1. Selection in C

2. If Statement: if ( condition) statement(s); One way decision:
Referred to as “Open Branch” in the book.
Syntax: Flowchart:
if ( condition)
statement(s);
Condition?
true
Statement(s)
false
Rest of the program

3. If / else statement: 2-way decision: Syntax: Flowchart: if (condition)
statement(s);
else
Condition?
false
true
Statement(s)
Statement(s)
Rest of the program

4. Exercise: Write a C program to..
Ask the user to enter his/her age.
Check the age, if it is > 21, display “ You may drink.”, otherwise, display “Do not drink”
regardless of the age, Display “Do not Drink and Drive”.

5. Explain the solution, using flowchart:
Start
Declare variable: age
Get user’s age
Age > 21?
false
true
May not drink
You may drink
Don’t drink & drive
end

6. Explain the solution using pseudo code:
Declare variable to store age.
Prompt the user to enter age,
Read the entered value into variable
If age is > 21 then
Display you may drink
Otherwise
Display you may not drink
5. Display: Do not drink and drive

7. Translate the logic into C code:

8. Nested If Statement:
An if statement can be nested in the if part or the else part of the if statement.
if (condition) ….
else …
How can you tell where an if statement begins or ends?

9. Ex1: Assume that: int a = 5, b = 7, c = 10;
Rule: else binds with the closest if
Ex1: Assume that: int a = 5, b = 7, c = 10;
if (a < b)
if (c <= a + b)
printf( " First place \n“);
else
printf( "2nd place \n“);
printf (“3rd place \n”);
Output:

10. Dangling else: Determine which if the else binds with?
Ex2: Assume: a = 12, b = 2, c = 10
if (a < b)
if (c <= a + b)
printf( " First place \n“);
else
printf( "2nd place \n“);
Output:

11. Ex: What is the outcome? float score = 75; float gpa = 2.5;
if (score > 80)
if (gpa > 2.0)
printf (“xxx”);
else
printf(“ ###”);

12. If / else if statement:
The else part of an if statement can be another if statement.
if (condition) …
else ….

13. Improved style  more clarity
Such nested if / else can be written as:
if (condition) …
else if (condition) ….
else

14. Example: Examine the student’s test score and assign a letter grade:
int score = …
char letter;
if (score >= 90) {
letter = ‘A’;
printf(“ Great job! \n”); }
else if (score >= 80)
letter = ‘B’;
else
if (score >= 70) …

15. Rewrite the code with improved style:
if (score >= 90) {
letter = ‘A’;
printf(“ Great job! \n”); }
else if (score >= 80)
letter = ‘B’;
else if (score >= 70)
letter = ‘C’;
else if (score >= 60)
letter = ‘D’;
else
letter = ‘F’;
Printf(“ Your letter grade is: %c”, letter); …

16. More complex conditions:
You may combine 2 or more conditions using logical operators:
Logical Operators:
Logical and: &&
C1 (condition1)
C2 (condition2)
C1 && C2
True
False

17. Application: Check the range of the data:
Ex: A healthy heart rate is between 60 and 85. Check the user’s heart rate, and inform the user whether s/he is healthy or not
int rate;
printf (“ Enter your heart rate: “);
Scanf(“%d”, &rate);
if ((rate >= 60) && (rate <= 85) ) …
else

18. Logical or: ||
C1 (condition1)
C2 (condition2)
C1 || C2
True
False

19. Applications: 1) Checking for invalid rage:
Ex1: Check the range of the test score. If it is outside the range of end the program:
int test;
test = 77;
if ( (test < 0) || (test > 100) ) {
printf (“ Invalid test score..”);
printf (“ Program will end now.”);
return 1; }

20. Cont…
Ex2: Assume that a company is considering a promotion for employees older than 60, or those who are paid under $20,000. Assume variables: salary and age are declared with some values stored in them.
If ( (salary < 20000) || (age > 60) )
printf (“ Give promotion..”);
else
printf (“ Does not qualify!”);

21. Cont… Logical not ! Operates on one condition.
It negates the value of the condition.
Ex1: ! (120 < 100) 
Ex2: int veteran = 1;
!veteran 
Notice:
! (a < b) is the same as (a >= b)
! (a == b) is the same as (a != b)

22. Precedence Rule: (..) Unary minus, ! * / % + -
* / %
+ -
<, <=, >, >=
==, !=
&& ||
= assignment operator

23. Evaluate the following expressions:
Assume int a = 5, b = 10, c = 15, flag = 1;
a < b && b < c
flag && b > 10
!(b <= 12) && (a %2 == 0)
!flag || c > a
int answer;
answer = (a > b) || (c > b);
Printf (“%d”,answer);
C >= a + b