Nested conditional statements

Previously discussed Conditional statements discussed so far: Syntax of the if-statement: if-statement if-else-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: Therefore: A conditional statement (i.e., an if-statement or an if- else-statement) is also a statement 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 if- else-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-else- statement, 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-else- statements 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: 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: 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 = 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 else- part 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: Each of the conditions are mutually excusive 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)

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 | } | One statement!! else | { | statements executed only when: | conditionX = false | and conditionY = 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 ) { S N-1 ; (one or more statements) } else { S N ; (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 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: 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