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The if-statement

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• The if-statement in Java
  • What is an if-statement:

    The if-statement is a conditional statement The statement is executed only if a given condition is satisfied Example: if ( condition is true ) then execute this statement;

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  • Purpose of an if-statement:

    An if-statement makes sure that the correct condition is satisfied before a certain action is performed

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• Syntax and meaning of the if-statement
  • Syntax of the if-statement:

    if ( CONDITION ) ONE-statement

    Explanation:

    The keyword if announces (to the Java compiler) that we started an if-statement A conditional clause ( CONDITION ) follows the keyword if This is the condition of the if-statement Following the condition clause, you can write (only) one statement This statement will only be executed if the condition is true

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• The condition clause of the if-statement
  • The condition clause of the if-statement is an expression that evaluates to true or false

  • Expressions that evaluates to true or false are known in Computer Science as:

    Boolean expressions

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  • We will learn more about Boolean expressions later

    Here, we will first use a few simple Boolean expressions to learn about the if-statement

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  • A simple Boolean expression:

    The expression a < 0 will return true if the variable a contains a negative value false otherwise (i.e., if the variable a contains zero or a positive value)

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• Example if-statement: find absolute value
  • Consider the following Java program:

    import java.util.Scanner; public class If1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); int a; System.out.print("Enter an integer value: "); a = in.nextInt(); // Read in an integer from keyboard System.out.print("Input value = "); System.out.println(a); if ( a < 0 ) // If-statement a = -a; // Negate a ONLY if a < 0 System.out.print("It's absolute value = "); System.out.println(a); } }

    Example outputs:

    cheung@HOME2(40)> java If1 Enter an integer value: -4 Input value = -4 It's absolute value = 4 cheung@HOME2(41)> java If1 Enter an integer value: 4 Input value = 4 It's absolute value = 4

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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 If1.java To run:          java If1

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• How is the if-statement executed ?
  • Instead of using words, we will use 2 representation techniques to illustrate how an if-statement is executed

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  • Representation techniques used:

    Flow chart: A flow chart is a diagram that represents a computer algorithm. It uses a rectangular box to represent an assignment statement It uses a diamond box to represent a condition Lines are used to indicate the flow of the steps in the algorithm Structure diagram A structure diagram is another type of diagram to represent a computer algorithm. It uses different shapes of boxes to represent assignment statements and conditions The structure diagram does not use lines

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• Flow chart of if-statement
  • Pseudo code of the program:

    a = read input from keyboard; if ( a < 0 ) a = -a; Print a;

  • Flow chart of the program:

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  • If a negative number (e.g., -4) is entered in the input, the program flow will be as follows:

    Explanation:

    Because the condition (test) a < 0 is satisfied, the statement a = −a; is executed a is updated to −a      (in example, with 4) The execution continues with the subsequent statement (which prints a) The value 4 is printed.

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  • If a non-negative number (e.g., 4) is entered in the input, the program flow will be as follows:

    Explanation:

    Because the condition (test) a < 0 is not satisfied, the statement a = −a; is not executed a retains the value 4 !!! The execution continues with the subsequent statement (which prints a) The value 4 is printed.

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• Structure diagram of if-statement
  • Pseudo code of the program:

    a = read input from keyboard; if ( a < 0 ) a = -a; Print a;

  • Structure diagram of the program:

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  • If a negative number (e.g., -4) is entered in the input, the program flow will be as follows:

    Explanation:

    Because the condition (test) a < 0 is satisfied, the statement a = −a; is executed a is updated to −a      (in example, with 4) The execution continues with the subsequent statement (which prints a) The value 4 is printed.

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  • If a non-negative number (e.g., 4) is entered in the input, the program flow will be as follows:

    Explanation:

    Because the condition (test) a < 0 is not satisfied, the statement a = −a; is not executed a retains the value 4 !!! The execution continues with the subsequent statement (which prints a) The value 4 is printed.

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  • Note:

    I will use the structure diagram technique to illustrate some of the algorithms discussed later in this course.

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• Computer jargon
  • Terminology:

    If-condition = the condition clause in the if-statement Then-part = the statement(s) that are executed when the if-condition is true Schematically:

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• Multiple statements in the "then"-part of the if-statement
  • Previously discussed: syntax of the if-statement:

    if ( CONDITION ) ONE-statement

    Notes:

    You can only use one statement in the then-part Fact: It is very common to use multiple statements in the then-part of an if-statement.

