• Frequently:
  • We need keep track of something that is changing
  • We want to refer to thing that is changing using one name
• Examples:
  • Age changes over time
  • Temperature changes over time
  • Position changes over space

• The Pre/Post Increment/Decrement Operators:
  • ++
  • --
• A Limitation:
  • They only change the variable by 1

• The Assignment Operator:
  • Takes the result of evaluating the expression on its right and puts it in the memory location identified by the variable on its left
• Some Examples:
  • javaexamples/programmingpatterns/Updating.java (Fragment: additionage)

                    int    currentAge, increment, initialAge;
            
            initialAge = 0;
            increment = 1;
            currentAge = initialAge + increment;
            

• An Observation:
  • These three assignment statements are particularly easy to understand because the expression on the right side of the assignment operator does not involve the variable on the left side
• A Different Situation:
  • javaexamples/programmingpatterns/Updating.java (Fragment: update1)

                    // Add age and 1 and assign the result to age
            age = age + 1;
            

• To the Non-Programmer:
  • This statement looks like a mistake
• To the Programmer:
  • Nothing about the syntax of assignment statements prevents this

• The Idea:
  • Use the same variable on the left and right sides of the assignment operator in order to change its value
• An Abstract Version:
  • value = value operator adjustment
• Why It Works:
  • The assignment operator has the lowest precedence so it is executed last
  • The assignment operator places the value on the right side into the variable on the left side

javaexamples/programmingpatterns/Updating.java (Fragment: grade)

                // Assign the initial grade
        grade = 85;

        // Reduce a grade by a late penalty
        grade = grade - latePenalty;
        

javaexamples/programmingpatterns/Updating.java (Fragment: sales)

                // Offer a 25% discount
        price = price - 0.25*price;
        

Tracing the Execution

javaexamples/programmingpatterns/Updating.java (Fragment: banking)

                // Earn 5% interest
        balance = 1.05 * balance;
        

• The Operators:
  • +=
  • *=
  • -=
  • /=
  • %=
• Examples:

  • 	  age += 1;
              price /= 2.0;
              grade -= penalty;
    	  

• A Question:
  • Will the statements i =+ 1; and j =- 1; compile?
• The Answer:
  Yes because they are the assignment operator followed by a unary positive/negative operator (but they are not compound assignment operators)