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Nested conditional statements

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• Previously discussed
  • Conditional statements discussed so far:

    if-statement if-else-statement

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  • Syntax of the if-statement:

    if ( CONDITION ) ONE-statement

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  • Syntax of the if-else-statement:

    if ( CONDITION ) ONE-statement else ONE-statement

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• Nested conditional statements
  • Fact:

    A conditional statement (i.e., an if-statement or an if-else-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 !!!

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  • Nested conditional statement:

    Nested conditional statement = a conditional statement where the then-part and/or the else-part contains another conditional statement

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  • 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

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• Programming example: determine the price for a hair cut                            (board)
  • 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

    Write a program that:

    Reads in the sex and the age Prints the price of the hair cut.

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  • Algorithm:

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  • 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 } 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); } }

    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 } 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; Graphically: path taken by program when input is sex = 'M' (male) and age = 11 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) Another example: path taken by program when input is sex = 'F' (female) and age = 25

    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

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac NestedIf01.java To run:          java NestedIf01

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• 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

    Write a program that:

    Reads in a number grade Prints the letter grade.

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  • Algorithm using non-nested if-statements:

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  • 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 System.out.print("Enter number 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); } }

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac Grade01.java To run:          java Grade01

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  • Short-coming of the algorithm:

    It is inefficient Because the algorithm will always perform 5 individual tests (in the 5 different if-statements 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 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

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

    Explanation:

    The first if-else statement breaks the cases into: ng ≥ 90 and ng < 90: If ng ≥ 90, we can assign lg = "A"; No further tests will be performed !!! (I.e.: much faster !!!) Otherwise, the execution will enter the else-part 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: 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 No further tests will be performed !!! (I.e.: again, much faster !!!) 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...

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  • 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 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); } }

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

  • 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:

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• 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.

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  • A common way to ensure that the 3 choices are mutually exclusive is as follows:

  • The 3-way selection construct can be implemented using the following nested if-else statements:

    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 exclusive

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  • 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 | } --+ }

    Since there is one statement in the else-part, we do not need to use a block

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  • 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 | } --+

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

    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) }

    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...

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  • 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 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); } }

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac Grade03.java To run:          java Grade03