Nested conditional statements Previously discussed Conditional statements discussed

Nested conditional statements

Previously discussed • Conditional statements discussed so far: • if-statement • if-else-statement • Syntax of the if-statement: if ( CONDITION ) ONE-statement

Previously discussed (cont. ) • Syntax of the if-else-statement: if ( CONDITION ) ONE-statement else ONE-statement

Nested conditional statements • Fact: • A conditional statement (i. e. , an if-statement or an ifelse-statement) is also a statement • Therefore: • We can use an if-statement or an if-else-statement in then-part (and in the else-part) of a conditional statement !!!

Nested conditional statements (cont. ) • Nested conditional statement: • Nested conditional statement = a conditional statement where then-part and/or the else-part contains another conditional statement

Nested conditional statements (cont. ) • Note: • You can nest conditional statement arbitrarily deep • Obviously, deeper nesting makes the program difficult to understand • You should: • Use nested conditional statements only when necessary • Try not to nest conditional statements too deeply

Programming example: determine the price for a hair cut • Hair cut pricing of a saloon: • Male customer: • Boys (age ≤ 13): $10 • Men (age > 13): $15 • Female customer: • Girls (age ≤ 13): $12 • Women (age > 13): $25

Programming example: determine the price for a hair cut (cont. ) • Write a program that: • Reads in the sex and the age • Prints the price of the hair cut.

Programming example: determine the price for a hair cut (cont. ) • Algorithm:

Programming example: determine the price for a hair cut (cont. ) • Java program: public class Nested. If 01 { public static void main(String[] args) { char sex; int age, price = 0; Scanner in = new Scanner(System. in); // Construct Scanner object sex = in. next(). char. At(0); // Read in next char into sex age = in. next. Int(); // Read in next integer into age if ( sex == 'M' ) { // Then-part of outer if-else statement if ( age <= 13 ) { price = 10; // Case: boy <-- then-part of inner if-else }

Programming example: determine the price for a hair cut (cont. ) else { price = 15; // Case: man <-- else-part of inner if-else } } else { // Else-part of outer if-else statement if ( age <= 13 ) { price = 12; // Case: girl <-- then-part of inner if-else } else { price = 25; // Case: woman <-- else-part of inner if-else } } System. out. println("Price = " + price); } }

Programming example: determine the price for a hair cut (cont. ) • Explanation: • Suppose the user enters: sex = 'M' and age = 11 • Then the condition of the outer if-else-statement is satisfied and executes only then-part of the outer ifelse-statement: // Then-part of outer if-else statement if ( age <= 13 ) { price = 10; // Case: boy <-- then-part of inner if-else } else { price = 15; // Case: man <-- else-part of inner if-else }

Programming example: determine the price for a hair cut (cont. ) • Since then-part of the outer if-else-statement is an if-elsestatement, and the condition age <= 13 is satisfied, it will only execute then-part: price = 10;

Programming example: determine the price for a hair cut (cont. ) • Graphically: path taken by program when input is sex = 'M' (male) and age = 11

Programming example: determine the price for a hair cut (cont. ) Notice that only the statement(s) in then-part of the inner and outer if-else-statements are executed (because the conditions in the inner and outer if-elsestatements are satisfied)

Programming example: determine the price for a hair cut (cont. ) • Another example: path taken by program when input is sex = 'F' (female) and age = 25

Programming example: determine the price for a hair cut (cont. ) • Note: • The expression in. next() reads in a string from the keyboard • The expression in. next(). char. At(0) returns the first character in a string from read from the keyboard

Programming example: determine the price for a hair cut (cont. ) • Example Program: (Demo above code) – Prog file: http: //mathcs. emory. edu/~cheung/Courses/170/Syllabus/06/Progs/ Nested. If 01. java • How to run the program: • Right click on link and save in a scratch directory • To compile: javac Nested. If 01. java • To run: java Nested. If 01

Programming example: assign a letter grade to a number grade • Letter grade assignment: • grade ≥ 90: A • 80 ≤ grade < 90: B • 70 ≤ grade < 80: C • 60 ≤ grade < 70: D • grade < 60: F

Programming example: assign a letter grade to a number grade (cont. ) • Write a program that: • Reads in a number grade • Prints the letter grade.

Programming example: assign a letter grade to a number grade (cont. ) • Algorithm using non-nested if-statements:

Programming example: assign a letter grade to a number grade (cont. ) • Java program: import java. util. Scanner; public class Grade 01 { public static void main(String[] args) { double ng; String lg = ""; Scanner in = new Scanner(System. in); // Construct Scanner object

Programming example: assign a letter grade to a number grade (cont. ) System. out. print("Enter numbger grade: "); ng = in. next. Double(); // Read in next number into ng if ( ng >= 90 ) lg = "A"; if ( 80 <= ng && ng < 90 ) lg = "B"; if ( 70 <= ng && ng < 80 ) lg = "C"; if ( 60 <= ng && ng < 70 ) lg = "D"; if ( ng < 60 ) lg = "F"; System. out. println("Letter grade = " + lg); } }

Programming example: assign a letter grade to a number grade (cont. ) • Example Program: (Demo above code) – Prog file: http: //mathcs. emory. edu/~cheung/Courses/170/Syllabus/06/Progs/ Grade 01. java • How to run the program: • Right click on link and save in a scratch directory • To compile: javac Grade 01. java • To run: java Grade 01

