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 the 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 the 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 NestedIf01 { public static void main(String[] args) { char sex; int age, price = 0; Scanner in = new Scanner(System.in); // Construct Scanner object sex = in.next().charAt(0); // Read in next char into sex age = in.nextInt(); // 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 the 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 the then-part of the outer if-else-statement is an if-elsestatement, and the condition age <= 13 is satisfied, it will only execute the 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().charAt(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/ NestedIf01.java • How to run the program: • Right click on link and save in a scratch directory • To compile: javac NestedIf01.java • To run: java NestedIf01 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 Grade01 { 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.nextDouble(); // 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/ Grade01.java • How to run the program: • Right click on link and save in a scratch directory • To compile: javac Grade01.java • To run: java Grade01 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 Grade02 { public static void main(String[] args) { double ng; String lg = ""; Scanner in = new Scanner(System.in); // Construct Scanner object ng = in.nextDouble(); // 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) S1; are executed only when condition1 = true • The statement(s) S2; are executed only when (condition1 = false and condition2 = true) • The statement(s) S3; are executed only when (condition1 = false and condition2 = false) Each of the conditions are mutually excusive A three-way selection construct (cont.) • The 3-way selection written in Java: if ( conditionX ) { statements executed only when: conditionX = true } else { if ( conditionY ) --+ { | statements executed only when: | conditionX = false | and conditionY = true | } | One statement!! else | { | statements executed only when: | conditionX = false | and conditionY = 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 ( conditionX ) { statements executed only when: conditionX = true } else if ( conditionY ) { statements executed only when: conditionX = false and conditionY = true } else { statements executed only when: conditionX = false and conditionY = false } --+ | | | | | One statement!! | | | | | --+ 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 ( condition1 ) { S1; (one or more statements) } else if ( condition2 ) { S2; (one or more statements) } else if ( condition3 ) { S3; (one or more statements) } ... else if ( conditionN-1 ) { SN-1; (one or more statements) } else { SN; (one or more statements) } The N-way selection construct (cont.) • Notes: • S1 will be executed (only) when condition1 = true • S2 will be executed (only) when (condition1 = false and condition2 = true) • S3 will be executed (only) when (condition1 = false, condition2 = false and condition3 = 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 Grade03 { public static void main(String[] args) { double ng; String lg = ""; Scanner in = new Scanner(System.in); // Construct Scanner object ng = in.nextDouble(); // 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/ Grade03.java • How to run the program: • Right click on link and save in a scratch directory • To compile: javac Grade03.java • To run: java Grade03