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  • Previously discussed: block ( click here)

    Block: Block = a pair of "{" and "}" braces that groups components in a Java program together Example:

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  • Statement block: multiple statements considered as one statement:

    Statement block: Statement block = multiple statements grouped together by braces { .... } Example: Java language fact: A statement block in Java is considered as one (single) statement Statement block are commonly used as the then-body of if-statements

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• Example if-statement: using a statement block in the then-body

  • Previously discussed: Java program that implements the a,b,c-formula

    import java.util.Scanner; // Import Scanner class (contains methods // for reading keyboard input) public class Abc2 { public static void main(String[] args) { double a, b, c, x1, x2; // Define 5 variable Scanner in = new Scanner(System.in); // Construct Scanner object a = in.nextDouble(); // Read in next number into a b = in.nextDouble(); // Read in next number into b c = in.nextDouble(); // Read in next number into c x1 = ( -b - Math.sqrt( b*b - 4*a*c ) ) / (2*a); x2 = ( -b + Math.sqrt( b*b - 4*a*c ) ) / (2*a); System.out.print("x1 = "); System.out.println(x1); System.out.print("x2 = "); System.out.println(x2); } }

    Shortcoming:

    The program does not work when b*b - 4*a*c < 0

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  • Solution:

    We will compute the solution only if   b*b - 4*a*c ≥ 0

    Schematically:

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  • Java program:

    import java.util.Scanner; public class If2 { public static void main(String[] args) { double a, b, c, x1, x2; // Define 5 variable Scanner in = new Scanner(System.in); // Construct Scanner object a = in.nextDouble(); // Read in next number into a b = in.nextDouble(); // Read in next number into b c = in.nextDouble(); // Read in next number into c x1 = x2 = 0; // Default solution /* ------------------------------------------------------ Only compute solution if determinant b*b - 4*a*c >= 0 ------------------------------------------------------ */ if ( b*b - 4*a*c >= 0 ) { x1 = ( -b - Math.sqrt( b*b - 4*a*c ) ) / (2*a); x2 = ( -b + Math.sqrt( b*b - 4*a*c ) ) / (2*a); } System.out.print("x1 = "); System.out.println(x1); System.out.print("x2 = "); System.out.println(x2); } }

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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 If2.java To run:          java If2

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• Some commonly made mistakes in if-statements
  • Common error 1: bogus semicolon after the if-condition

    Example:

    import java.util.Scanner; public class If3 { public static void main(String[] args) { Scanner in = new Scanner(System.in); int a; System.out.print("Enter an integer value: "); a = in.nextInt(); // Read in an integer from keyboard System.out.print("Input value = "); System.out.println(a); if ( a < 0 ) ; // Bogus ; a = -a; System.out.print("It's absolute value = "); System.out.println(a); } }

    Structure diagram of this erroneous program:

    Notes:

    The then-part contains no statement !! The statement a = -a; is not part of the then-part !!! Therefore, the statement a = -a; is always executed.

    Example output:

    Enter an integer value: 4 Input value = 4 It's absolute value = -4 (Because a = -a; is always executed)

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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 If3.java To run:          java If3

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  • Common error 2: forgetting to use statement block in the then-part

    Example:

    import java.util.Scanner; public class If2 { public static void main(String[] args) { double a, b, c, x1, x2; // Define 5 variable Scanner in = new Scanner(System.in); // Construct Scanner object a = in.nextDouble(); // Read in next number into a b = in.nextDouble(); // Read in next number into b c = in.nextDouble(); // Read in next number into c x1 = x2 = 0; // Default solution /* ------------------------------------------------------ Only compute solution if determinant b*b - 4*a*c >= 0 ------------------------------------------------------ */ if ( b*b - 4*a*c >= 0 ) x1 = ( -b - Math.sqrt( b*b - 4*a*c ) ) / (2*a); x2 = ( -b + Math.sqrt( b*b - 4*a*c ) ) / (2*a); System.out.print("x1 = "); System.out.println(x1); System.out.print("x2 = "); System.out.println(x2); } }

    Structure diagram of this erroneous program:

    Notes:

    The then-part contains only the first of the 2 statements !! (Remember: the syntax rule says that the then-part consists of only one statement !!) The second assignment statement is not part of the then-part !!! (Therefore, the second assignment statement is always executed - and it will cause an error)

    Example output: (use: a = 1, b = 1, c = 1)

    x1 = 0.0 x2 = NaN <----- x2 was computed with sqrt(neg. value)

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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 If4.java To run:          java If4