1. Program Control Flow: Making Decision
CHAPTER 4B
Program Control Flow: Making Decision

2. Chapter GOAL:
To be able to implement decisions using if /else statements
To understand statement blocks
To learn how to apply relational and logical operator, (compare integers, floating-point numbers).
To develop strategies for processing input and handling errors
To understand the Boolean data type
To avoid infinite loops and off-by-one errors
To recognize the correct ordering of decisions in multiple branches
if/else-if
switch

3. 4.1 The if Statement Syntax 1: if statement if (condition) statement
Example: if (x >= 0) y = sqrt(x);
Purpose: Execute the statement if the condition is true.
Syntax 2: block statement {
statement1;
statement2; …
statementn; }
Example: {
double length = sqrt(area);
cout << area << “\n”;
Purpose: Group several statements into a block that can be controlled by another statement..

4. 4.1 The if Statement Usage: implement a decision
It has two parts: a test and a body. Ex:
Multiple statements can be grouped together in a block statement by enclosing them in braces { }:
area <0?
Print message
false
true
if (area < 0)
cout << “Error: Negative area. \n”;
if (area < 0) {
cout << “Error: Negative area. \n”;
return 1; }

5. 4.1 The if Statement
Example: If the area is negative, then print an error message and returns an error code (use return 1 in main()).
area1.cpp

6. 4.1 The if Statement
Exercise:: Write a program that reads in a student’s grade. Prints ‘Passed” if the student’s grade is higher or equal than 60.
print “Passed”
[grade < 60]
[grade >= 60]
If single-selection statement activity diagram

7. 4.2 The if/else Statement Syntax 3: if/else statement
if (condition) statement1 else statement2
Example: if (x >= 0) y = sqrt(x); else cout <<“Bad input\n”;
Purpose: Execute the statement if the condition is true, the second statement if the condition is false.

8. 4.2 The if/else Statement
The if/else consists of a condition of two alternatives.
The first condition is performed is the condition is true.
The second condition is performed if the condition is false.
The if/else statement is a better choice than a pair of if statements with complementary conditions.
area <0?
Compute & print root
Display error message
true
false

9. 4.2 The if/else Statement
area2.cpp

10. 4.2 The if/else Statement
Exercise:: Write a program that reads in a student’s grade. Prints ‘Passed” if the student’s grade is higher or equal than 60, but prints “Failed” if the student’s grade is less than 60.
print “Failed”
print “Passed”
[grade < 60]
[grade >= 60]
If …else double-selection statement activity diagram

11. 4.2 The if/else Statement
Exercise:: Write a program that prints all solutions to the quadratic equation ax2 + bx + c =0.
Read in a, b, c and use the quadratic formula. If the discriminant b2-4ac is negative, display a message stating that there are no solutions.
Test with these data: a b c x 1 3 2
-1, -2 5 25
-4.877, 6 9
-1, -0.5
No solution

12. 4.3 Relational Operators
C++ has six relational operators to implement conditions:

13. 4.3 Relational Operators Common error: confusing = and ==
Remember!!! Use == inside tests Use = outside tests
a = 5; /* assign 5 to a */
if (a == 5) /* test whether a equals to 5 */

14. 4.3 Relational & Logic Operator

15. 4.4 Boolean Operations:
Combine test conditions:
Reverse a value:
AND
&& OR
NOT ! ||
&& : combines several tests into new test that passes only when ALL the conditions are true?
|| : combines several conditions and succeeds if AT LEAST ONE of the conditions is true.
! : returns its opposite

16. 4.4 Boolean Operations:
The && and || operators are computed using lazy evaluation.
The expressions are evaluated from left to right, and evaluation stops as soon as the truth value is determined.

17. 4.4 Boolean Operations: Test yourself if ( (2 < x) && (x < 7) )
x = 2 False
x = 5 True
x = 10 False
if ( ( x = = 1) | | ( x = = y) )
x = -1, y = -1 True
x = 1, y = 1 True
x = y True
!( x < y)
x = 2, y = 4 False
x = 5, y = -2 True
x = 1, y = 1 True
!(x = = y)
x = 2, y = 4 True
x = 1, y = 1 False

18. 4.4 Relational & Logic Operator

19. EXERCISE
Write a program that survey friendship by two questions. Display a conclusion message as below:
Question
Answer
Would you encourage your friends to study? Y N
Would you help your friend to cheat in exam?
Output message
You are not a true friend!
You are an angel!
You should be a more concerned friend
You are a devil!

