Upload File

if / else statements

35
1 if/else statements

Upload: velika

Post on 14-Jan-2016

30 views

Category:

Documents


0 download

Report

DESCRIPTION

if / else statements. Conditionals. “If you eat your vegetables, then you can have dessert.” “If you do your homework, then you may go outside to play, or else you’ll be grounded for life.”. The if statement. - PowerPoint PPT Presentation

TRANSCRIPT

Page 1: if / else statements

1

if/else statements

Page 2: if / else statements

2

Conditionals

“If you eat your vegetables, then you can have dessert.”

“If you do your homework, then you may go outside to play, or else you’ll be grounded for life.”

Page 3: if / else statements

3

The if statement

if statement: A control structure that executes a block of statements only if a certain condition is true.

General syntax:if (<test>) { <statement(s)> ;}

Example (with grade inflation):if (gpa >= 2.0) { System.out.println("You get an A!");}

Page 4: if / else statements

4

if statement flow chart

Page 5: if / else statements

5

The if/else statement

if/else statement: A control structure that executes one block of statements if a certain condition is true, and a second block of statements if it is false. We refer to each block as a branch.

General syntax:if (<test>) { <statement(s)> ;} else { <statement(s)> ;}

Example:if (gpa >= 3.0) { System.out.println("Welcome to Temple!");} else { System.out.println("Try applying to Penn.");}

Page 6: if / else statements

6

if/else statement flow chart

Page 7: if / else statements

7

if/else: Exercise

Write code to read a number from the user and print whether it is even or odd using an if/else statement.

Example executions:

Type a number: 42

Your number is even

Type a number: 17

Your number is odd

Page 8: if / else statements

8

Chained if/else statements

Chained if/else statement: A chain of if/else that can select between many different outcomes based on several tests.

General syntax:if (<test>) { <statement(s)> ;} else if (<test>) { <statement(s)> ;} else { <statement(s)> ;}

Example:if (number > 0) { System.out.println("Positive");} else if (number < 0) { System.out.println("Negative");} else { System.out.println("Zero");}

Page 9: if / else statements

9

Chained if/else variations

A chained if/else can end with an if or an else. If it ends with else, one of the branches must be taken. If it ends with if, the program might not execute any branch.

if (<test>) { <statement(s)>;} else if (<test>) { <statement(s)>;} else { <statement(s)>;}

if (<test>) { <statement(s)>;} else if (<test>) { <statement(s)>;} else if (<test>) { <statement(s)>;}

Page 10: if / else statements

10

Chained if/else flow chartif (<test>) { <statement(s)>;} else if (<test>) { <statement(s)>;} else { <statement(s)>;}

Page 11: if / else statements

11

Chained if/else if flow chartif (<test>) { <statement(s)>;} else if (<test>) { <statement(s)>;} else if (<test>) { <statement(s)>;}

Page 12: if / else statements

12

Chained if/else variations

if (place == 1) { System.out.println("You win the gold medal!");} else if (place == 2) { System.out.println("You win a silver medal!");} else if (place == 3) { System.out.println("You earned a bronze medal.");}

Are there any cases where this code will not print a message?

How could we modify it to print a message to non-medalists?

Page 13: if / else statements

13

Sequential if flow chart

if (<test>) { <statement(s)>;}if (<test>) { <statement(s)>;}if (<test>) { <statement(s)>;}

Page 14: if / else statements

14

Summary: if/else structures

Choose 0 or 1 set of statements

if (<test>) { <statement(s)>;} else if (<test>) { <statement(s)>;} else if (<test>) { <statement(s)>;}

Choose 0, 1, or more set of statements

if (<test>) { <statement(s)>;}if (<test>) { <statement(s)>;} if (<test>) { <statement(s)>;}

Choose exactly 1 set of statements

if (<test>) { <statement(s)>;} else if (<test>) { <statement(s)>;} else { <statement(s)>;}

Page 15: if / else statements

15

Which if/else construct to use? Reading the user's GPA and printing whether the student is on

the dean's list (3.8 to 4.0) or honor roll (3.5 to 3.8)if / else if

Printing whether a number is even or oddif / else

Printing whether a user is lower-class, middle-class, or upper-class based on their income

if / else if / else Determining whether a number is divisible by 2, 3, and/or 5

if / if / if Printing a user's grade of A, B, C, D, or F based on their

percentage in the courseif / else if / else if / else if / else

Page 16: if / else statements

16

Boolean Arithmetic

Page 17: if / else statements

17

Recall: Java's Primitive Data Types

Discrete Typesbyteshortintlong

Continuous Typesfloatdouble

Non-numeric Typesbooleanchar

Page 18: if / else statements

18

The boolean Type

The boolean type has two possible values:true and false

boolean variables are declared and initialized just like other primitive data types:

boolean iAmSoSmrt = false; //just like int i = 2;

boolean minor = (age < 21); //just like int x = y*2;

Page 19: if / else statements

19

Relational expressions

Relational expressions have numeric arguments and boolean values.

