if Statements ICS 111 Introduction to Computer Science

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if Statements • ICS 111: Introduction to Computer Science I – William Albritton •

if Statements • ICS 111: Introduction to Computer Science I – William Albritton • Information and Computer Sciences Department at the University of Hawai‘i at Mānoa • “I do not fear computers. I fear the lack of them. ” – Isaac Asimov 10/2/2020 © 2007 William Albritton 1

Terminology • Boolean – Models truth values • Either true or false – Examples

Terminology • Boolean – Models truth values • Either true or false – Examples • Boolean sushi. Is. Fresh = true; sushi. Is. Fresh = false; • Boolean ipod. Is. On = false; ipod. Is. On = true; – Named in honor of British logician George Boole (1815 -1864) • See example code at Boolean. Examples. java 10/2/2020 © 2007 William Albritton 2

Boolean Expression • A combination of operators & operands that returns true or false

Boolean Expression • A combination of operators & operands that returns true or false – Used in selection (if) & repetition (loop) statements – Evaluated from left to right – Operands are what the operators “operate” upon • 3 Kinds of Boolean operators 1. Equality operators: ==, != 2. Relational operators: >, >=, <, <= 3. Logical operators: &&, ||, ! (and, or, not) 10/2/2020 © 2007 William Albritton 3

Equality Operators • Equal or not equal – ==, != • Evaluated left to

Equality Operators • Equal or not equal – ==, != • Evaluated left to right • Returns true or false • Operands can be any primitive data type • Example code – Boolean is. Equal = (5 == 5); //is. Equal is true is. Equal = (7 != 5); //is. Equal is true 10/2/2020 © 2007 William Albritton 4

== And equals() For Objects • The equals() method is a boolean method –

== And equals() For Objects • The equals() method is a boolean method – Returns true or false based on the object’s data fields (values) • The == operator is a boolean operator – Returns true or false based on the object’s address • See example code for how the results may differ, depending on which you use – In Boolean. Examples. java, see method 3() • You only have to think about the differences when both operators are variables with address to objects 10/2/2020 © 2007 William Albritton 5

Relational Operators • Greater than, greater than or equal, less than or equal –

Relational Operators • Greater than, greater than or equal, less than or equal – >, >=, <, <= • Evaluated left to right • Returns true or false • Operands can be any primitive data type • Example code – Boolean is. Less = (5 < 5); //is. Equal is false Boolean is. Less. Or. Equal = (5 <= 5); //is. Less. Or. Equal is true 10/2/2020 © 2007 William Albritton 6

Logical Operators • and, or, not – &&, ||, ! • Evaluated left to

Logical Operators • and, or, not – &&, ||, ! • Evaluated left to right • Returns true or false • Operands have to be Boolean 10/2/2020 © 2007 William Albritton 7

Logical Operators • Logical and (&&) – Has 2 operands – Returns true if

Logical Operators • Logical and (&&) – Has 2 operands – Returns true if both operands are true, otherwise false • Logical or (||) – Has 2 operands – Returns true if at least one operands is true, otherwise false • Logical not (!) – Has only 1 operand – Returns the opposite of the operand 10/2/2020 © 2007 William Albritton 8

Logical Operators • Example code – Boolean bool 1 = (3 != 3) &&

Logical Operators • Example code – Boolean bool 1 = (3 != 3) && (1 <= 4); //false Boolean bool 2 = (!true) || (1. 1 < 2. 2); //true Boolean bool 3 = !(bool 1 && bool 2); //true Boolean bool 4 = (bool 1 || false) || ((bool 2 && bool 3) && (!bool 3)); //false 10/2/2020 © 2007 William Albritton 9

Class Exercise 1 • What is the output of the following code? – See

Class Exercise 1 • What is the output of the following code? – See Exercise 1. java • Try to solve it on your own • Then discuss the problem with your fellow students 10/2/2020 © 2007 William Albritton 10

Terminology • if statement – Uses a Boolean expression to choose which statements to

Terminology • if statement – Uses a Boolean expression to choose which statements to execute • Also called “conditional statement”, or “selection statement” 10/2/2020 © 2007 William Albritton 11

if Statements • Syntax – if(Boolean expression){ //code to be executed } • How

if Statements • Syntax – if(Boolean expression){ //code to be executed } • How does it work? – First, the Boolean expression is evaluated – If it is true, then the body (code within the curly brackets) gets executed – If it is false, then the body of the if statement DOES NOT get executed 10/2/2020 © 2007 William Albritton 12

REMEMBER THIS • BOOLEAN values (true or false) MUST be used inside the parentheses

REMEMBER THIS • BOOLEAN values (true or false) MUST be used inside the parentheses of “if statements” • Code example – See Simple. If. Statements. java • Here are two examples of very simple if statements • This program doesn’t do much • This program is just trying to emphasize that BOOLEAN values (true or false) MUST be used inside the parentheses of “if statements” 10/2/2020 © 2007 William Albritton 13

