1. Other Conditional Methods

2. Conditional Operator

3. Conditional Operator (?:)
Ternary operator: 3 arguments
expression1 ? expression2 : expression3
If expression1 is
true, result is expression2
false, result is expression3

4. Conditional Operator (?:)
Example: int max = (a >= b) ? a : b;
Short for: int max; if (a >= b) max = a; else max = b;

5. Common Use
Pick one of two values inside larger expression:

6. Switch Statement

7. Comparing if…else with ifs
Method 1:
if( month == 1) cout << "Jan"; if( month == 2) cout << "Feb"; if( month == 3) cout << "Mar"; if( month == 4) cout << "Apr"; …

8. Comparing if…else with ifs
Method 2:
if( month == 1) cout << "Jan"; else if( month == 2) cout << "Feb"; else if( month == 3) cout << "Mar"; else if( month == 4) cout << "Apr"; …

9. Comparing if…else with ifs
Method 2 is more efficient… but messy

10. Switch Statement switch choses from list of integer options
If 1 do this
If 2 do this
If 3 do this …

11. switch Structures switch expression is evaluated first
Must be integer value

12. switch Structures switch expression evaluated Jump to matching value
Must be integral value
Jump to matching value

13. switch Structures switch expression is evaluated first
Must be integral value
Jump to matching value
Execute any instructions in that case

14. switch Structures switch expression is evaluated first
Must be integral value
Jump to matching value
Execute any instructions in that case
Continue until break
break jumps to }

15. switch version Switch month select:
int month = magic value switch(month) { case 1: cout << "Jan"; break; case 2: cout << "Feb"; break; … }

16. switch version
int month = magic value switch(month) { case 1: cout << "Jan"; cout << "1st month"; break; case 2: … }

17. switch version
Default is the "else" switch(month) { … case 11: cout << "Nov"; break; case 12: cout << "Dec"; break; default: cout << "Unknown month"; }

18. switch version No break – continues executing statements!
Jan and Feb!!!
int month = magic value switch(month) { case 1: cout << "Jan"; case 2: cout << "Feb"; break; … }

19. switch version Fall through used well: switch (day) {
case 1: // Fall to through to the next case
case 2: // Fall to through to the next case
case 3: // Fall to through to the next case
case 4: // Fall to through to the next case
case 5: cout << "Weekday"; break;
case 0: // Fall to through to the next case
case 6: cout << "Weekend"; }

20. Switch Gotchas No way to represent ranges: switch (day) {
case <= 5: cout << "Weekday"; //NO
Easy to mess up
forget break statement

21. Logical Operators

22. If you make over $35000 and are married
Boolean Operators
Compound condition:
If you make over $35000 and are married
Boolean/Logic Operators combine/manipulate Boolean values

23. Not ! ! : Not : Logical unary negation operator
Evaluates to opposite of input False  True and True False
Examples (assume age = 24, weight = 140)
Expression
Value
age > 18
True
!(age > 18)
False
weight == 150
!( weight == 150)

24. OR || || : OR : Logical Or operator
True if either or both operands is true
Examples (assume age = 24, weight = 140)
A || B
Value
Expression A
Expression B
(age > 34) || (weight <= 140)
True
age > 34
False
weight <= 140
(age > 34) || (weight >= 150)
weight >= 150

25. AND && && : AND : Logical And operator True if both operands are true
Examples (assume age = 24, weight = 140)
A && B
Value
Expression A
Expression B
(age > 18) && (weight <= 140)
True
age > 18
weight <= 140
(age > 18) && (weight >= 150)
False
weight >= 150

26. When to use:
Same code Expressed Twice!!!
Yuck:

27. When to use:
Better:
Conditional expresses logic for leap year in one expression

28. When NOT to use: Yuck: Same information Expressed Twice!!!
double score = 75;
if (score >= 90.0)
cout << "Grade is A";
if (score < 90.0 || score >= 80.0)
cout << "Grade is B";
if (score < 80.0 || score >= 70.0)
cout << "Grade is C";
if (score < 70.0 || score >= 60.0) double score = 75;
if (score < 90.0 && score >= 80.0)
if (score < 80.0 && score >= 70.0)
if (score < 70.0 && score >= 60.0)
cout << "Grade is D";
if (score < 60.0)
cout << "Grade is F";

29. When NOT to use: Better: Each boundary only defined once
double score = 75;
if (score >= 90.0)
cout << "Grade is A";
else if (score >= 80.0)
cout << "Grade is B";
else if (score >= 70.0)
cout << "Grade is C";
else if (score >= 60.0)
cout << "Grade is D";
else
cout << "Grade is F";

30. Booleans Nasty Details

31. Short Circuit
C++ only evaluates right half of logical statement if needed
Short Circuit evaluation
bool result = x > 5 && y == 2;
Not evaluated if x <= 5

32. Short Circuit Short circuit used to "guard" an operation:
bool result = fileExists && fileContains("assign")…
Don't read if doesn't exist

33. Short Circuit Short circuit can cause bugs:
Avoid side effects in second statement
bool result = x > 2 && y++ > 1;
x > 2, we add one to y
x <= 2, nothing happens to y

34. Precedence
What goes when?
Left to right in category

35. Parens Parentheses are GoodTM (true || true) && false

36. Parens Parentheses are GoodTM && before || without parens!
(true || true) && false
true || true && false
(true) && false
true || false
&& before || without parens!

37. Parens Parentheses are GoodTM Parens make desired ordering obvious
(true || true) && false
true || true && false
(true) && false
true || false
false
true
Parens make desired ordering obvious

38. Order of ops important! ( ) !
relational operators ==, <, >, etc…
&& II
Left to right in each category
if (!2 < 4)
if (!(2 < 4))
if ( false < 4 )
if ( !(true) )
if ( 0 < 4 )
if ( false )
if ( true )

39. Compound Warning
This does not work:
0 <= num <= 10
num = 20

40. Compound Warning Each half of compound conditional MUST stand alone:
NO!!!
if(60 <= grade < 90)
YES
if(grade <= 60 && grade < 60)

41. Logical Equivalencies
Many ways to write same thing
!( x > 5) x <= 5
Use easiest form
No: !( x != 5)
Yes: x == 5

42. DeMorgan's Can convert AND  OR Use to simplify logic:
Not both means not one or not the other !(x && y) == !x || !y
Not either means not one and not the other !(x || y) == !x && ! y
Use to simplify logic:
!(x != 5 || y > 2) !(x !=5) && !(y > 2) x == 5 && y <= 2