Selection in C

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

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

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”.

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

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

Translate the logic into C code:

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?

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:

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:

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

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

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

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) …

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); …

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

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

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

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

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!”);

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)

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

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