Assignment statements using the same variable in LHS and RHS

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Assignment statements using the same variable in
LHS and RHS
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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
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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

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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)
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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 (x contains 1.0)
1.0 4.0 5.0
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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 5.0

• 2. Then the result 5.0 is assigned to the
  receiving variable x

x 5.0
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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/Sylla
  bus/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

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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)

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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

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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
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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("P
rinciple after 3 year ")
System.out.println(p3)
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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.

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Example program interest computation (cont.)

• Example
• Prog file
• http//mathcs.emory.edu/cheung/Courses/170/Sylla
  bus/04/Progs/Interest01.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

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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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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
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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)

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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 !!!

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Improved program to compute interest (cont.)

• Example
• Prog file
• http//mathcs.emory.edu/cheung/Courses/170/Sylla
  bus/04/Progs/Interest02.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