L EC. 03: C ONTROL STATEMENTS Fall. 2014 0 Java Programming.
-
Upload
julianna-williamson -
Category
Documents
-
view
215 -
download
0
Transcript of L EC. 03: C ONTROL STATEMENTS Fall. 2014 0 Java Programming.
Java Program
ming
1
LEC. 03: CONTROL STATEMENTS
Fall. 2014
2
Java Program
ming
CONTENT
Using System.in object to input data Definition of Java statements Categories of flow control statements Condition constructs: if and switch Iteration constructs: for, while and do-while Nested constructs Special control instructions: break and continue
Fall. 2014
3
Java Program
ming
A SIMPLE WAY FOR INPUTTING DATA
System.in is an object representing the input stream associated with the console input, i.e. the keyboard.
When it receives the read() message, it waits until the user presses a key and then returns the result to the message sender. Browsing the methods defined in java.io.InputStream class!
The character returned from read() is an int, so it must be cast into a char to assign it to a char variable.
Fall. 2014
4
Java Program
ming
EXAMPLE OF USING System.in// Read a character from the keyboard.
class KbIn {
public static void main(String args[])
throws java.io.IOException {
char ch;
for(;;) {
System.out.print("Press a key followed by ENTER: ");
ch = (char) System.in.read(); // get a char
System.out.println("Your key is: " + ch);
if(ch == ‘z’) break;
}
}
}
Fall. 2014
This declaration will be discussed later!
DO NOT remove it, otherwise, you will get
error message.
5
Java Program
ming
JAVA STATEMENTS
A Java statement forms a complete unit of execution. Each statement is terminated by a semicolon (;). Types of statements
Expression statements Declaration statements Control flow statements (the major topic of this lecture)
Fall. 2014
6
Java Program
ming
EXPRESSION STATEMENTS
A expression statement is one of the following expressions. Assignment expressions Any use of ++ or -- Method invocations Object creation expressions
Examples val = 3.2; ++x; System.out.println(); new Integer(3);
Fall. 2014
7
Java Program
ming
DECLARATION STATEMENTS
A declaration statement declares a variable. Example
Double obj; double dVal;
Fall. 2014
8
Java Program
ming
CATEGORIES OF FLOW CONTROL Sequential
Sequential is the flow type where instructions are executed one after another, from top to bottom.
Selection Selection is the flow type where one path out of a number of possibilities is
taken. Iteration
Iteration is the flow type where one or more instructions is executed repeatedly if certain conditions are fulfilled.
Transfer Transfer is the flow type where the point of execution jumps to a different
point in the program. Using transfer is considered as a poor programming style and makes the code
maintenance difficult. Java only supports the forward transfer, which transfer the execution point to
a point beyond the current execution point. Sometimes forward transfer makes the code less complex.
Fall. 2014
9
Java Program
ming
if-CONSTRUCT Syntax form
if(<condition>) statementelse statement
if and else parts control a single statement. If more than one statement needed to be controlled, use code block.
The else part is optional. The else part must be associated with an if-part if it exists.
The condition expression must be a boolean expression. If the conditional expression is true, the target of the if will be
executed; otherwise, if it exists, the target of the else will be executed.
Fall. 2014
if( i > 0 ) a++; // this if statement is terminated here b++; // since the target of if statement is a single statement // if( i > 0 ) statement is terminated here else c++; // no if part to match, so it’s illegal
10
Java Program
ming
FLOWCHART FOR if-CONSTRUCT
Fall. 2014
<condition>
Statement
true
Next statement
false
Statement
11
Java Program
ming
A GUESSING GAME
// Guess the letter game.class Guess { public static void main(String args[]) throws java.io.IOException { char ch, answer = 'K'; System.out.println("I'm thinking of a letter between
A and Z."); System.out.print("Can you guess it: "); // read a char from the keyboard ch = (char) System.in.read(); if(ch == answer) System.out.println("** Right **"); else System.out.println("...Sorry, you're wrong."); }}
Fall. 2014
12
Java Program
ming
NESTED if-CONSTRUCT
A nested if-construct is an if statement that is the target of another if or else.
An else statement always refers to the nearest if statement that is within the same block as the else and not already associated with an else.
