AP Computer Science A:

Compound Boolean Expressions

Logical Operators

We can incorporate additional boolean expressions to our if statements with logical operators.

Logical operators allow programs to make decisions based on multiple conditions

Logical Operators

NOT !

AND &&

OR ||

​

The !(not) Operator

! evaluates a condition to the opposite boolean value

The && (and) Operator

&& evaluates whether all conditions are true

The || (or) Operator

|| evaluates whether one condition is true

Operator Shortcuts!

true

false

||

is

true

If any boolean expression evaluates to true in an || expression, the final expression will be true!

true

false

&&

is

false

If any boolean expression evaluates to false in an && expression, the final expression will be false!

Operator Shortcuts!

true

anything

||

is

true

If any boolean expression evaluates to true in an || expression, the final expression will be true!

anything

false

&&

is

false

If any boolean expression evaluates to false in an && expression, the final expression will be false!

Operator Shortcuts!

anything

true

anything

||

is

true

Because these expressions will always evaluate to false and true regardless of what the other expression evaluates to, the boolean expression doesn’t evaluate the second expression.

false

&&

is

false

Operator Shortcuts!

true

anything

||

is

true

Evaluating only the first boolean expression is called short circuit evaluation.

anything

false

&&

is

false

Short Circuit Evaluation

• first && second
• if first is false, then the whole expression must be false. Don’t bother evaluating second.

​

• first || second
• if first is true, then the whole expression must be true. Don’t bother evaluating second.

​

Short Circuit Example

int numSlices = 10;

int numPeople = 0;

​

if(numSlices / numPeople > 0)

{

System.out.println(“There’s enough pizza!”);

}

else

{� System.out.println(“Not enough pizza.”);

}

Short Circuit Example

​

int numSlices = 10;

int numPeople = 0;

​

if(numPeople != 0 && numSlices / numPeople > 0)

{

System.out.println(“There’s enough pizza!”);

}

else

{� System.out.println(“Not enough pizza.”);

}

false

not evaluated

Example

int numSlices = 10;

int numPeople = 5;

​

if(numPeople != 0 && numSlices / numPeople > 0)

{

System.out.println(“There’s enough pizza!”);

}

else

{� System.out.println(“Not enough pizza.”);

}

Short Circuit Example

​

int numSlices = 10;

int numPeople = 5;

​

if(numPeople != 0 && numSlices / numPeople > 0)

{

System.out.println(“There’s enough pizza!”);

}

else

{� System.out.println(“Not enough pizza.”);

}

true

evaluates!

int total=2;

boolean flipper = true;

​

if(flipper || total>4)

{

out.println("short");

}

out.println("check");

OUTPUT

short�check

Short Circuit Evaluation

int total=9, num=13;�

if (total>4 ||++num>15 && total>0)

{

out.println("short");

}

out.println(num);

OUTPUT

short�13

The && never happens!

Short Circuit Evaluation

Order of Operations

Boolean Operators also have an order of operations!

Order of Operations

Boolean operators also have an order of operations:

​

! is evaluated before &&

&& is evaluated before ||

NOT !

AND &&

OR ||��NAO = “NOW”

​

Nesting if Statements

Nesting if statements means we can place if statements within other if statements!

num = 67;�if(num>50)

if(num>60)

if(num>70)

System.out.println(">70");

else

System.out.println(">60 and <70");

else

System.out.println(">50 and <60");

else

System.out.println("<50");

OUTPUT

>60 and <70

Nested Ifs

int p = 74;

if(p > 30)

{

System.out.println("aplus");

if(p > 40)

System.out.println("comp");

}

else

System.out.println("sci");

OUTPUT

aplus�comp

Nesting ifs

int num=1;�if(num>2)

{

if(num<10)

System.out.println(">2<10");

}

else{

System.out.println("<2");

}

OUTPUT

<2

Nesting ifs

int num=11;�if(num>2)

if(num<10)

System.out.println(">2<10");

else

System.out.println("<2");

​

OUTPUT

<2

Which if-statement does the else belong to?

“Dangling” Else

Concepts Learned this Lesson