if Statements

1
Programming

• if Statements

2
Review Rules for Division

• C treats integers different than doubles.
• 100 is an int.
• 100.0 , 100.0000, and 100. are doubles.
• The general rule for division of int and double
  types is
• double/double -gt double (normal)
• double/int -gt double (normal)
• int/double -gt double (normal)
• int/int -gt int (special case any decimal
  places discarded)

3
Rules for Division

• Example
• 220. / 100.0 double/double -gt double result is
  2.2
• 220. / 100 double/int -gt double result is 2.2
• 220 / 100.0 int/double -gt double result is 2.2
• 220 / 100 int/int -gt int result is 2
• Summary division is normal unless both the
  numerator and denominator are int, then the
  result is an int (the decimal places are
  discarded).

4
Forcing a Type Change

• You can change the type of an expression with a
  cast operation
• Syntax
• variable1 type(variable2)
• variable1 type(expression)
• Example
• int x1, y2
• double result1 x/y // 1/2 0 -gt 0.0
• double result2 double(x)/y // 1.0/2 0.5 -gt
  0.5
• double result3 x/double(y) // 1/2.0 0.5 -gt
  0.5
• double result4 double(x)/double(y) // 1.0/2.0
  -gt 0.5
• double result5 double(x/y)
• // double(1/2) double(0) -gt 0.0
• int cents int(result4100) // cents is 50

5
Three Program Structures

• Sequence - executable statements which the
  computer processes in the given order
• Choice - sequence(s) selected depending on some
  condition
• if ltcondition existsgt
• ltdo Pgt
• Iteration - repetitively executed sequences
• while ltcondition existsgt
• ltdo Pgt

6
Sequence

• It is natural to write a program as a sequence of
  program structures such as sequences, choices,
  and iterations

• Program
• Structure 1

• Program
• Structure 2

• Program
• Structure 3

7
Choice Constructs

• Provide
• Ability to control whether a statement list is
  executed
• Two constructs
• if statement
• if
• if-else
• if-else-if
• switch statement

8
The Basic if Statement

• Syntax
• if(Expression)
• Action
• If the Expression is true then execute Action
• Action is either a single statement or a group of
  statements within braces
• Example absolute value
• if(value lt 0)
• value -value

Expression
true
false
Action
9
Absolute Value

• // program to read number print its absolute
  value
• include ltiostreamgt
• using namespace std
• int main()
• int value
• cout ltlt "Enter integer "
• cin gtgt value
• if(value lt 0)
• value -value
• cout ltlt "The absolute value is " ltlt value ltlt
  endl
• return 0

10
Choice (if)

• Put multiple action statements within braces
• if ltit's raininggt
• lttake umbrellagt
• ltwear raincoatgt

11
Sorting Two Numbers

• int value1
• int value2
• int temp
• cout ltlt "Enter two integers "
• cin gtgt value1 gtgt value2
• if(value1 gt value2)
• temp value1
• value1 value2
• value2 temp
• cout ltlt "The input in sorted order "
• ltlt value1 ltlt " " ltlt value2 ltlt endl

12
Relational Operators

• Relational operators are used to construct a
  logical expression
• Math C Plain English
• equals example if(ab)
• (ab) means put the value of b into a
• lt lt less than
• ? lt less than or equal to
• gt gt greater than
• ? gt greater than or equal to
• ? ! not equal to

13
Relational Expressions

• Examples
• numberOfStudents lt 200
• 10 gt 20
• 20 j 10 i

14
Operator Precedence

• Which comes first?
• /
• -
• lt lt gt gt
• !

Answer
15
The if-else Statement

• Syntax
• if(Expression)
• Action1 else Action2
• If Expression is true thenexecute Action1
  otherwiseexecute Action2
• Example
• if(v 0)
• cout ltlt "v is 0"
• else
• cout ltlt "v is not 0"

Expression
false
true
Action1
Action2
16
Choice (if and else)

• if ltit's sunnygt
• ltgo to beachgt
• else
• lttake umbrellagt

17
Finding the Big One

• int value1
• int value2
• int larger
• cout ltlt "Enter two integers "
• cin gtgt value1 gtgt value2
• if(value1 gt value2)
• larger value1
• else
• larger value2
• cout ltlt "Larger of inputs is " ltlt larger ltlt endl

18
Selection

• Often we want to perform a particular action
  depending on the value of an expression
• Two ways to do this
• if-else-if statement
• if-else statements glued together
• switch statement
• An advanced construct

19
if-else-if Statements

• if ltcondition 1 existsgt
• ltdo Qgt
• else if ltcondition 2 existsgt
• ltdo Rgt
• else if ltcondition 3 existsgt
• ltdo Sgt
• else
• ltdo Tgt

Q
R
T
S
20
if-else-if Statements

• Top10

• if ltMon, Wed, or Fri AMgt
• ltgoto COMP 104gt
• else if ltTues, Thurs AMgt
• ltgoto MATH 113gt
• else if lt1PM or 7PMgt
• lteatgt
• else
• ltsleepgt