Fall. 2014
13
Java Program
ming
EXAMPLES OF NESTED IF-CONSTRUCT
Example 1
Example 2
Fall. 2014
if(i == 10) { if(j < 20) a = b; if(k > 100) c = d; else a = c; // this else refers to if(k > 100)}else a = d; // this else refers to if(i == 10)
if(i == 10) { if(j < 20) { a = b; if(k > 100) c = d; } else a = c; // this else refers to if(j<20)}else a = d; // this else refers to if(i == 10)
Java’s syntax is free of position!Using proper indentation makes
programs clearer!
14
Java Program
ming
switch-CONSTRUCT
The switch-construct enables a program to select among several alternatives.
Syntax form
Fall. 2014
switch(<expression>) { case <constant1>: statement sequence break; case <constant2>: statement sequence break; case <constant3>: statement sequence break; … default: statement sequence}
15
Java Program
ming
Executing switch-CONSTRUCT
Evaluating <expression> and comparing the result sequentially, from top to bottom, with constants following case-clauses.
When a match is found, the statements associated with that case are executed until the break is encountered or, in the case of default or the last case, until the end of the switch is reached.
The default statement sequence, if it exists, is executed if no case constant matches the expression. The default-clause is optional. If default-clause is not present, no action takes place if all
matches fail.
Fall. 2014
16
Java Program
ming
SYNTAX RULES FOR switch-CONSTRUCT
Prior to JDK 7, the <expression> controlling the switch must be of type byte, short, int, char, or enum.
Beginning with JDK 7, <expression> can also be of type String.
Each value specified in the case clauses must be a unique constant expression. A constant expression is a expression which can be evaluated
during the compiling time. The type of each value specified in the case clauses must
be compatible with the type of <expression> .
Fall. 2014
17
Java Program
ming
DEMO SEGMENT FOR switch-CONSTRUCT switch (month) { case 1: case 3: case 5: case 7: case 8: case 10: case 12 : System.out.println(“There are 31
days"); break; case 2: System.out.println(“There are 28 or
29 days"); break; case 4: case 6: case 9: case 11: System.out.println(“There are 30
days"); break; default: System.out.println("Invalid month."); }
Fall. 2014
18
Java Program
ming
LOOPING
Java provides four types of looping (iteration) statements Traditional for while do-while Enhanced for
Supported after Java 2 version 1.5
All looping have 4 parts Initialization Iteration condition Body Termination condition
Fall. 2014
TRADITIONAL for-CONSTRUCT
Fall. 2014
19
Java Program
ming
Syntax of while-statement for ( <init_exp> ; <test_exp> ; <post_exp> ) single-statement;
Test_ exp
statement
true
Next statement
false
init-_exp
post-_exp
Example
int sum = 0;
for(int count=0; count<=n; ++count ) {
sum += count;
} // count does not exist after this point
20
Java Program
ming
MORE ON for-CONSTRUCT
The type of control variable can be any numeric types and char.
The loop control variable can be modified by any amount in <post-exp>.
It is a pre-condition looping. The <post-exp> is tested at the beginning of each loop. The code inside the loop may not be executed at all.
The <init-exp> and <post-exp> may consist of more than one expression separated by comma.
Fall. 2014
int i, j;for(i=0, j=i+10; i < j; i++, j--) System.out.println("i and j: " + i + " " + j);
21
Java Program
ming
MORE ON for-CONSTRUCT
The <test-exp> must be a boolean expression. It does not need to involve the loop control variable.
All of the <init-exp> , <test-exp> and <post-exp> are option, but all three semicolons are not.
Missing <test-exp> means the result of <test-exp> is always true. The simplest for-construct for(;;); will perform nothing for
ever. When you declare a variable inside a for loop, the scope
of that variable ends when the for statement does.
Fall. 2014
for( int i = 0; i < 100; i++) sum += i;System.out.print(sum / i); // illegal since i is not declared
22
Java Program
ming
while-CONSTRUCT
Fall. 2014 Syntax of while statement
while ( <condition> ) single-statement;
<condition>
statement
true
Next statement
false
Example
int sum = 0;int count = 0;while ( count <= n ) { sum += count; count++;}
The while statement is a pre-test looping.
