Slide 1 of 64 Lecture C Lesson 3: Conditions and Loops Unit 1: The if Statement.

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Lesson 3: Conditions and LoopsUnit 1: The if Statement

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The if Statement

• The Java if statement has the following syntax:

if (boolean-condition)

statement;

• If the Boolean condition is true, the statement is executed; if it is false, the statement is skipped

• This provides basic decision making capabilities

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Tempreture

class Temperature { static final int THRESHOLD = 65;

public static void main(String[] args) { InputRequestor input = new InputRequestor(); int temperature =

input.requestInt(“Enter the temperature:”);

System.out.println(“Current temperature “+ temperature); if (temperature < THRESHOLD) System.out.println(“It’s cold in here!”); }}

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If statement flow diagram

if (condition)

statement;

conditioncondition

statementstatement

truetrue

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Boolean Expressions

• The condition of an if statement must evaluate to a true or false result

• Java has several equality and relational operators:

• More complex Boolean expressions are also possible

OperatorMeaning

==equal to

!=not equal to

<less than

<=less than or equal to

>greater than

>=greater than or equal to

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Block Statements

• Several statements can be grouped together into a block statement

• Blocks are delimited by braces• A block statement can be used wherever a statement is

called for in the Java syntax

if (boolean-condition){

statement1;

statement2;

…

}

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Example - Temperature2

class Temperature2 { static final int THRESHOLD = 65;

public static void main(String[] args) { InputRequestor input = new InputRequestor(); int temperature =

input.requestInt(“Enter the temperature:”); System.out.println(“Current temperature “+

temperature); if (temperature < THRESHOLD) {

System.out.println(“It’s cold in here!”); System.out.println(“But we’ll survive.”); } }}

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If .. Else Statement

• An else clause can be added to an if statement to make it an if-else statement:

if (condition)

statement1;

else

statement2;

• If the condition is true, statement1 is executed; if the condition is false, statement2 is executed

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Example - Temperature3

class Temperature3 { static final int FREEZING_POINT = 32;

public static void main(String[] args) { InputRequestor input = new InputRequestor(); int temperature = input.requestInt(“Enter temperature:”); if (temperature <= FREEZING_POINT) System.out.println(“It’s freezing!”); else System.out.println(“Above freezing.”); }}

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If/else flow diagram

if (condition)

statement1;

else

statement2;

conditioncondition

statement2statement2

truetrue

statement1statement1

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Nested If statements

• Since an “If” statement is a statement, it can appear inside another if statement.

if (condition1) if (condition2) statement;

• It can also appear in an “else” clause

if (condition1) statement1; else if (condition2) statement2;

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Nested If Example// Reads 2 integers and compares themclass CompareExample { public static void main(String[] args) { InputRequestor input = new InputRequestor(); int a = input.requestInt(“First number:”); int b = input.requestInt(“Second number:”); if (a != b){ if (a > b) System.out.println(a+” is greater”); else System.out.println(b+” is greater”); }else System.out.println(“the numbers are equal”); }}

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Checking your Input• When requesting input from the user, keep in mind that the input may be

invalid. • It is good practice to check the validity of user input

int numberOfItems = input.requestInt(“Enter number of items:”); if (numberOfItems < 0) { System.out.println( “Number of items must be positive!”);} else { double price = numberOfItems * ITEM_PRICE; System.out.println(“The total price is:“ +price);}

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Lesson 3: Conditions and Loops

Unit 2: Boolean Expressions

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Logical Operators

• Boolean expressions may be combined using logical operators

• There are three logical operators in Java:

• They all take Boolean operands and produce Boolean results

• Logical NOT is unary (one operand), but logical AND and OR are binary (two operands)

OperatorOperation

!Logical NOT

&&Logical AND

||Logical OR

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Logical NOT

• The logical NOT is also called logical negation or logical complement

• If a is true, !a is false; if a is false, then !a is true• Logical expressions can be shown using truth tables

a!a

falsetrue

truefalse

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Logical AND

• The expression a && b is true if both a and b are true, and false otherwise

• Truth tables show all possible combinations of all terms

aba && b

falsefalsefalse

falsetruefalse

truefalsefalse

truetruetrue

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Logical OR

• The expression a || b is true if a or b or both are true, and false otherwise

aba || b

falsefalsefalse

falsetruetrue

truefalsetrue

truetruetrue

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Logical Operators

• Logical operators are used to form more complex logical expressions

• Logical operators have precedence relationships between themselves and other operators

if (a<1 || a%2!=0) System.out.println( “The input should be an even even number!”);

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Logical Operators

• Full expressions can be evaluated using truth tables

a < 1a%2!=0a<1 || a%2=0

falsefalsefalse

falsetruefalse

truefalsefalse

truetruetrue

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Boolean variables

• Boolean expressions can be assigned to Boolean variables

• Boolean variables are Boolean expressions

boolean b, c;

b = (x > 17);

c = (x>17) && (x<60);

boolean b, c;

b = (x > 17);

c = b && (x<60);

if (c)

