loops (cont.). loop statements while statement do statement for statement while ( condition )...
TRANSCRIPT
Loops (cont.)
Loop Statements
while statement
do statement
for statement
while ( condition ) statement;
do { statement list;} while ( condition );
for ( initialization ; condition ; increment ) statement;
Flowchart of a while Loop
statement
true
conditionevaluated
false
while ( condition ) statement;
Example
// set initial value of month so that the while condition // below is false initially
int month = -1;
while (month < 1 || month > 12){
System.out.print( “Enter a month (1 to 12): “);
month = scan.nextInt();
}
System.out.print( “Enter a month (1 to 12): “);
int month = scan.nextInt();
while (month < 1 || month > 12){
System.out.println( month + “ is not a valid month.” );
System.out.print( “Enter a month (1 to 12): “);
month = scan.nextInt();
}
The do Statement: Syntax
do{ statement;}while ( condition );
BothBoth dodo and andwhilewhile
arearereservedreserved
wordswords
The The statementstatement is executed once initially, then the is executed once initially, then the conditioncondition is evaluated is evaluated
The The statementstatement is repetitively executed until the is repetitively executed until the conditioncondition becomes false becomes false
Flowchart of a do Loop
truetrue
conditionevaluated
statement
falsefalse
do{ statement;}while ( condition );
Comparing the while and do Loops
statement
truetrue
conditionevaluated
falsefalse
while loopwhile loop
truetrue
conditionevaluated
statement
falsefalse
do loopdo loop
A do loop is similar to a while loop, except that the condition is evaluated after the body of the loop is executed Therefore the body of a do loop will execute at least once
Example
int month; // no need to initialize month
do
{
System.out.print( “Enter a month (1 to 12): “);
month = scan.nextInt();
} while (month < 1 || month > 12);
// beginning of the next statement
The for Statement: Syntax
for ( initialization ; condition ; increment ) statement;
ReservedReservedwordword
The The initializationinitialization portion portionis executed onceis executed once
before the loop beginsbefore the loop begins
The statement isThe statement isexecuted until theexecuted until the
conditioncondition becomes false becomes false
The The incrementincrement portion is executed at the end of each iteration portion is executed at the end of each iteration
Both semi-colons are always Both semi-colons are always requiredrequired
The for Statement: Syntax
• 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
– if the increment is left out, no increment operation is performed
Flowchart of a for loop
statement
truetrue
conditionevaluated
falsefalse
increment
initialization
for ( initialization ; condition ; increment ) statement;
The for Statement
• A for loop is equivalent to the following while loop structure:
initialization;while ( condition ){ statement; increment;}
for ( initialization ; condition ; increment ) statement;
The for Statement: Example
counter++
Establish initial value of control variable.
Determine if final value of control variable has been reached.
counter <= max sum+= counter
true
false
int counter = 1
Body of loop (this may be multiple statements)
Increment the control variable.
int sum = 0;
for (int counter = 1; counter <= max; counter++)
sum += counter;
// beginning of the next statement
Summary: Loop Statements Syntax
while statement
do statement
for statement
while ( condition ) statement;
do { statement list;} while ( condition );
for ( initialization ; condition ; increment ) statement;
15
Comments about Loop Statements
• Choosing which loop statement to use will depend on the specific situation and personal taste
• Checking and updating the condition– in most cases, the body of a loop must eventually make the
condition false• if not, it is an infinite loop, which will execute until the user interrupts
the program– Ctrl-C in command line
• this is a common type of logical error, and you should always double check to ensure that your loops will terminate normally
– pay attention to the condition to avoid an “off by 1” problem
Designing a Loop: A (Rough) Template
• Think of state (captured by variables) that you need to keep track across multiple iterations– e.g., counter, sum– e.g., boolean variables: thisRoundIsOver, canBeDivided …
• Initialize state (variables)
• Inside the loop– process
• the processing may depend on the current state– update state
• e.g., increase the counter, add to sum, set the flag
• Termination condition: in general, it is a boolean expression involving the state variables
Example: Check if a Number is Prime
// check if a positive integer n is prime
boolean canBeDivided = false;for (int i = 2; i < n && !canBeDivided; i++) { if (n % i == 0) canBeDivided = true;}
if (!canBeDivided){ System.out.println (n + “ is not a prime!”);} else{ System.out.println (n + “ is a prime!”);}
Example: Reverse a Number
1 2 543 7 6
number reverse
The state is captured by number and reverse.