They use one of the following six relational operators:

true5.0 >= 5.0greater than or equal to>=

false126 <= 100less than or equal to<=

true10 > 5greater than>

false10 < 5less than<

true3.2 != 2.5does not equal!=

true1 + 1 == 2equals==

ValueExampleMeaningOperator

Page 20: if / else statements

20

Evaluating Relational expressions Relational operators have lower precedence than math

operators.5 * 7 >= 3 + 5 * (7 - 1)5 * 7 >= 3 + 5 * 635 >= 3 + 3035 >= 33true

Relational operators cannot be chained (unlike math operators)

2 <= x <= 10true <= 10 error!

Page 21: if / else statements

21

Logical operators

Logical operators have boolean arguments and boolean values

!(7 > 0)

(2 == 3) || (-1 < 5)

(9 != 6) && (2 < 3)

Example

not

or

and

Description

!

||

true&&

ResultOperator

Page 22: if / else statements

22

What is the result of each of the following expressions?int x = 42;

int y = 17;

int z = 25;

y < x && y <= z x % 2 == y % 2 || x % 2 == z % 2 x <= y + z && x >= y + z !(x < y && x < z) (x + y) % 2 == 0 || !((z - y) % 2 == 0)

Answers: true, false, true, true, false

Boolean expressions

Page 23: if / else statements

23

Subtleties of if/else

Page 24: if / else statements

24

Scope

scope: The portion of a program where a given variable exists. A variable's scope is from its declaration to the end of the { }

braces in which it was declared.

public class ScopeExample {

public static void main(String[] args) {

int x = 3;

int y = 7;

if(x > 0 && y > 0) {

int sumPositives = x + y;

} else {

sumPositives = 0; // illegal: sumPositives is out of scope

}

// illegal: sumPositives is out of scope

System.out.println("sum = " + sumPositives);

}

Why not just have the scope of a variable be the whole program?

Page 25: if / else statements

25

Variable initialization

String message;if (gpa >= 3.0) { message = "Welcome to Temple!";}if (gpa >= 2.0) { message = "Have you considered applying to Penn?";}if (gpa < 2.0) { message = "I hear Harvard still needs students...";}System.out.println(message);

The compiler will complain that "variable message might not have been initialized". Why?

Page 26: if / else statements

26

Variable initialization

The solution:

String message;if (gpa >= 3.0) { message = "Welcome to Temple!";} else if (gpa >= 2.0) { message = "Have you considered applying to Penn?";} else { // gpa < 2.0 message = "I hear Harvard still needs students...";}System.out.println(message);

Page 27: if / else statements

27

Putting it all together: Exercises Write a program named NumUnique that reads two

integers from the user and displays how many unique values were passed. For example, if the user enters 3 and 7, NumUnique displays 2

because 3 and 7 are two unique numbers, but if the user enters 4 and 4, it displays 1 because 4 and 4 only represent one unique number.

Write a program named DividesEvenly that reads two integers from the user and displays “true” if the first evenly divides the second, and false otherwise. For example, if the user enters 7 and 28, DividesEvenly

displays true because 7 is a factor of 28.

Page 28: if / else statements

Magic Numbers

Magic numbers are BAD NEWS.

They are numeric constants other than 0, 1, and 2 that appear in the body of a program.

Page 29: if / else statements

Magic Number Example

public class Magic {public static void main(String [] args) {

for(int i=0; i<10; i++) {if(i % 3 ==0) {

System.out.println(“Divisible by 3”);}

}}

}

MagicMagic

Page 30: if / else statements

Class Constants

A class constant is a variable whose scope is the entire class, and whose value can never change after it has

been initialized.

To give it the right scope, simply declare it right inside the class:

public class MyClass {public static final int myConstant = 4;

}

The final keyword means its value can’t be changed.

Page 31: if / else statements

Magic Number Example: with Class Constants

public class Magic {public static final int upperBound = 10;public static final int divisor = 3;

public static void main(String [] args) {for(int i=0; i<upperBound; i++) {

if(i % divisor ==0) {System.out.println(“Divisible by ” +

divisor);}

}}

}

Page 32: if / else statements

32

Factoring if/else

Page 33: if / else statements

33

Factoring if/else

factoring: extracting common/redundant code

Factoring if/else code reduces the size of the if and else statements

Factoring tips: If the start of each branch is the same, move it before the if/else. If the end of each branch is the same, move it after the if/else.

