Nested conditional statements. Previously discussed Conditional statements discussed so far: Syntax...
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Transcript of Nested conditional statements. Previously discussed Conditional statements discussed so far: Syntax...
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:
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 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 ( 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