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Assignment statements using the same variable in LHS and RHS

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• Previously discussed
  • Assignment statement:

    The assignment statement in the Java programming language instructs the computer to update the value stored in a variable

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  • Syntax of an assignment statement:

    VariableName = Expression ;

  • Meaning of the assignment statement:

    The expression on the RHS of the "=" operator is first computed The computed value is then assigned to the receiving variable

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• Assignment statements with the same variable on the LHS and RHS
  • Consider the following program:

    public class Assign01 { public static void main(String[] args) { double x = 1.0; // x = 1.0 System.out.print("Before: x = "); System.out.println(x); x = x + 4.0; // x is used in the LHS and RHS System.out.print("After: x = "); System.out.println(x); } }

    Explanation:

    You need to realize that the symbol "=" does not represent an equality relationship The symbol "=" denotes an assignment operation: The computer will first evaluate the LHS "x + 4.0": LHS = x + 4.0 (x contains 1.0) = 1.0 + 4.0 = 5.0 Then the result 5.0 is assigned to the receiving variable x: x = 5.0; Therefore, after executing the statement "x = x + 4.0", the variable x contains the value 5.0

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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 Assign01.java To run:          java Assign01

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• Example program: interest computation
  • Problem statement:

    Write a Java program that read in the following information: A principle amount principle A interest rate interest_rate (given in percents) The program prints the principle ammount for the next 3 years (with the interest added to the principle)

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  • Algorithm:

    Let p0 = principle and i = interest_rate The principle amount after 1 year is:   p1 = (1.0 + i/100.0)×p0 The principle amount after 2 year is:   p2 = (1.0 + i/100.0)×p1 The principle amount after 3 year is:   p3 = (1.0 + i/100.0)×p2

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  • A preliminary version of the Java program:

    import java.util.Scanner; public class Interest01 { public static void main(String[] args) { double principle, interest_rate; double p1, p2, p3; Scanner in = new Scanner(System.in); // Construct a Scanner object System.out.print("Enter principle = "); principle = in.nextDouble(); // Read in principle System.out.print("Enter interest rate = "); interest_rate = in.nextDouble(); // Read in interest rate p1 = (1.0 + interest_rate/100.0) * principle; System.out.print("Principle after 1 year = "); System.out.println(p1); p2 = (1.0 + interest_rate/100.0) * p1; System.out.print("Principle after 2 year = "); System.out.println(p2); p3 = (1.0 + interest_rate/100.0) * p2; System.out.print("Principle after 3 year = "); System.out.println(p3); } }

    Comments:

    This program works, but it uses an unnecessarily large number of variables We can use the variable principle to record the year-to-year principle amount.

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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 Interest01.java To run:          java Interest01

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• Advice in writing programs
  • Programming principle:

    Each variable in the program stores one piece of information A good programming practice is to assign a meaning to each variable in the program Giving a meaning to each variable will help you understand the steps of the algorithm

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• Improved program to compute interest
  • We will re-write the preliminary version of the program using the following meaning of the variables:

    principle = the current amount of principle interest_rate = interest rate paid

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  • Java program:

    import java.util.Scanner; public class Interest01 { public static void main(String[] args) { double principle, interest_rate; Scanner in = new Scanner(System.in); // Construct a Scanner object System.out.print("Enter principle = "); principle = in.nextDouble(); // Read in principle System.out.print("Enter interest rate = "); interest_rate = in.nextDouble(); // Read in interest rate principle = (1.0 + interest_rate/100.0) * principle; System.out.print("Principle after 1 year = "); System.out.println(principle); principle = (1.0 + interest_rate/100.0) * principle; System.out.print("Principle after 2 year = "); System.out.println(principle); principle = (1.0 + interest_rate/100.0) * principle; System.out.print("Principle after 3 year = "); System.out.println(principle); } }

    Explanation:

    By storing the computed value "(1.0 + interest_rate/100.0) * principle" back into the variable principle, this variable principle will contains the correct value to be used in the next computation !!!

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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 Interest02.java To run:          java Interest02