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Assignment statements using the
same variable in LHS and RHS
Previously discussed
• Assignment statement:
• The assignment statement in the Java programming
language instructs the computer to update the value
stored in a variable
• Syntax of an assignment statement:
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 + 4.0
= 1.0 + 4.0
= 5.0
(x contains 1.0)
Assignment statements with the same
variable on the LHS and RHS (cont.)
2. 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
Assignment statements with the same
variable on the LHS and RHS (cont.)
• Example Program: (Demo above code)
– Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/04/Progs/
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:
• A principle amount principle
• A interest rate interest_rate (given in percents)
The program prints the principle amount for the next 3 years
(with the interest added to the principle)
Example program: interest computation
(cont.)
• 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
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-toyear principle amount.
Example program: interest computation
(cont.)
• Example:
– Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/04/Progs/I
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:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/04/Progs/I
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