Selection

1
Selection
2
Structured Components

• There are three basic constructs that can be used
  to build programs
• Sequence
• One statement follows another.
• Selection
• Choose what statements to execute.
• Repetition
• Repeat statements.

3
Selection In C

• If statement
• Many forms
• Switch statement
• Implements a case structure

4
Compound Statement

• Groups a set of statements together under one
  scope.
• Used in selections.

statement1 statement2 statement3
5
Function Body

• Uses a compound statement.

int MyFunction(int a, int b, int c) int
ReturnValue ReturnValue a b c return
ReturnValue
Compound Statement
6
The if statement
if Condition do if true else do if false

• Condition is an expression that evaluates to
  false (0) or true (?0).
• If it evaluates to true, the first compound
  statement is executed.
• If it evaluates to false, the second compound
  statement is executed.

7
So, What Is A Condition?

• variable relational-operator variable
• a lt b
• variable relational-operator constant
• a gt 3
• variable equality-operator variable
• d e
• variable equality-operator constant
• fire ! 12

8
Relational Operators

• lt less than
• lt less than or equal to
• gt greater than
• gt greater than or equal to

9
Equality Operators

• Equal to.
• ! Not equal to.
• Be sure to remember both signs on the equal to
  operator.
• Be sure to review Table 4.2
• This author calls 1 true. Since 1 is ? 0, hes
  right. But 2 is true, 3 is true, 129 is true.
  Anything other than 0.

10
Logical Expression

• rel_exress logical_operator rel_express
• Logical Operators
• And
• Or
• ! Not
• If ( a lt b c gt d )
• If a lt b and c gt d do true compound statement.
  If either is false, do false compound statement.

11
Logical Expression (cont.)

• if (e gt f gift snow)
• If either is true, execute true compound
  statement.
• If both are false, execute false compound
  statement
• if ! (cost lt 12.0)
• If cost is less than 12.0, execute false.
• If cost is greater than or equal to 12.0 execute
  true.

12
Watch comparing float to zero

• NEVER
• NEVER
• NEVER
• NEVER
• NEVER
• NEVER if ( float_variable 0.0 ) ..

13
Did I say NEVER!!

• The reason is that internal representations for
  float or double variables can have a bit set that
  will fail a comparison to zero when the value is
  basically zero.
• Better to do the following
• if ( fabsf(float_variable) lt some_epsilon )
• Trust Me! (or trust someone else!) Just Trust!
• www.cygnus-software.com/papers/comparingfloats/com
  paringfloats.htm

14
Precedence?

• Study the book carefully.
• Or, just use parentheses.

15
Short Circuit Evaluation

• If the first relational expression in an AND
  logical expression is false, the rest of the
  condition is not executed. The result is false.
• If the first relational expression in an OR
  logical expression is true, the rest of the
  condition is not executed. The result is true.

16
Example

• if (altb cgtd)
• If altb is false, cgtd is not evaluated.
• If (e ! f glth)
• If e ! f is true, glth is not evaluated.
• With relational, this isnt to significant. But
  if the second relational also changes a value,
  this can be very significant. (More Later)

17
Character Relational

• Lexicographic (alphabetical) order is used.
• a gt c is false
• d lt f is true

18
Logical Assignment

• The book shows using an int.
• Most people define a separate variable type bool
  to make programs more readable.
• result a lt b
• The comparison is made and the true or false is
  stored in result.
• if (result)

19
DeMorgans Theorem

• To complement an expression, not both the
  relationals and change the to .
• To complement an expression, not both the
  relationals and change the to .
• !(altb cgtd) is the same as
• !(altb) !(cgtd)

20
Dont Use Flow Charts

• They are cumbersome.
• They are old fashioned.
• They limit your thinking.
• Use pseudocode.

21
IF ( ) THEN ELSE
if (a lt b) compound1 lt- If altb is true
execute this. else compound2 lt- If altb is
false execute this.
22
One Statement ? No
If (fltg) statement1
if (dlte) statement1 else statement2
Null else branch!
If (hi) else statement2
Seldom used!
23
Book Case Studies

• They have some nice case study examples.
• Dont lose the benefits by skipping over them too
  lightly.
• If you see something funny, bring it up in class.

24
Nested If Statements

• Obviously, a compound statement may contain more
  if statements.
• Logic can be a bit tricky if they get too deep!

25
Switch Statement
switch( size ) case 2 case 3
printf(small\n) break case 4
printf(medium\n) break

• A range of choices.