23
Java Program
ming
do-while-CONSTRUCT
Fall. 2014 Syntax of do-while statement
do single-statement; while ( <condition> );
<condition>
statement
true
Next statement
false
Example
int sum = 0;int count = 0;if ( count <= n ) do { sum += count; count++; } while ( count <= n );
The do-while statement is a post-test looping.
24
Java Program
ming
break STATEMENT
The break statement is used to force an immediate exit from a loop enclosing the statement, bypassing any remaining code in the body of the loop and the loop’s conditional test.
The break statement can be only used within a loop body or the body of switch-construct.
Fall. 2014
25
Java Program
ming
DEMO PROGRAM FOR BREAK STATEMENT
// Using break with nested loops.class Break3 { public static void main(String args[]) { for(int i=0; i<3; i++) { System.out.println("Outer loop count: " + i);
System.out.print(" Inner loop count: ");
int t = 0; while(t < 100) { if(t == 10) break;//terminate while loop
if t is 10 System.out.print(t + " "); t++; } System.out.println(); } System.out.println("Loops complete."); }}
Fall. 2014
26
Java Program
ming
break STATEMENT WITH LABEL The break statement with label can be used to terminate the execution
of a code block. The code block terminated by using a break statement with label does not
need to be the body of loop or switch. Syntax form for break statement with label
break <label>; A label is the identifier used to identify a code block or a statement. Syntax form for labeling code block
<label>: { <statement sequences> } When this form of break executes, control is transferred out of the
named code block or statement. The labeled code block or statement must enclose the break statement,
but it does not need to be the immediately enclosing block. This means that you can use a labeled break statement to exit from a set of
nested blocks.
Fall. 2014
27
Java Program
ming
DEMO PROGRAM FOR break STATEMENT WITH LABEL // Using break with a label.class Break4 { public static void main(String args[]) { int i; for(i=1; i<4; i++) { one: { two: { three: { System.out.println("\ni is " + i); if(i==1) break one; if(i==2) break two; if(i==3) break three; System.out.println("won't print"); // this is never reached } System.out.println("After block three."); } System.out.println("After block two."); } System.out.println("After block one."); } System.out.println("After for."); }}
Fall. 2014
28
Java Program
ming
TRY THIS …class Break6 { public static void main(String args[]) { int x=0, y=0; // here, put label before for statement. stop1: for(x=0; x < 5; x++) { for(y = 0; y < 5; y++) { if(y == 2) break stop1; // stop the for loop System.out.println("x and y: " + x + " " + y); } } System.out.println(); // now, put label immediately before { for(x=0; x < 5; x++){ stop2: { for(y = 0; y < 5; y++) { if(y == 2) break stop2; // stop the body of for, not for System.out.println("x and y: " + x + " " + y); } } } }}
Fall. 2014
29
Java Program
ming
continue-CONSTRUCT
The continue statement forces the next iteration of the loop to take place, skipping any code between itself and the conditional expression that controls the loop.
In while and do-while loops, a continue statement will cause control to continues the loop by directly evaluating the <test-exp>.
In the case of the for, the <post-exp> of the loop is evaluated, then the loop continues by evaluating the <test-exp>.
Fall. 2014
30
Java Program
ming
DEMO PROGRAM FOR continue-CONSTRUCT
// Use continue.
class ContDemo {
public static void main(String args[]) {
int i;
// print even numbers between 0 and 100
for(i = 0; i<=100; i++) {
if((i%2) != 0) continue;
// skipping the println and
// continuing the next loop
System.out.println(i);
}
}
}
Fall. 2014
The boolean expression is true for
odd number.
31
Java Program
ming
continue STATEMENT WITH LABEL
The continue may specify a label to describe which enclosing loop to continue.
The statement labeled by a label used with continue statement must be a loop statement.
Fall. 2014
32
Java Program
ming
DEMO PROGRAM FOR continue STATEMENT WITH LABEL
// Use continue with a label.
class ContToLabel {
public static void main(String args[]) {
outerloop:
for(int i=1; i < 10; i++) {
System.out.print("\nOuter loop pass " + i + ", Inner loop: ");
for(int j = 1; j < 10; j++) {
if(j == 5) continue outerloop;
// continue outer loop
System.out.print(j);
}
}
}
}
Fall. 2014