20. 4.5 Input Validation An important application for the if statement
Purpose: verifies the user has given reasonable input.
Example: user types "five" and hits return! (Causing cin to fail).
GOOD PRACTICE: After every input, test for the failed state: The actual input should also be validated, even if type is correct.
double area;
cin >> area;
if (cin.fail()) {
cout << “Error: Bad input\n”;
return 1; }
if (area < 0)

21. 4.5 Input Validation
area3.cpp

22. 4.5 Input Validation Alternative: if (cin)
/* the stream did not fail */
else
/* the stream failed */

23. Modify your program in [2. 7] (Chapter 2 slide no
Modify your program in [2.7] (Chapter 2 slide no. 45) so that the program could be able to validate user input. If input such as “fifty” is inserted, the program display “Bad inputs” and ends the program.

24. 4.6 Multiple Alternatives:
if/else-if statement (conditional structure)
switch statement (selective structure)

25. 4.6 Multiple Alternatives:
if/else-if statement
Distinguish between multiple alternatives by using collections of if/else statements
Syntax:
if (condition1)
statement1;
else if (condition2)
statement2; …
else
statementN; 1 2 3 4 5 6 7

26. 4.6 Multiple Alternatives:
if/else-if statement
Example: Telling if the value currently stored in x is positive, negative or none of them (i.e. zero):
if (x > 0)
cout << "x is positive";
else if (x < 0)
cout << "x is negative";
else
cout << "x is 0"; 1 2 3 4 5 6

27. 4.6 Multiple Alternatives:
if/else-if statement
if/else-if ensure the alternatives are exclusive. → remaining else-if statement will be ignored once a condition is fulfilled.
Independent if statements may cause a single input to print several messages.
Consider the example:
Month
Category
1-3
Winter
4-6
Spring
7-9
Summer
10-12
Autumn 
winter: december to february spring: march to may summer: june to august fall: september to november

28. 4.6 Multiple Alternatives:
Comparison:
if/else-if
Independent if
if (month < 0)
cout << “Invalid Input";
else if (month <= 3)
cout << “Winter";
else if (month <= 6)
cout << “Spring";
else if (month <= 9)
cout << “Summer";
else if (month <= 12)
cout << “Autumn";
else 1 2 3 4 5 6 7 8 9 10 11 12
if (month < 0)
cout << “Invalid Input";
if (month <= 3)
cout << “Winter";
if (month <= 6)
cout << “Spring";
if (month <= 9)
cout << “Summer";
if (month <= 12)
cout << “Autumn";
if (month > 12) 1 2 3 4 5 6 7 8 9 10 11 12
What is the output if month is 6?

29. 4.6 Multiple Alternatives:
if/else-if statement
Since the alternatives are exclusive, therefore order of the condition is important.
Considering the previous example with the conditions reversed:
if (month <= 12)
cout << “Autumn ";
else if (month <= 9)
cout << “Summer ";
else if (month <= 6)
cout << “Spring";
else if (month <= 3)
cout << “Winter ";
else if (month < 0)
cout << “Invalid Input";
else 1 2 3 4 5 6 7 8 9 10 11 12
What is the output if month is 6?

30. 4.6 Multiple Alternatives:
if/else-if statement
Example:
(order not important)
The user enters the name of a coin, and the program returns the value.
Program output: (bold – user input) 1¢ 5¢
Enter coin name: penny
Penny equal to $0.01
nickel
penny
Enter coin name: penn
Invalid Input!!!
10¢
25¢
quarter
dime

31. Case 1: Solution
Order not important!!
coin5.cpp

32. 4.6 Multiple Alternatives:
if/else-if statement
Example:
(order is important)
Develop a program that prompt the user for the value of magnitude on the Richter scale and displays a description of likely impact of an earthquake based on its magnitude on the Richter scale.
Note that the order of the tests ensures that the right results are printed
Program output: (bold – user input)
Enter a magnitude on the Richter scale: 6.5
Many buildings considerably damaged, some collapse.