26
Hand Trace
int main ( void ) int a3, b4, c5,
d6,total2 if( altb cgtd ) total
3 else total 4 switch (total)
case 2 printf(hello\n)
break case 3 printf(good-bye\n)
break default
printf(so long\n)
Given this code, what prints?
27
Hand Trace
int main ( void ) int a3, b4, c5,
d6,total2 if( altb cgtd ) total
3 else total 4 switch (total)
case 2 printf(hello\n)
break case 3 printf(good-bye\n)
break default
printf(so long\n)
Given this code, what prints?
a
b
c
d
total
altb
cgtd

Output
28
Hand Trace
int main ( void ) int a3, b4, c5,
d6,total2 if( altb cgtd ) total
3 else total 4 switch (total)
case 2 printf(hello\n)
break case 3 printf(good-bye\n)
break default
printf(so long\n)
Given this code, what prints?
a
b
c
d
total
altb
cgtd

3
4
5
6
2
Output
29
Hand Trace
int main ( void ) int a3, b4, c5,
d6,total2 if( altb cgtd ) total
3 else total 4 switch (total)
case 2 printf(hello\n)
break case 3 printf(good-bye\n)
break default
printf(so long\n)
Given this code, what prints?
a
b
c
d
total
altb
cgtd

3
4
5
6
2
T
F
Output
30
Hand Trace
int main ( void ) int a3, b4, c5,
d6,total2 if( altb cgtd ) total
3 else total 4 switch (total)
case 2 printf(hello\n)
break case 3 printf(good-bye\n)
break default
printf(so long\n)
Given this code, what prints?
a
b
c
d
total
altb
cgtd

3
4
5
6
2
T
F
F
4
Output
31
Hand Trace
int main ( void ) int a3, b4, c5,
d6,total2 if( altb cgtd ) total
3 else total 4 switch (total)
case 2 printf(hello\n)
break case 3 printf(good-bye\n)
break default
printf(so long\n)
Given this code, what prints?
a
b
c
d
total
altb
cgtd

3
4
5
6
2
T
F
F
4
Output
32
Hand Trace
int main ( void ) int a3, b4, c5,
d6,total2 if( altb cgtd ) total
3 else total 4 switch (total)
case 2 printf(hello\n)
break case 3 printf(good-bye\n)
break default
printf(so long\n)
Given this code, what prints?
a
b
c
d
total
altb
cgtd

False
3
4
5
6
2
T
F
F
4
Output
33
Hand Trace
int main ( void ) int a3, b4, c5,
d6,total2 if( altb cgtd ) total
3 else total 4 switch (total)
case 2 printf(hello\n)
break case 3 printf(good-bye\n)
break default
printf(so long\n)
Given this code, what prints?
a
b
c
d
total
altb
cgtd

3
4
5
6
2
T
F
F
4
False
Output
34
Hand Trace
int main ( void ) int a3, b4, c5,
d6,total2 if( altb cgtd ) total
3 else total 4 switch (total)
case 2 printf(hello\n)
break case 3 printf(good-bye\n)
break default
printf(so long\n)
Given this code, what prints?
a
b
c
d
total
altb
cgtd

3
4
5
6
2
T
F
F
4
True
Output
35
Hand Trace
int main ( void ) int a3, b4, c5,
d6,total2 if( altb cgtd ) total
3 else total 4 switch (total)
case 2 printf(hello\n)
break case 3 printf(good-bye\n)
break default
printf(so long\n)
Given this code, what prints?
a
b
c
d
total
altb
cgtd

3
4
5
6
2
T
F
F
4
so long
Output
36
Hand Trace
int main ( void ) int a3, b4, c5,
d6,total2 if( altb cgtd ) total
3 else total 4 switch (total)
case 2 printf(hello\n)
break case 3 printf(good-bye\n)
break default
printf(so long\n)
Given this code, what prints?
a
b
c
d
total
altb
cgtd

3
4
5
6
2
T
F
F
4
so long
Output
37
Hand Trace
int main ( void ) int a3, b4, c5,
d6,total2 if( altb cgtd ) total
3 else total 4 switch (total)
case 2 printf(hello\n)
break case 3 printf(good-bye\n)
break default
printf(so long\n)
Given this code, what prints?
a
b
c
d
total
altb
cgtd

3
4
5
6
2
T
F
F
4
so long
Output
main exits