Lesson 5

If Statement Body, Increment/Decrement, Compound-Assignment

Author: Brandon Fajardo (9/1/17). Updated (8/1/19)

Introduction

Review: The last lesson covered mathematical equations and accessing the Math class for more complex functions.

Lesson topic: In this lesson, you will learn how to apply what you have learned thus far about conditions and equations to the bodies of if statements.

Topic 1 - Using Mathematical Expressions in Conditions

Subtopic 1 - Restricting the Domain/Range

When working with mathematical equations, we can use the condition of an if statement to restrict domain or range.

EX: Here, we restrict the domain by using the condition to prevent division by 0 where it would be undefined.

int x;
if (x!=0) {
 (x+2)/x
}

EX: Here, the print statement only runs if the number inputted is positive and not 0.

int y;
if (y>0) {
 System.out.println(“The number is positive and not 0”);
}

EX: Here, we test to see if the modulus (remainder) of 9 is 0. If it is true, z must be evenly divisible by 9.

int z;
if (z%9 == 0) {
 System.out.println(z + “is evenly divisible by 9”);
}

Problem #1:

Try creating an if statement condition which tests if a value is even. Write another for when the value is odd.

*Solutions at bottom of this document

Subtopic 2 - Using Mathematical Expressions in the Condition

You can also use mathematical expressions within the condition.

EX:

double d;
if (d*2>=10) {
 System.out.println(d/2);
}

*The test value is first multiplied by 2 before checking if it is greater than or equal to 10.

Topic 2 - Redefining Variables

You may also redefine the value of a variable within the body of an if statement.

Subtopic 1 - Declare a New Value

You can declare a new value for a variable.

EX:

boolean b = true;
if(b) {
 b = false; *Here, b is redefined to be false
 System.out.println(b);
}

Subtopic 2 - Modify a Current Value

You can also add to or subtracting from the value of a variable. This involves equating a variable to itself along with some modification. This is often written as x = x + y, where x is the original variable and y is the modification.

EX:

int i = 4;
if (i >-1) {
 i = i + 4; //Here, the variable becomes 4 greater and is now equal to 8 
 System.out.println(i);
}

EX:

String s = “First”;
if (s == ”First”) {
 s = s + “ Last”;
 System.out.println(s);
}

*Here, “ Last” is added to the end of the variable s. The variable s now equals “First Last”

Topic 3 - Notations to Modify Variables

In Java, there are built-in notations to more easily add to or subtract from variables. These include: Increment, Decrement, and Compound-Assignment Operators

Subtopic 1 - Increment and Decrement

Increment/Decrement Operations:

Symbol	Name	Notation	Notation Equivalent	Use
++	Increment	x++;	x = x + 1;	Adds 1 to itself
--	Decrement	x--;	x = x - 1;	Subtracts 1 from itself

These operations are shorthand ways to increase or decrease a mathematical variable by 1.

*x++ and x --can not be used within print statements.

EX: If it is your birthday, your age increases by 1

int age = 16;
boolean Birthday;
if (Birthday) {
 age++;
}

EX: If you eat a cookie, the number of cookies decreases by 1

int cookie;
boolean eat;
if (eat) {
 cookie--;
}

Subtopic 2 - Compound-Assignment Operators

Compound-Assignment Operators are additional shorthand notations used to modify mathematical variables in different ways.

Symbol	Name	Notation	Notation Equivalent	Use
+=	Plus equals	x+=v;	x = x + v	Adds a value v to x
-=	Minus equals	x-=v;	x = x - v	Subtracts a value v from x
*=	Times equals	x*=v;	x = x * v	Multiplies a value v and x *The product result become x
/=	Divide equals	x/=v;	x = x / v	Divides x by a value v *The dividend becomes x
%=	Modulus equals	x%=v;	x = x % v	Finds the modulus of x by the value v *The remainder becomes x

This original variable is represented by x here.

EX: Adds 100 points to your score if you kill an enemy.

double score;
boolean killenemy;
if (killenemy) {
 score+=100;
}

Problem #2: Compound-Assignment Operators

Try using each of the compound-assignment operators in its own if statement.

Things to note:

• You cannot apply most Compound-Assignment Operators onto Strings or booleans.

• The exception is that += will work with Strings.

Conclusion

Recap: In this lesson, you learned how to combine if statement conditions with mathematical equations, two concepts you have previously learned.

Next: The next lesson will cover how to use multiple components in an if statement condition. This includes the use of nested if statements and logical operators (||, &&).

Solutions

Problem #1:

Even: if(z%2==0)

Odd: if(z%2!=0)

Problem #2:

No solutions given