1 2 543
number
6 7 7 6 345
reverse
2 1
Example: Reverse a Number
1 2 43 7 6
number reverse
{ lastDigit = number % 10; reverse = reverse * 10 + lastDigit;
}
5 5
number % 10 reverse = reverse * 10 + number % 10
number = number / 10;
reverse = 0;while (number > 0)
int reverse , lastDigit;
int intReverse (int number){
}
Example: Reverse a Number
1 2 43 7 6
number reverse
reverse = 0;while (number > 0){ lastDigit = number % 10; reverse = reverse * 10 + lastDigit; number = number / 10;}
5
Example: Play Games// initialize global state variables// such as statisticsboolean userQuits = false;do{ // initialize state variables for one game // such as number of guesses allowed boolean thisRoundIsOver = false; do { // get user input // process input, update states // determine if thisRoundIsOver // userQuits implies thisRoundIsOver } while ( !thisRoundIsOver ); // update statistics of preceding game} while ( !userQuits );// report total statistics
Using Break: Loop-and-a-Half Idiom
Initialize total to zeroInitialize counter to zero
While (true) { Input next grade (possibly the sentinel) If ( the user has entered the sentinel) break;
Add this grade into the running total Add one to the grade counter }
If the counter is not equal to zero Set the average to the total divided by the counter Print the averageElse Print “No grades were entered”
Initialize total to zeroInitialize counter to zero
Input the first grade (possibly the sentinel)
While (grade != sentinel) { Add this grade into the running total Add one to the grade counter Input next grad (possibly the sentinel) }
If the counter is not equal to zero Set the average to the total divided by the counter Print the averageElse Print “No grades were entered”
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Exercise : AverageGrade
• Write a program to compute the average of a sequence of (numerical) student grades entered by the user– If the user types something else than a
number between 0.0 and 4.0, the program should abort.
Exercise : Reverse a Number
• Write a program that outputs a reverse of the positive integers the user types.– If the user types something else, the program
should abort.
25
Exercise : PalindromeTester
• Write a program to read in a sequence of strings; for each string, determine whether it is a palindrome
Exercise : Stars
• Write a program to print a triangle formed by *
*
**
***
The program should read in the number of rows from the user; the row should be between 1 to 10
Methods
Methods
• A useful program can be long and contains many statements
• A method groups a sequence of statements and should provide a well-defined, easy-to-understand functionality
– a method takes input, performs actions, and produces output
• Recall: In Java, each method is defined within specific class
Method Declaration: Header
• A method declaration begins with a method header
methodmethodnamename
returnreturntypetype
parameter listparameter list
The parameter list specifies the typeThe parameter list specifies the typeand name of each parameterand name of each parameter
The name of a parameter in the methodThe name of a parameter in the methoddeclaration is called a declaration is called a formal argumentformal argument
class MyClass { static int min ( int num1, int num2 )
…
propertiesproperties
Method Declaration: Body
The header is followed by the method body:
static int min(int num1, int num2){ int minValue = num1 < num2 ? num1 : num2; return minValue;}
class MyClass{
…
…
}
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The return Statement
• The return type of a method indicates the type of value that the method sends back to the calling location– A method that does not return a value has a void return type
• The return statement specifies the value that will be returned– Its expression must conform to the return type
Calling a Method• Each time a method is called, the values of the actual
arguments in the invocation are assigned to the formal arguments
static int min (int num1, int num2)
{ int minValue = (num1 < num2 ? num1 : num2); return minValue;}
int num = min (2, 3);
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Method Overloading
• A class may define multiple methods with the same name---this is called method overloading– usually perform the same task on different data types
• Example: The PrintStream class defines multiple println methods, i.e., println is overloaded:
println (String s) println (int i) println (double d)
…• The following lines use the System.out.print method for
different data types:
System.out.println ("The total is:"); double total = 0; System.out.println (total);
34
Method Overloading: Signature
• The compiler must be able to determine which version of the method is being invoked
• This is by analyzing the parameters, which form the signature of a method– the signature includes the type and order of the
parameters• if multiple methods match a method call, the compiler picks the
best match• if none matches exactly but some implicit conversion can be
done to match a method, then the method is invoke with implicit conversion.
– the return type of the method is not part of the signature
Method Overloading
double tryMe (int x){ return x + .375;}
Version 1Version 1
double tryMe (int x, double y){ return x * y;}
Version 2Version 2
result = tryMe (25, 4.32)
InvocationInvocation
More Examples
double tryMe ( int x ) { return x + 5;}
double tryMe ( double x ) { return x * .375;}
double tryMe (double x, int y){ return x + y;}
tryMe( 1 );
tryMe( 1.0 );
tryMe( 1.0, 2);
tryMe( 1, 2);
tryMe( 1.0, 2.0);
Which tryMe will be called?
Variable Scoping
Three variable types
• There are can be three types of variables in a method– local variables
• those declared in the method
– formal arguments
– class variables• those defined in the class but not in the method
Example of Variable Typespublic class Box{ private int length, width; …
public int widen (int extra_width) {
private int temp1;size += extra_width;…
} public int lenghten (int extra_lenth) {
private int temp2;size += extra_length;…
} …
}
• class variables• formal arguments• local variables
Scope of Variables• Class variables are
valid in all methods of the class
• A formal argument is valid within its method
• Local variables are valid from the point of declaration to the end of the enclosing block
public class Box{ private int length, width; …
public int widen (int extra_width) {
private int temp1;size += extra_width;…
} public int lenghten (int extra_lenth) {
private int temp2;size += extra_length;…
} …
}
Two Types ofParameter Passing
• If a modification of the formal argument has no effect on the actual argument,– it is call by value
• If a modification of the formal argument can change the value of the actual argument,– it is call by reference
Call-By-Value and Call-By-Reference in Java
• Depend on the type of the formal argument
• If a formal argument is a primitive data type, a modification on the formal argument has no effect on the actual argument– this is call by value, e.g. num1 = min(2, 3);
num2 = min(x, y);
• If a formal argument is not a primitive data type, an operation on the formal argument can change the actual argument– this is call by reference– more discussion in the later part of the course…