System.out.println(“x is in range”);

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Example - RightTriangle// Receives the length of the edges of a triangle// and determine if this is a right triangleclass RightTriangle { public static void main(String[] args) { InputRequestor input = new InputRequestor(); float a = input.requestInt(“Edge1:”); float b = input.requestInt(“Edge2:”); float c = input.requestInt(“Hypotenuse:”); boolean test = a*a+b*b == c*c; if (test) System.out.println(“It’s a right triangle”); else System.out.println(“It’s not a right triangle”); }}

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Lesson 3: conditions and loops

Unit C3: The while Statement

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The while statement

• A while statement has the following syntax:

while (condition)

statement;

• If the condition is true, the statement is executed; then the condition is evaluated again

• The statement is executed over and over until the condition becomes false

• If the condition of a while statement is false initially, the statement is never executed

• Therefore, we say that a while statement executes zero or more times

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While statement flow diagram

while (condition)

statement;

conditioncondition

statementstatement

truetrue

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Example - Counter

// Counts from 1 to 5class Counter { static final int LIMIT = 5;

public static void main(String[] args) { int count = 1; while (count <= LIMIT) { System.out.println(count); count = count + 1; } System.out.println(“done”); }}

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Examples - Factors// Gets an integer and prints its factorsclass FactorsExample { public static void main(String[] args) { InputRequestor input = new InputRequestor(); int a = input.requestInt(“Enter a number:”); int i = 1; System.out.println(“The divisors of “+a+” are:”); while (i <= a) { if (a%i == 0) { System.out.println(i); } i = i + 1; } }}

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Infinite Loops

• The body of a while loop must eventually make the condition false

• If not, it is an infinite loop, which will execute until the user interrupts the program

• This is a common type of logical error -- always double check that your loops will terminate normally

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Example - Forever

// This program contains an infinite loopclass Forever { static final int LIMIT = 25;

public static void main(String[] args) { int count = 1; while (count <= LIMIT) { System.out.println(count); count = count - 1; } }}

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Unit 4: More conditionals

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The Conditional Operator

• Java has a conditional operator that evaluates a Boolean condition that determines which of two expressions is evaluated

• The result of the chosen expression is the result of the entire conditional operator

• Its syntax is:

condition ? expression1 :

expression2

• If the condition is true, expression1 is evaluated; if it is false, expression2 is evaluated

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• It is similar to an if-else statement, except that it is an expression that returns a value

• For example:

• If a is greater that b, then a is assigned to max; otherwise, b is assigned to max

• The conditional operator is ternary, meaning it requires three operands

int max = (a > b) ? a : b;

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• Another example:

• If count equals 1, "Dime" is printed, otherwise "Dimes" is printed

System.out.println ("Your change is " + count + ((count == 1) ? "Dime" : "Dimes”));

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Another Selection Statement

• The if and the if-else statements are selection statements, allowing us to select which statement to perform next based on some Boolean condition

• Another selection construct, called the switch statement, provides another way to choose the next action

• The switch statement evaluates an expression, then attempts to match the result to one of a series of values

• Execution transfers to statement list associated with the first value that matches

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The switch Statement

• The syntax of the switch statement is:

switch (expression) {

case value1:

statement-list1

case value2:

statement-list2

case …

}

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• The expression must evaluate to an integral value, such as an integer or character

• The break statement is usually used to terminate the statement list of each case, which causes control to jump to the end of the switch statement

• A default case can be added to the end of the list of cases, and will execute if no other case matches

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/** * A client that enables you to connect to the * bank server and make remote banking operations... */public class BankClient { public static final int VIEW_BALANCE = 1; public static final int VIEW_SAVINGS = 2; public static final int CASH_TRANSFER = 3; public static final int VIEW_LAST_OPERATIONS =

4; // ...

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// Inside the main loop of the client: int option = InputRequestor.requentInt(“Enter your choice:”); switch(option) { case VIEW_BALANCE: showBalance(); break; case VIEW_SAVINGS: showSavings(); break; default: output.showMessage(“No such option!”); }

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Unit 5: Shorthand Operators

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Shorthand Operators

• Many operations are very commonly used

• Java has shorthand notations for these increment and decrement operators assignment operators

x = x + 1;sum = sum + x;

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The Increment and Decrement Operators

• The increment operator (++) adds one to its integer or floating point operand

• The decrement operator (--) subtracts one• The statement

is essentially equivalent to

count++;

count = count + 1;

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• The increment and decrement operators can be applied in prefix (before the variable) or postfix (after the variable) form

• When used alone in a statement, the prefix and postfix forms are basically equivalent. That is,

is equivalent to

count++;

++count;

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• When used in a larger expression, the prefix and postfix forms have a different effect

• In both cases the variable is incremented (decremented)• But the value used in the larger expression depends on

the form

ExpressionsOperationValue Of expression

count++add 1old value

++countadd 1new value

count--subtract 1old value

--countsubtract 1new value

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• If count currently contains 45, then

assigns 45 to total and 46 to count

• If count currently contains 45, then

assigns the value 46 to both total and count

total = count++;

total = ++count;

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• If sum contains 25, what does this statement print?

