Boolean Expressions and Control Structures

Boolean: a primitive data type that is either true or false

boolean computersAreCool = true;
boolean paperIsCool = false;

Boolean Expression: an expression that evaluates as either true or false

1==1                    // is true, because 1 will always be equal to 1
a < 1                   // will sometimes be true, because not all values of a will be less than 1
“paper” == “good”       // is false, because the string “paper” is not equal to the string “good”

We can use boolean expressions in control structures to run programs when certain conditions are met.

if Statements

if statements will run a code block if a particular condition is true.

Structure:

if(condition) {
    // code will run when condition is true
}

// an if statement will run when the condition is true
if(true) {
	System.out.println("this will print");
} 
if(1==1) {
	System.out.println("this will print");
}

// an if statement will not run if the condition is false
if(false) {
	System.out.println("this will not print");
} 
if("paper" == "good") {
	System.out.println("this will not print");
}

this will print
this will print

if, else if, and else

A single if statement only takes one condition, but what if we wanted to specify more conditions? We can add else if and else statements to accomplish this.

Structure:

if(condition) {
    // code to execute when condition is met
} else if (condition 2) {
    // code to execute when condition is false but condition 2 is true
} else {
    // code to execute when condition and condition 2 are false
}

int time = 8; // 24-hour clock, 8 AM

if(time < 8 ) {  // will run if it is before 8 AM
    System.out.println("School will start in a few hours!");
} else if ( time >= 8 && time < 16) { // will run betwee 8 AM and 4 PM
    System.out.println("You should be in school!");
} else { // will run from 4 PM to 12 AM
    System.out.println("You are at home!");
}

You should be in school!

switch statements

switch statements check the value of a condition and run code blocks for different scenarios.

Structure:

switch(condition) {
    case value1:
        // code
        break;
    case value2:
        // code
        break;
    default;
        // code that should run for unspecified case
        break;
}

switch statements are especially useful for creating menus.

int menuOption = 0;

// print menu for user to see options they can choose from
System.out.println("---- Menu Options ---");
System.out.println("0: print hello world");
System.out.println("1: print goodbye world\n");

switch(menuOption) { // check the value of menuOption
    case 0: // what to run when menuOption=0
        System.out.println("hello world");
        break; 
    case 1: // what to run when menuOption=1
        System.out.println("goodbye world");
        break;
    default: // default code to run
        System.out.println("menu options not found!");
        break;
}

---- Menu Options ---
0: print hello world
1: print goodbye world

hello world

The two programs below accomplish the same thing: given an object, the program will rate it as "cool" or "uncool". The first uses if-else if-else, and the other uses switch-case.

if statements:

String object = "computer"; // object is "computer"

// the if statement will print different statements depending on the value of object
if(object == "computer") {
    System.out.println("cool");
} else if (object == "keyboard") {
    System.out.println("cool");
} else if (object == "phone") {
    System.out.println("cool");
} else if (object == "paper") {
    System.out.println("uncool");
} else if(object == "pencil") {
    System.out.println("uncool");
} else {
    System.out.println("object not found!");
}

// since object = "computer" , this program should print "cool"

cool

switch-case:

String object = "computer"; // object is "computer"

// the switch statement will test different values of object and print a result depending on the value of object
switch(object) {
    case "computer":
        System.out.println("cool");
        break;
    case "keyboard":
        System.out.println("cool");
        break;
    case "phone":
        System.out.println("cool");
        break;
    case "paper":
        System.out.println("uncool");
        break;
    case "pencil":
        System.out.println("uncool");
        break;
    default:
        System.out.println("object not found!");
        break;
}

// since object = "computer", this program should print "cool"

cool

DeMorgan's Law

DeMorgan's Laws show how to deal with the negation of a conditional statements. When ! is applied:

• true will become false
• false will become true
• and (&&) will become or (||)

Examples:

Expression	Applying DeMorgan's Law	result
!true	opposite of true --> false	false
!(true || false)	false && true	false
!((false && true) || (!false && true))	(true || false) && (false || true) --> true && true	true

// DeMorgan's law practice

if(!true) {
    System.out.println("!true = false, so this statement should not print");
}

if(!((true && false) || (false && false))) {
    // using DeMorgan's law, this statement is equal to (false || true) && (true || true), which is true
    System.out.println("!((true && false) || (false && false)) is true, so this should print!");
}

if(!(false || true)) {
    // using DeMorgan's law, this statement is equal to true && false, which is false
    System.out.println("this should not print!");
}

!((true && false) || (false && false)) is true, so this should print!