Programming example: assign a letter grade to a number grade (cont. ) • Short-coming of the algorithm: • It is inefficient Because the algorithm will always perform 5 individual tests (in the 5 different if-statements

Programming example: assign a letter grade to a number grade (cont. ) • Fact: • The different cases are mutually disjoint E. g. , when the first condition passes (i. e. , grade ≥ 90), then the other 4 subsequent tests will all fail

Programming example: assign a letter grade to a number grade (cont. ) The algorithm will still have to go through these "useless" tests to reach the print statement • We can make use of the if-else-statement to shorten the execution

Improved algorithm to find numeric grade • Improved algorithm to assign a letter grade:

Improved algorithm to find numeric grade (cont. ) • Explanation: • The first if-else statement breaks the cases into: ng ≥ 90 and ng < 90:

Improved algorithm to find numeric grade (cont. ) If ng ≥ 90, we can assign lg = "A"; Otherwise, the execution will enter the else-part

Improved algorithm to find numeric grade (cont. ) • The second if-else statement knows that ng < 90 !!! It uses the test ng >= 80 to break the cases into: 80 ≤ ng < 90 and ng < 80:

Improved algorithm to find numeric grade (cont. ) Because we know that ng < 90 when we are inside the elsepart of the first if-else statement, when ng ≥ 80, we can assign lg = "B"; because 80 < ng ≤ 90 Otherwise, the execution will enter the else-part of the second if-else statement Inside the else-part of the second if-else statement, we know (for sure) that ng < 80. • The rest of the algorithm follows the same reasoning process. . .

Improved algorithm to find numeric grade (cont. ) • Program in Java: (I omitted the import statement for brevity) public class Grade 02 { public static void main(String[] args) { double ng; String lg = ""; Scanner in = new Scanner(System. in); // Construct Scanner object ng = in. next. Double(); // Read in next number into ng

Improved algorithm to find numeric grade (cont. ) if ( ng >= 90 ) lg = "A"; else { if ( ng >= 80 ) lg = "B"; else { if ( ng >= 70 ) lg = "C"; else { if ( ng >= 60 ) lg = "D"; else lg = "F"; } } } System. out. println("Letter grade = " + lg); } }

Two-way selection • A two-way selection is a choice between 2 mutually exclusive cases:

Two-way selection (cont. ) • The if-else-statement is known as a two-way selection statement because it can use to make a choice between 2 mutually exclusive cases:

A three-way selection construct • A two-way selection is a choice between 3 mutually exclusive cases: The choices of A, B and C are mutually exclusive.

A three-way selection construct (cont. ) • A common way to ensure that the 3 choices are mutually exclusive is as follows:

A three-way selection construct (cont. ) • The 3 -way selection construct can be implemented using the following nested if-else statements:

A three-way selection construct (cont. ) • Notes: • The statement(s) S 1; are executed only when condition 1 = true • The statement(s) S 2; are executed only when (condition 1 = false and condition 2 = true) • The statement(s) S 3; are executed only when (condition 1 = false and condition 2 = false) Each of the conditions are mutually excusive

A three-way selection construct (cont. ) • The 3 -way selection written in Java: if ( condition. X ) { statements executed only when: condition. X = true } else { if ( condition. Y ) --+ { | statements executed only when: | condition. X = false | and condition. Y = true | } | One statement!! else | { | statements executed only when: | condition. X = false | and condition. Y = false | } --+ }

A three-way selection construct (cont. ) • Since there is one statement in the else-part, we do not need to use a block

A three-way selection construct (cont. ) • The popular way to write a 3 -way selection in Java: if ( condition. X ) { statements executed only when: condition. X = true } else if ( condition. Y ) --+ { | statements executed only when: | condition. X = false | and condition. Y = true | } | One statement!! else | { | statements executed only when: | condition. X = false | and condition. Y = false | } --+

The N-way selection construct • The 3 -way selection construct can be generalized into an N -way selection

The N-way selection construct (cont. ) A N-way selection construct looks like the following: if ( condition 1 ) { S 1; (one or more statements) } else if ( condition 2 ) { S 2; (one or more statements) } else if ( condition 3 ) { S 3; (one or more statements) } . . . else if ( condition. N-1 ) { SN-1; (one or more statements) } else { SN; (one or more statements) }

The N-way selection construct (cont. ) • Notes: • S 1 will be executed (only) when condition 1 = true • S 2 will be executed (only) when (condition 1 = false and condition 2 = true) • S 3 will be executed (only) when (condition 1 = false, condition 2 = false and condition 3 = true) • And so on. . .

The N-way selection construct (cont. ) • Example: the number grade program can be written using an N-way selection public class Grade 03 { public static void main(String[] args) { double ng; String lg = ""; Scanner in = new Scanner(System. in); // Construct Scanner object ng = in. next. Double(); // Read in next number into ng

The N-way selection construct (cont. ) if ( ng >= 90 ) { lg = "A"; } else if ( ng >= 80 ) { lg = "B"; } else if ( ng >= 70 ) { lg = "C"; } else if ( ng >= 60 ) { lg = "D"; } else { lg = "F"; } System. out. println("Letter grade = " + lg); } }

The N-way selection construct (cont. ) • Example Program: (Demo above code) – Prog file: http: //mathcs. emory. edu/~cheung/Courses/170/Syllabus/06/Progs/ Grade 03. java • How to run the program: • Right click on link and save in a scratch directory • To compile: javac Grade 03. java • To run: java Grade 03