Iteration Statements in java

Java’s iteration statements are for, while, and do-while. These statements create what we commonly call loops

while

The while loop is Java’s most fundamental loop statement. It repeats a statement or block while its controlling expression is true

while(condition) {
// body of loop
}

The condition can be any Boolean expression. The body of the loop will be executed as long as the conditional expression is true. When condition becomes false, control passes to the next line of code immediately following the loop. The curly braces are unnecessary if only a single statement is being repeated.

Demonstrate the while loop

class While {
public static void main(String args[]) {
int n = 10;
while(n > 0) {
System.out.println("tick " + n);
n--;
}
}
}

Output

tick 10
tick 9
tick 8
tick 7
tick 6
tick 5
tick 4
tick 3
tick 2
tick 1

do-while

The do-while loop always executes its body at least once, because its conditional expression is at the bottom of the loop

do {
// body of loop
} while (condition);

Each iteration of the do-while loop first executes the body of the loop and then evaluates the conditional expression. If this expression is true, the loop will repeat. Otherwise, the loop terminates. As with all of Java’s loops, condition must be a Boolean expression

Demonstrate the do-while loop

class DoWhile {
public static void main(String args[]) {
int n = 10;
do {
System.out.println("tick " + n);
n--;
} while(n > 0);
}
}

Using a do-while to process a menu selection

class Menu {
public static void main(String args[])
throws java.io.IOException {
char choice;
do {
System.out.println("Help on:");
System.out.println(" 1. if");
System.out.println(" 2. switch");
System.out.println(" 3. while");
System.out.println(" 4. do-while");
System.out.println(" 5. for\n");
System.out.println("Choose one:");
choice = (char) System.in.read();
} while( choice < '1' || choice > '5');
System.out.println("\n");
switch(choice) {
case '1':
System.out.println("The if:\n");
System.out.println("if(condition) statement;");
System.out.println("else statement;");
break;
case '2':
System.out.println("The switch:\n");
System.out.println("switch(expression) {");
System.out.println(" case constant:");
System.out.println(" statement sequence");
System.out.println(" break;");
System.out.println(" // ...");
System.out.println("}");
break;
case '3':
System.out.println("The while:\n");
System.out.println("while(condition) statement;");
break;
case '4':
System.out.println("The do-while:\n");
System.out.println("do {");
System.out.println(" statement;");
System.out.println("} while (condition);");
break;
case '5':
System.out.println("The for:\n");
System.out.print("for(init; condition; iteration)");
System.out.println(" statement;");
break;
}
}
}

Output

Help on:

  1. if
  2. switch
  3. while
  4. do-while
  5. for
    Choose one:
    4
    The do-while:
    do {
    statement;
    } while (condition);

for

The for loop operates as follows. When the loop first starts, the initialization portion of the loop is executed. Generally, this is an expression that sets the value of the loop control variable, which acts as a counter that controls the loop. It is important to understand that the initialization expression is only executed once. Next, condition is evaluated. This must be a Boolean expression. It usually tests the loop control variable against a target value. If this expression is true, then the body of the loop is executed. If it is false, the loop terminates. Next, the iteration portion of the loop is executed. This is usually an expression that increments or decrements the loop control variable. The loop then iterates, first evaluating the conditional expression, then executing the body of the loop, and then executing the iteration expression with each pass. This process repeats until the controlling expression is false.

Example

class ForTick {
public static void main(String args[]) {
int n;
for(n=10; n>0; n--)
System.out.println("tick " + n);
}
}

for loop Using the Comma

class Comma {
public static void main(String args[]) {
int a, b;
for(a=1, b=4; a<b; a++, b--) {
System.out.println("a = " + a);
System.out.println("b = " + b);
}
}
}

Output

a = 1
b = 4
a = 2
b = 3

Nested Loops

Loops may be nested.

class Nested {
public static void main(String args[]) {
int i, j;
for(i=0; i<10; i++) {
for(j=i; j<10; j++)
System.out.print(".");
System.out.println();
}
}
}

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