21
if-else-if Statement

• Example
• if(score gt 90)
• cout ltlt "Grade A" ltlt endl
• else if(score gt 80)
• cout ltlt "Grade B" ltlt endl
• else if(score gt 70)
• cout ltlt "Grade C" ltlt endl
• else if(score gt 60)
• cout ltlt "Grade D" ltlt endl
• else
• cout ltlt "Grade F" ltlt endl

22
switch Statement

• switch(int(score)/10)
• case 10
• case 9 cout ltlt "Grade A" ltlt endl
• break
• case 8 cout ltlt "Grade B" ltlt endl
• break
• case 7 cout ltlt "Grade C" ltlt endl
• break
• case 6 cout ltlt "Grade D" ltlt endl
• break
• default cout ltlt "Grade F" ltlt endl

23

• int left
• int right
• char oper
• cout ltlt "Enter simple expression "
• cin gtgt left gtgt oper gtgt right
• cout ltlt left ltlt " " ltlt oper ltlt " " ltlt right
• ltlt " "
• switch (oper)
• case '' cout ltlt left right ltlt endl break
• case '-' cout ltlt left - right ltlt endl break
• case '' cout ltlt left right ltlt endl break
• case '/' cout ltlt left / right ltlt endl break
• default cout ltlt "Illegal operation" ltlt endl

24
Boolean Type

• C contains a type named bool which can have one
  of two values
• true (corresponds to non-zero value)
• false (corresponds to zero value)
• Boolean operators can be used to form more
  complex conditional expressions
• The and operator is
• The or operator is
• The not operator is !
• Warning
• and are bit-wise operators
• please do NOT confuse them with boolean operators

25
A Boolean Type

• Example logical expressions
• bool P truebool Q falsebool R
  truebool S P Qbool T !Q Rbool U
  !(R !Q)

26
More Operator Precedence

• Precedence of operators (from highest to lowest)
• Parentheses ( )
• Unary operators !
• Multiplicative operators /
• Additive operators -
• Relational ordering lt lt gt gt
• Relational equality !
• Logical and
• Logical or
• Assignment

27
More Operator Precedence

• Parentheses ( )
• Unary operators !
• Multiplicative operators /
• Additive operators -
• Relational ordering lt lt gt gt
• Relational equality !
• Logical and
• Logical or
• Assignment

• Examples
• 5 ! 6 7 lt 3
• (5 !6) (7 lt 3)
• 5 15 4 13 12 lt 19 !false 5 lt 24
• 5 15 4 13 12 lt 19 (!false) 5 lt
  24
• (5 15) 4 13 12 lt 19 true 5 lt 24
• ((5 15) 4) 13 12 lt 19 true 5 lt
  24
• ((5 15) 4) 13 (12 lt 19) true (5
  lt 24)
• (((5 15) 4) 13) true (true true)
• false true true
• (false true) true
• true

28
Finding the largest number

• Consider three numbers, A, B and C How many ways
  can they be ordered?
• A gt B gt C
• A gt C gt B
• B gt A gt C
• B gt C gt A
• C gt A gt B
• C gt B gt A

29
Whats Wrong?
int a3, b2, c1 if ((a gt b gt c) (a gt c gt
b)) cout ltlt a ltlt endl else if ((b gt a gt c)
(b gt c gt a)) cout ltlt b ltlt endl else if ((c gt a
gt b) (c gt b gt a)) cout ltlt c ltlt endl

• a gt b gt c
• (a gt b) gt c
• (3 gt 2) gt 1
• true gt 1
• 1 gt 1
• false

• a gt c gt b
• (a gt c) gt b
• (3 gt 1) gt 2
• true gt 2
• 1 gt 2
• false

30
Correct program
int a2, b2, c1 if ((a gt b) (b gt c) (a gt
c) (c gt b)) cout ltlt a ltlt endl else if ((b gt
a) (a gt c) (b gt c) (c gt a)) cout ltlt b
ltlt endl else if ((c gt a) (a gt b) (c gt b)
(b gt a)) cout ltlt c ltlt endl
31
Nested if Statements

• Nested means that one complete statement is
  inside another (nested scopes)
• if (condition_1)
• if (condition_2)
• if (condition_3)
• cout ltlt 123
• cout ltlt 12
• cout ltlt 1

Scope of if-statement
32
Dangling Else Problem

• Problem Nested if statements can seem ambiguous
  in their meaning.
• What is the value of c after the following is
  executed?
• int a-1, b1, c1
• if(agt0)
• if(bgt0)
• c 2
• else
• c 3

33
Dangling Else Problem

• C groups a dangling else with the most recent
  if.
• The following indentation shows how C would
  group this example (answer c1).
• int a-1, b1, c1
• if(agt0)
• if(bgt0)
• c 2
• else // dangling else grouped to nearest if
• c 3

34
Dangling Else Problem

• Use extra brackets to clarify the intended
  meaning, even if not necessary.
• int a-1, b1, c1
• if(agt0)
• if(bgt0)
• c 2
• else // use to avoid dangling else
• c 3