Lesson 3: Conditions and Loops
Unit 1: The if Statement
Lecture C
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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
Lecture C
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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!”);
}
}
Lecture C
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If statement flow diagram
if (condition)
statement;
condition
true
statement
Lecture C
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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:
Operator
==
!=
Meaning
equal to
not equal to
<
<=
>
less than
less than or equal to
greater than
>=
greater than or equal to
• More complex Boolean expressions are also possible
Lecture C
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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;
…
}
Lecture C
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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.”);
}
}
}
Lecture C
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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
Lecture C
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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.”);
}
}
Lecture C
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If/else flow diagram
if (condition)
statement1;
else
statement2;
true
statement1
Lecture C
condition
statement2
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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;
Lecture C
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Nested If Example
// Reads 2 integers and compares them
class 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”);
}
}
Lecture C
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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);
}
Lecture C
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Lesson 3: Conditions and Loops
Unit 2: Boolean Expressions
Lecture C
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Logical Operators
• Boolean expressions may be combined using logical
operators
• There are three logical operators in Java:
Operator
!
&&
Operation
Logical NOT
Logical AND
||
Logical OR
• They all take Boolean operands and produce Boolean
results
• Logical NOT is unary (one operand), but logical AND and
OR are binary (two operands)
Lecture C
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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
false
true
Lecture C
!a
true
false
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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
a
b
a && b
false
false
true
true
false
true
false
true
false
false
false
true
Lecture C
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Logical OR
• The expression a || b is true if a or b or both are true,
and false otherwise
a
b
a || b
false
false
true
true
false
true
false
true
false
true
true
true
Lecture C
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Logical Operators
• Logical operators are used to form more complex logical
expressions
if (a<1 || a%2!=0)
System.out.println(
“The input should be an even even number!”);
• Logical operators have precedence relationships between
themselves and other operators
Lecture C
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Logical Operators
• Full expressions can be evaluated using truth tables
a < 1
false
false
a%2!=0
false
true
a<1 || a%2=0
false
false
true
true
false
true
false
true
Lecture C
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Boolean variables
• Boolean expressions can be assigned to Boolean
variables
boolean b, c;
b = (x > 17);
c = (x>17) && (x<60);
• Boolean variables are Boolean expressions
boolean b, c;
b = (x > 17);
c = b && (x<60);
if (c)
System.out.println(“x is in range”);
Lecture C
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Example - RightTriangle
// Receives the length of the edges of a triangle
// and determine if this is a right triangle
class 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”);
}
}
Lecture C
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Lesson 3: conditions and loops
Unit C3: The while Statement
Lecture C
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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
Lecture C
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While statement flow diagram
while (condition)
statement;
condition
true
statement
Lecture C
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Example - Counter
// Counts from 1 to 5
class 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”);
}
}
Lecture C
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Examples - Factors
// Gets an integer and prints its factors
class 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;
}
}
}
Lecture C
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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
Lecture C
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Example - Forever
// This program contains an infinite loop
class 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;
}
}
}
Lecture C
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Lesson 3: conditions and loops
Unit 4: More conditionals
Lecture C
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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
Lecture C
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The Conditional Operator
• It is similar to an if-else statement, except that it is an
expression that returns a value
• For example:
int max = (a > b) ? a : b;
• 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
Lecture C
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The Conditional Operator
• Another example:
System.out.println ("Your change is " + count +
((count == 1) ? "Dime" : "Dimes”));
• If count equals 1, "Dime" is printed, otherwise
"Dimes" is printed
Lecture C
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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
Lecture C
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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 …
}
Lecture C
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The switch Statement
• 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
Lecture C
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The switch Statement
/**
* 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;
// ...
Lecture C
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The switch Statement
// 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!”);
}
Lecture C
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Lesson 3: conditions and loops
Unit 5: Shorthand Operators
Lecture C
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Shorthand Operators
• Many operations are very commonly used
x = x + 1;
sum = sum + x;
• Java has shorthand notations for these


increment and decrement operators
assignment operators
Lecture C
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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
count++;
is essentially equivalent to
count = count + 1;
Lecture C
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The Increment and Decrement Operators
• 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,
count++;
is equivalent to
++count;
Lecture C
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The Increment and Decrement Operators
• 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
Expressions
count++
++count
count--
Operation
add 1
add 1
subtract 1
Value Of expression
old value
new value
old value
--count
subtract 1
new value
Lecture C
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The Increment and Decrement Operators
• If count currently contains 45, then
total = count++;
assigns 45 to total and 46 to count
• If count currently contains 45, then
total = ++count;
assigns the value 46 to both total and count
Lecture C
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The Increment and Decrement Operators
• If sum contains 25, what does this statement print?
System.out.println (sum++
+
"
" +
++sum
+
"
" +
sum
+
"
" +
sum--);
• Prints the following result:
25
27
27
27
• sum contains 26 after the line is complete
Lecture C
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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
sum += value;
is equivalent to
sum = sum + value;
Lecture C
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Assignment Operators
• There are many such assignment operators, always
written as op= , such as:
Operator
+=
Example
x+=y
Equivalent to
x = x + y
-=
*=
/=
x-=y
x*=y
x/=y
x = x - y
x = x * y
x = x / y
%=
x%=y
x = x % y
Lecture C
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Assignment Operators
• 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;
result
/= total-MIN;
is equivalent to
result = result / (total-MIN);
Lecture C
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Lesson 3: conditions and loops
Unit C6: More Repetition
Lecture C
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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
Lecture C
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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
Lecture C
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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
Lecture C
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Do Statement Example
// Gets an integer and prints its factors
class 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);
}
}
Lecture C
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The do Statement flow diagram
statement
true
condition
false
Lecture C
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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;
}
Lecture C
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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);
}
Lecture C
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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
Lecture C
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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);
}
Lecture C
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The for Statement flow diagram
initialization
condition
false
true
statement
increment
Lecture C
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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();
}
}
}
Lecture C
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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
Lecture C
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Break and Continue Example
class 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
Lecture C
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Break and Continue Example (2)
System.out.println(“The average is “+sum/count);
}
}
Lecture C
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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();
}}}
Lecture C
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