Using if Statements • Typically, we use “if statements” to make choices • Code

Using if Statements • Typically, we use “if statements” to make choices • Code example – See Spam. Musubi. java • Note that the code in only one of the “if statement” bodies executed each time the program is run • Also note that the last statement, which is outside all the “if statement” bodies, is always executed 10/2/2020 © 2007 William Albritton 14

Class Exercise 2 • Implement the following algorithm – Ask the user to input

Class Exercise 2 • Implement the following algorithm – Ask the user to input two integers – Check to see if the 1 st integer is larger, the 2 nd integer is larger, or if they are equal – Output an appropriate message 10/2/2020 © 2007 William Albritton 15

Scope • Similar to methods, the scope for variables declared within an if statement

Scope • Similar to methods, the scope for variables declared within an if statement body is limited from the variable declaration to the closing right curly bracket ("}") – These variables are also called “local variables” – See Scope. Example. java • Local variables radius & area are not visible outside the if statement body 10/2/2020 © 2007 William Albritton 16

Bug Using == Or equals() • An easy mistake to make is using the

Bug Using == Or equals() • An easy mistake to make is using the equality operator (==) instead of the equals operator to compare objects – Double x = new Double(3. 0); – Double y = new Double(3. 0); – (x == y) returns false • This compares the address of the two objects – (x. equals(y)) returns true • This compares the values stored within the two objects 10/2/2020 © 2007 William Albritton 17

Class Exercise 3 • See Exercise 3. java – What is the output of

Class Exercise 3 • See Exercise 3. java – What is the output of the program? – What are the local variables? – What is the scope of each local variable? 10/2/2020 © 2007 William Albritton 18

Nested If Statements • You can put it if statements within if statements –

Nested If Statements • You can put it if statements within if statements – Don’t forget to match the opening & closing curly brackets • if(Boolean expression. A){//A open //code to be executed if(Boolean expression. B){//B open //code to be executed }//B close }//A close – See example Nested. java 10/2/2020 © 2007 William Albritton 19

if Statement Bugs • Common programming errors – Forgetting to match the left &

if Statement Bugs • Common programming errors – Forgetting to match the left & right parenthesis • if(x%2)==0) // error! • if((x%2)==0) //correct – Using an expression other than boolean (true or false) • if(x%2) //error! • if((x%2)==0) //correct 10/2/2020 © 2007 William Albritton 20

if Statement Bugs II • Common programming errors – Forgetting to match the left

if Statement Bugs II • Common programming errors – Forgetting to match the left & right curly brackets • if(x==0){ System. out. println("Zero"); //error! • if(x==0){ System. out. println("Zero"); } //correct • Avoid these bugs by using the “Generate CSD” button as often as possible – You should check after writing every 2 -3 lines of code 10/2/2020 © 2007 William Albritton 21

ICS Statusudtalelse FOR SAMARBEJDSPARTNERE Hvad er ICS ICSICS Statusudtalelse FOR SAMARBEJDSPARTNERE Hvad er ICS ICS
2 Overview ICS Evaluation Procedures 2 ICS ICS2 Overview ICS Evaluation Procedures 2 ICS ICS
Interfaces ICS 111 Introduction to Computer Science IIInterfaces ICS 111 Introduction to Computer Science II
Applets ICS 111 Introduction to Computer Science IIApplets ICS 111 Introduction to Computer Science II
String Output ICS 111 Introduction to Computer ScienceString Output ICS 111 Introduction to Computer Science
File IO ICS 111 Introduction to Computer ScienceFile IO ICS 111 Introduction to Computer Science
Classes ICS 111 Introduction to Computer Science IClasses ICS 111 Introduction to Computer Science I
String Input ICS 111 Introduction to Computer ScienceString Input ICS 111 Introduction to Computer Science
Exceptions ICS 111 Introduction to Computer Science IExceptions ICS 111 Introduction to Computer Science I
for Loop ICS 111 Introduction to Computer Sciencefor Loop ICS 111 Introduction to Computer Science
while Loop ICS 111 Introduction to Computer Sciencewhile Loop ICS 111 Introduction to Computer Science
TwoDimensional Arrays ICS 111 Introduction to Computer ScienceTwoDimensional Arrays ICS 111 Introduction to Computer Science
Classes Constructors ICS 111 Introduction to Computer ScienceClasses Constructors ICS 111 Introduction to Computer Science
Program Control ICS 111 Introduction to Computer ScienceProgram Control ICS 111 Introduction to Computer Science
Arrays ICS 111 Introduction to Computer Science IArrays ICS 111 Introduction to Computer Science I
Methods Parameters ICS 111 Introduction to Computer ScienceMethods Parameters ICS 111 Introduction to Computer Science
project 111 midland Project 111 project 111 midlandproject 111 midland Project 111 project 111 midland
Surface Observations 111 Group Land Observations 111 111Surface Observations 111 Group Land Observations 111 111
Assignment Statements Operator Precedence Blanca Polo ICS 111Assignment Statements Operator Precedence Blanca Polo ICS 111
Methods Return Values ICS 111 Introduction to ComputerMethods Return Values ICS 111 Introduction to Computer