Page 34: if / else statements

34

Factoring: Before

if (money < 500) { System.out.println("You have, $" + money + " left."); System.out.print("Caution! Bet carefully."); System.out.print("How much do you want to bet? "); bet = console.nextInt();} else if (money < 1000) { System.out.println("You have, $" + money + " left."); System.out.print("Consider betting moderately."); System.out.print("How much do you want to bet? "); bet = console.nextInt();} else { System.out.println("You have, $" + money + " left."); System.out.print("You may bet liberally."); System.out.print("How much do you want to bet? "); bet = console.nextInt();}

Page 35: if / else statements

35

Factoring: After

System.out.println("You have, $" + money + " left.");

if (money < 500) { System.out.print("Caution! Bet carefully.");} else if (money < 1000) { System.out.print("Consider betting moderately.");} else { System.out.print("You may bet liberally.");}

System.out.print("How much do you want to bet? ");bet = console.nextInt();


CP104 Introduction to Programming Selection structures_2 Lecture 10 __ 1 If statement (preview version) Syntax form: if ( condition) Statements else Statements

Chapter 3:Decision Structures. 3.1 The if Statement 3.2 The if-else Statement 3.3 The if-else-if Statement 3.4 Nested if Statements 3.5 Logical

Chapter 4 Introduction to Control Statements Section 1 - Additional Java Operators Section 2 - If Statements Section 3 - If-Else Statements Section 4

If-Else Statements PowerPoint

VB.Net Decisions. The If … Then Statement If condition Then Statements End If If condition Then Statements Else Statements End If Condition: –Simple condition:

EPSII 59:006 Spring 2004. Topics Using TextPad If Statements Relational Operators Nested If Statements Else and Elseif Clauses Logical Functions For Loops

PYTHON - TOPS Technologies · MODULE 1 [Core Python Concepts] Introduction of Python Programming Style Conditional Statements If, If- else, Nested if-else Looping, For, While, Nested

1 CIS 230 25-Jan-06. 2 Overview Selection Statements –If Statement –Else –Nested If-Else –Switch Repetition Statements –While statement –For Statement

Lecture 7: if-else if-else Statements

Outline if...else...elseif Statements Switch Loops Functions Arrays Forms

Conditional Statements if and else if statements if (expression) if (expression) statements statements { else if (expression) { else if (expression)

Additional control structures. The if-else statement The if-else statement chooses which of two statements to execute The if-else statement has the form:

2006 Pearson Education, Inc. All rights reserved. 1 3 3 Control Statements: Part 1: Selection statements: if, if…else, switch

1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

Control Statements - Sabancı Üniversitesimyweb.sabanciuniv.edu/gulsend/files/2009/04/control.pdfNested if statements if/else statements are inside other if/else statements Method

1.5 Conditional Statements academy.zariba.com 1. Lecture Content 1.If-else statements 2.If-else-if statements 3.Switch-case statements 2

Compound Statements If you want to do more than one statement if an IF- else case, you can form a block of statements, or compound statement, by enclosing

1-Dec-15 Additional control structures. 2 The if-else statement The if-else statement chooses which of two statements to execute The if-else statement

Control Statements (Decision Making). Control Statements Control statements Selection Statements The if else statement The switch statements Iteration

Monday, Week 3 Conditional Statements! If-then statements If-then-else statements Nested if-statements Some examples… Mini-Lab 3!

CIS162AD - C# If Statements 04_decisions.ppt. CIS162AD2 Overview of Topic Pseudocode Flow Control Structures Flowcharting If, If-Else Nested

Parsing - Indian Institute of Technology Madrasrupesh/teaching/compiler/jul18/3-parsing.pdfif if = else then then = else else then = if + else if if = else then then = else else then

Processing Decisions Chapter 3. If…Then…Else… Statements If decSalesAmount > 1000 Then sngTaxRate =.05 Else sngTaxRate =.07 End If

Nested conditional statements. Previously discussed Conditional statements discussed so far: Syntax of the if-statement: if-statement if-else-statement

Week 4 - Wednesday. What did we talk about last time? if statements else statements Nested selection statements

Unit 3: Boolean Expressions, if statements · Unit 3: Boolean Expressions, if statements if-else if-else statements Adapted from: 1) Building Java Programs: A Back to Basics Approach

CSE 401/M501 –Compilers•Fix the grammar to separate if statements with else clause and if statements with no else –Done in Java reference grammar –Adds lots of non-terminals

CONTROL STRUCTURE The if, elseif, and else & switch Statements 1

Lecture 3 Decision Structures. 4-2 Topics – The if Statement – The if - else Statement – The PayRoll class – Nested if Statements – The if - else - if

Conditional Statements - pnucs313.files.wordpress.com€¦ · 2019-01-05  · The conditional statements if and if-else provide conditional execution of blocks of code Constantly

Various Descriptions of Majority Gate When-else If-else Case, select and other conditionals in sequential and concurrent descriptions Concurrent Statements

CS 303E Lecture 6 Selection / Conditionals: if and if-else statements

Selection Control Structures (L08) * Selection Criteria * if Statements * The if-else Statement Selection Control Structure Dr. Ming Zhang

#7: Else Statements and Nesting · Use else and elif statements to make multiple-option decisions Use nesting to combine if statements with other if statements or functions. Conditional

Unit 1: Introduction to Computer Fundamentals … · Selection Statements Control Statements / Branch Statements IF-ELSE Nested IF-ELSE Conditional Expression Switch statements