• actions
• The switch statement can be used only for testing
  a constant integral expression.

switch (grade) case gt 90 case gt
80 ..
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Selection Structures - switch

• switch statement with break statements
• switch ( controlling expression )
• case value1
• actions
• break
• case value2
• actions
• break
• default
• actions
• break
• It causes program control to continue with the
  first statement after the switch statement.

10
Selection Structures - switch

• switch statement with break statements
• Example
• switch ( n )
• case 1
• printf( The number is 1\n)
• break
• case 2
• printf( The number is 2\n)
• break
• case 3
• printf( The number is 3\n)
• break
• default
• printf( The number is not 1, 2, or
  3\n)
• break

11
Selection Structures - switch

• switch statement with break statements
• Example
• switch ( n )
• case 1
• printf( The number is 1\n)
• case 2
• printf( The number is 2\n)
• break
• case 3
• printf( The number is 3\n)
• break
• default
• printf( The number is not 1, 2, or
  3\n)
• break

12
Repetition Structures

• Two types of repetition structures
• Counter-controlled repetition
• Definite repetition know how many times loop
  will execute
• Control variable used to count repetitions
• Sentinel-controlled repetition
• Indefinite repetition
• Used when number of repetitions not known
• Sentinel value indicates "end of data entry
• Loop ends when user inputs the sentinel value

13
Repetition Structures

• Common error
• Infinite loops are caused when the
  loop-continuation condition in a while, for or
  do...while statement never becomes false. To
  prevent
• a) Make sure there is not a semicolon
  immediately
• after the header of a while statement.
• b) In a counter-controlled loop, make sure the
  control variable is incremented (or decremented)
  in the loop.
• c) In a sentinel-controlled loop, make sure the
  sentinel value is eventually input.

j 1 while (j lt 10) printf(d\n,
--j) j
j 1 while (j lt 10) printf(d\n, j)
j--
j 1 while (j lt 10) printf(d\n, j)
j
j 1 while (j lt 10) printf(d\n, j)
14
Repetition Structures - while

• while loop repeated until condition becomes false
• Counter-controlled repetition
• Sentinel-controlled repetition
• Format
• while ( condition )
• statement(s)
• Flowchart

15
Repetition Structures - while

• Counter-controlled while loop
• Loop repeated until counter reaches a certain
  value
• Count the number of iterations

j 1 while (j lt 10) printf(Good
Luck\n) j
j 0 while (j lt 10) printf(Good
Luck\n) j
j 0 while (j lt 10) printf(Good
Luck\n) j
j 1 while (j lt 10) printf(Good
Luck\n) j 2
j -1 while (j lt 10) printf(Good
Luck\n) j 3
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Repetition Structures - while

• Sentinel-controlled while loop
• Loop repeated until user inputs the sentinel
  value
• Sentinel value chosen so it cannot be confused
  with a regular input
• Examples
• Simple calculator
• ATM menu interface
• Prime number

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Repetition Structures - dowhile

• Similar to the while statement
• Condition for repetition only tested after the
  body of the loop is performed.
• All actions are performed at least once.
• Format
• do
• statement(s)
• while ( condition )
• Flowcharting the dowhile repetition statement

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Repetition Structures - dowhile

• Examples

counter 1 while (counter lt 10) printf(
d\n, counter) counter
counter 1 do printf( d\n, counter)
counter while (counter lt 10)
counter 12 while (counter lt 10) printf(
d\n, counter) counter
counter 12 do printf( d\n, counter)
counter while (counter lt 10)
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Repetition Structures - for

• Can loop a known number of times
• Format when using for loops
• for ( initialization loopContinuationTest
  increment )
• statement(s)
• initialization
• while ( loopContinuationTest )
• statement(s)
• increment

for (j 1 j lt 10 j) printf(d\n,
j)
for (j 10 j gt 0 j--) printf(d\n,
j)
for (j 10 j gt 0 j - 2) printf(d\n,
j)
Increasing/decreasing other than 1
increment
decrement
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Nested Control Loops
j

• Nested while loops
• i 1
• while (i lt 5)
• j 1
• while (j lt 5)
• printf(d d d\n, i, j, ij)
• j
• i
• Nested for loops
• for (i 1 i lt 5 i)
• for (j 1 j lt 5 j)
• printf(d d d\n, i, j, ij)

i
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Nested Control Loops
j

• Nested while loops
• i 1
• while (i lt 5)
• j 1
• while ( j lt i )
• printf(d d d\n, i, j, ij)
• j
• i
• Nested for loops
• for (i 1 i lt 5 i)
• for (j 1 j lt i j)
• printf(d d d\n, i, j, ij)

i
j lt i
j lt i
22
Nested Control Loops
j

• Nested while loops
• i 1
• while (i lt 5)
• j 1
• while ( j lt 5 )
• printf(d d d\n, i, j, ij)
• j
• i 2
• Nested for loops
• for (i 1 i lt 5 i 2)
• for (j 1 j lt 5 j)
• printf(d d d\n, i, j, ij)

i
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Operators

• Arithmetic operators
• , -, , /,
• Assignment operator
• Arithmetic assignment operators
• , -, , /,
• Increment / decrement operators
• , --
• Relational operators
• lt, lt, gt, gt
• Equality operators
• , !
• Logical operators
• , , !

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and

• Swapping both operators does not ordinarily cause
  syntax errors.

int payCode 0 payCode 4 if (payCode 4)
printf(You get a bonus!\n)
int payCode 0 payCode 4 if (payCode
4) printf(You get a bonus!\n)
int payCode 0 payCode 3 if (payCode 4)
printf(You get a bonus!\n)
int payCode 0 payCode 3 if (payCode 4)
printf(You get a bonus!\n)
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Pre-increment and Post-increment

• Pre-increment/Pre-decrement
• Operator is used before the variable (c or --c)
  
• Variable is changed before the expression it is
  in is evaluated
• Post-increment/Post-decrement
• Operator is used after the variable (c or c--)
• Expression executes before the variable is
  changed.

c 5 printf(d\n, c) printf(d\n, c)
c 5 printf(d\n, c) printf(d\n, c)
c 5 c printf(d\n, c)
c 5 c printf(d\n, c)
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Logical Operators
x

• Logical AND -
• Logical OR -
• Logical NOT - !

y
x
y
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Logical Operators

• Examples
• int F 0, T 1
• int x, y
• x !240
• x !0
• x (F 1) (T F)
• y (T !F) (!F T)
• x !(T F 1)
• if (grade gt 80 grade lt90)
• printf(B\n)
• while ((x gt 0) (y gt 10))
• x--
• y--

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scanf and printf

• scanf()
• Used to obtain a value (values) from the user
  using standard input (usually keyboard).
• Examples
• int num, j
• float x
• scanf( d, num)
• scanf( dd, num, j)
• scanf( f, x)
• scanf( ddf, num, j, x)
• scanf( d, num)
• scanf( dd, num)

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scanf and printf

• printf()
• int num 3, j 2
• float x 3.0
• printf( Good Luck!\n) / prints a string /
• printf( num d\n, num) / prints the value
  of a variable /
• printf( d d d\n, num, j, numj)
• / prints the value of an expression /
• printf( c\n, b) / prints the character
  b /
• j a
• printf( c\n, j) / prints the character a
  /
• printf( x f\n, x) / prints a
  floating-point number /
• printf( x .2f\n, x )
• / prints result with 2 digits after decimal
  point /