Assignment statements using the same variable in LHS and RHS

Previously discussed Assignment statement: Syntax of an assignment statement: The assignment statement in the Java programming language instructs the computer to update the value stored in a variable VariableName = Expression ;

Previously discussed (cont.) 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

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); }

Assignment statements with the same variable on the LHS and RHS (cont.) Explanation: You need to realize that the symbol "=" does not represent an equality relationship The symbol "=" denotes an assignment operation: 1. The computer will first evaluate the LHS "x + 4.0": LHS = x (x contains 1.0) = = 5.0

Assignment statements with the same variable on the LHS and RHS (cont.) Therefore, after executing the statement "x = x + 4.0", the variable x contains the value Then the result 5.0 is assigned to the receiving variable x: x = 5.0;

Assignment statements with the same variable on the LHS and RHS (cont.) Example Program: (Demo above code) –Prog file: Assign01.java How to run the program: Right click on link and save in a scratch directory To compile: javac Assign01.java To run: java Assign01

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

Example program: interest computation (cont.) Algorithm: Let p 0 = principle and i = interest_rate The principle amount after 1 year is: p 1 = (1.0 + i/100.0)×p 0 The principle amount after 2 year is: p 2 = (1.0 + i/100.0)×p 1 The principle amount after 3 year is: p 3 = (1.0 + i/100.0)×p 2

Example program: interest computation (cont.) 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

Example program: interest computation (cont.) 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); }

Example program: interest computation (cont.) 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.

Example program: interest computation (cont.) Example: –Prog file: nterest01.java How to run the program Right click on link and save in a scratch directory To compile: javac Interest01.java To run: java Interest01

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

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

Improved program to compute interest (cont.) 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

Improved program to compute interest (cont.) 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); }

Improved program to compute interest (cont.) 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 !!!

Improved program to compute interest (cont.) Example: –Prog file: nterest02.java How to run the program: Right click on link and save in a scratch directory To compile: javac Interest02.java To run: java Interest02