• Prints the following result: 25 27 27 27

• sum contains 26 after the line is complete

System.out.println (sum++ + " " +

++sum + " " +

sum + " " +

sum--);

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Assignment Operators

• Often we perform an operation on a variable, then store the result back into that variable

• Java provides assignment operators that simplify that process

• For example, the statement

is equivalent to

sum += value;

sum = sum + value;

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• There are many such assignment operators, always written as op= , such as:

OperatorExampleEquivalent to

+=x+=yx = x + y

-=x-=yx = x - y

*=x*=yx = x * y

/=x/=yx = x / y

%=x%=yx = x % y

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• The right hand side of an assignment operator can be a complete expression

• The entire right-hand expression is evaluated first, then combined with the additional operation

• Therefore result /= total-MIN;

is equivalent to

result /= total-MIN;

result = result / (total-MIN);

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Unit C6: More Repetition

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More Repetition Constructs

• In addition to while loops, Java has two other constructs used to perform repetition:

the do statement the for statement

• Each loop type has its own unique characteristics• You must choose which loop type to use in each situation

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The do Statement

• The do statement has the following syntax:

do

statement

while (condition);

• The statement is executed until the condition becomes false

• It is similar to a while statement, except that its termination condition is evaluated after the loop body

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The do Statement

• The key difference between a do loop and a while loop is that the body of the do loop will execute at least once

• If the condition of a while loop is false initially, the body of the loop is never executed

• Another way to put this is that a while loop will execute zero or more times and a do loop will execute one or more times

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Do Statement Example

// Gets an integer and prints its factorsclass AvgExample { public static void main(String[] args){ InputRequestor input = new InputRequestor(); double x, sum=0, count=-1; do { x = input.RequestDouble(“Next number:”); sum += x; count++; } while (x != 0); // 0 is a flag indicating end of input System.out.println(“The average is “+sum/count); }}

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The do Statement flow diagram

statement

condition

false

true

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The for Statement

• Many loops have a common pattern, captured by the for statement

• The syntax of the for loop is

for (intialization; condition; increment)

statement;

• This is equivalent to

initialization;

while (condition) {

statement;

increment;

}

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The for Statement: examples

• Examples:

for (int count=1; count < 75; count++) { System.out.println (count); }

for (int num=1; num <= max; num = num * 2) { System.out.println (“Next power of 2: “ + num); }

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The for Statement

• The initialization is always performed once• The condition of a for statement is tested prior to

executing the loop body (like in the while statement)• Therefore, a for loop will execute zero or more times• For loops are well suited for cases where the number of

iterations is known beforehand• The increment is executed after each iteration of the loop

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Omitting parts in a for Statement

• Each expression in the header of a for loop is optional If the initialization is left out, no initialization is performed If the condition is left out, it is always considered to be true, and

therefore makes an infinite loop If the increment is left out, no increment operation is performed

• Both semi-colons are always required

for (;;) {// an infinite loop System.out.println (“beep”); } // compute a value count for (; count < max ; count ++ ) { System.out.println (count); }

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The for Statement flow diagram

statement

conditionfalse

true

initialization

increment

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Multiplication Table Example

class MultiplicationTable { public static void main(String[] args){ for(int j=1 ; j <= 10 ; j++) { for(int k=1 ; k <= 10 ; k++) System.out.print(j*k); System.out.println(); } }}

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The break and continue statements

• The break statement, which we used with switch statements, can also be used inside a loop

• When the break statement is executed, control jumps to the statement after the loop (the condition is not evaluated again)

• A similar construct, the continue statement, can also be executed in a loop

• When the continue statement is executed, control jumps to the end of the loop and the condition is evaluated

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Break and Continue Exampleclass AvgExample2 { public static void main(String[] args){ InputRequestor in = new InputRequestor(); double x, sum = 0; count = 0; while(true){ x = in.RequestDouble(); if (x == 0) break; if (x < 0) { System.out.println(“Only positive numbers!”); continue; } sum += x ; count ++ ; } // continued on next page

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Break and Continue Example (2)

System.out.println(“The average is “+sum/count);

}

}

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Why do We Need Indentation?

class Mystery {

public static void main(String[] args) {InputRequestor in = new InputRequestor();int dimension = in.requestInt(“Please enter the dimension”);

for (int j = 0; j < dimension; j++) {for (int k = 1; k < dimension - j; k++) {System.out.print(" ");} for (int k = 0; k < j; k++) {System.out.print("*");} System.out.println();}}}

Slide 1 of 64 Lecture C Lesson 3: Conditions and Loops Unit 1: The if Statement.

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