33. 4.6 Multiple Alternatives:
Richter Scale Information:
Information:
Richter scale is a measurement for the strength of an earth quake. Every step in the scale, for example , signifies a tenfold increase in the strength of the quake.
In 2004 Indian Ocean earthquake which kills more than 230,000 casualties rated 9.3 on the Richter scale.
Magnitude
Earthquake effects
≥ 8.0
Most structures falls
7.0–7.9
Many buildings destroyed
6.0–6.9
Many buildings considerably damaged, some collapse
4.5–5.9
Damage to poorly constructed buildings
3.5–4.4
Felt by many people, no destruction
0–3.3
Generally not felt by people

34. Case 2: (order is important)
richter.cpp

35. Case 2: (order is important)
All +ve values fall into the 1st case!!!

36. 4.6 Multiple Alternatives:
if/else-if statement
Exercise:
Create a program that asks the user to key in any number which is within 1 to 9. The user inputs ‘1’ , for instance, and the program prints the English word “one”.
Program output: (bold – user input)
Enter a number (1-9): 2
Two
Enter a number (1-9): -1
Invalid number
Enter a number (1-9): 100
Invalid number

37. 4.6 Multiple Alternatives:
if/else-if statement
Exercise:
Write a program to convert scores to their respective grade according to the below table:
Score
Grade
≥90 ( ) A
≥80 ( ) B
≥70 ( ) C
≥60 ( ) D
<60 (0 - 59)
Fail
Program output: (bold – user input)
Enter your test score(0-100): 75
Your grade is C

38. 4.6 Multiple Alternatives:
if/else-if statement
Exercise:
Create a program that reads the user’s age and then prints “You are a child.” if age < 18, “You are an adult.” if 18 ≤ age < 65, “You are a senior citizen.” if age ≥ 65
Program output: (bold – user input)
Enter your age: 44
You are an adult

39. 4.6 Multiple Alternatives:
switch statement
Compares a single integer value against several constant alternatives → like a sequence of if/else-if
Syntax:
switch (expression) {
case constant1:
group of statement1;
break;
case constant2:
group of statement2;
...
default:
default group of statements; }

40. 4.6 Multiple Alternatives:
switch statement
Example 1:
Test cases must be constant
Each branch terminated by break
break:
loop is immediately terminated, and program control resumes at the next statement following the loop
Try remove the break?

41. 4.6 Multiple Alternatives:
switch statement
Example 2:

42. 4.6 Multiple Alternatives:
switch statement
Exercise:
Create a program that simulates a simple calculator. It reads two integers and a character representing the operation.
If the character is “ + ”, the sum is printed; if “ - ”, the difference is printed; if “ * ”, the product is printed; if “ / ”, the quotient is printed; and if it is a %, the remainder is printed.
Use a switch statement. Hint: The character should be the control variable for the switch statement.
Program output: (bold – user input)
Enter two integers: 30 13
Enter an operator (+, -, *, /, %): %
Answer: 30 % 13 = 4

43. 4.7 Nested Branches:
Nested if/else statements can be used when there are two (or more) levels of decision making.
Example: Taxes
There is a different table for each marital status (decision 1).
Once you have found the right table, your are taxed differently according to your income (decision 2).

44. 4.7 Nested Branches:
The two-level decision process is reflected in two levels of if statements.
We say the income test is nested inside the test for filing status

45. What is the program code?
4.7 Nested Branches:
What is the program code? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
if (marital_status == "s") {
if (income <= SINGLE_LEVEL1)
tax = RATE1 * income;
else if (income <= SINGLE_LEVEL2)
tax = SINGLE_TAX1
+ RATE2 * (income - SINGLE_LEVEL1);
else
tax = SINGLE_TAX2
+ RATE3 * (income - SINGLE_LEVEL2); }
if (income <= MARRIED_LEVEL1)
else if (income <= MARRIED_LEVEL2)
tax = MARRIED_TAX1
+ RATE2 * (income - MARRIED_LEVEL1);
tax = MARRIED_TAX2
+ RATE3 * (income - MARRIED_LEVEL2);

46. 4.7 Nested Branches: else always belongs to the closest if
COMMON ERROR: The Dangling else problem
Example: Postage Rate (Domestic, International)
Peninsular
Penang
International
RM 5.00
RM 10.00
RM 20.00
double shipping_charge = 5.00;
if(country == "MALAYSIA")//RM5 inside continental Malaysia
if (state == "Penang") //Penang is more expensive
shipping_charge = 10.00;
else //Pitfall
shipping_charge = 20.00; //RM20 outside continental
else always belongs to the closest if

47. The following algorithm yields the season (Spring, Summer, Fall, or Winter) for a given month and day.
Write a program that prompts the user for a month and day and then prints the season, as determined by this algorithm.

48. WAP to print sum of four integers which are key in by the user.
Key in four integers: 3 4 -5 10
Sum is 12
WAP to print maximum integer among four integers which are key in by the user.
Key in four integers: 3 4 -5 10
Max is 10