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REPETITION CONTROL STRUCTURES - LOOPS (do-while, for)

TOPICS

· The do-while Statement: Definition, Syntax, Terminology, Examples

· do-while vs. while Comparison

· The for Statement: Definition, Syntax, Terminology, Examples

· The break and continue Statements.

· Choosing the most appropriate looping statement for a given problem.

· Additional Java Programming Concepts: Generating Random Numbers

· Exercises and Sample Programs

OUTLINE

1. The do-while statement

Looping control structure in which the loop condition/test/expression is tested at the end (or bottom of the loop). Executes statements repeatedly while a condition is true, testing it at the end of each iteration.
Similar to a while loop, except that its body statement(s) will always execute the first time, regardless of whether the condition is true or false. In other words, the format of a do-while guarantees that the loop body executes at least once.
Syntax:

do{
    <statement>
    <statement>
    ... //loop body
    ...
} while(Expression);

Note1: The loop body can be a single statement or a block.
Note2: do-while ends with a semicolon (;).
How it works:

The statements between do and while are executed as long as Expression still has the value true at the end of the loop.
When the Expression is tested and found to be false, the loop is exited and control passes to the statement that follows the do-while statement.

$Description: Description: Description: C:\Courses-NOW\Webpage\236\LectureNotes\L7-DoWhile.JPG$

Example 1:

       ...
   int counter = 1;
   do {
       System.out.print(counter + " ");
       counter++;
   } while(counter <= 10);
   System.out.println();

Example 2: Input controls

//Some HR application: employee info
//Input validation with do-while
...
Scanner input = new Scanner(System.in);
char gender;
int age;
...
do {
    System.out.print("Enter age: ");
    age = input.nextInt();
    if(age <= 16)
        System.out.print("\nInvalid! ");
} while(age <= 0);

do {
    System.out.print("Enter gender(M/F): ");
    gender = input.next().charAt(0); ;
    if(gender != 'M' && gender != 'F')
        System.out.print("\nInvalid, should be M or F. ");
} while(gender != 'M' && gender != 'F');
...

Note: A do-while statement does not require priming read, so the input steps don't appear twice (once before the loop, once inside the do-while body, at the end of the loop), but it does test the input value twice.

· do-while loop vs. while loop


DO-WHILE (post-test loop)	WHILE (pre-test loop)
POST-TEST loop (exit-condition)	PRE-TEST loop (entry-condition)
The looping condition is tested AFTER executing the loop body.	The looping condition is tested BEFORE executing the loop body.
Loop body is always executed at least once.	Loop body may not be executed at all.

Example 1: Comparing a while loop with a do-while loop

while loop:

do-while loop:

int age;
Scanner input = new Scanner(System.in);

System.out.print("Enter age: ");
age = input.nextInt();
while(age <= 0){
   System.out.println("Must be positive. ");
   System.out.print(" Enter age: ");
   age = input.nextInt();
}

int age;
Scanner input = new Scanner(System.in);

do{
   System.out.print("Enter age: ");
   age = input.nextInt();
   if(age <= 0)
       System.out.println("Must be positive! ");
} while(age <= 0);

Example 2: Comparing a while loop with a do-while loop to sum all integers between 1 and n (input from user)


while loop:	do-while loop:
// ... Scanner input = new Scanner(System.in); int sum = 0, n; int count = 1; n = input.nextInt(); while (count <= n){ sum = sum + count; count++; } System.out.println("sum = " + sum);	// ... Scanner input = new Scanner(System.in); int sum = 0, n; int count = 1; n = input.nextInt(); do{ sum = sum + count; count++; } while(count <=n); System.out.println("sum = " + sum);

Note: The output may not be the same for all values of n. If n > 0, both versions have the same output. But, if n <= 0, the 2 loops give different outputs. With while,sum = 0 (loop body never entered). With do-while,sum = 1 (the body executes once and the condition is tested after).

2. The for statement

Simplified count-controlled loops - a compact and flexible notation for a while loop with a counter (definite loop).
Syntax:

for(initialization;test expression; update){
0 or more statements to repeat
}

Equivalent to:

initialization;
while(test expression){
0 or more statements to repeat
update;
}

The for loop contains

An initialization
An expression to test for continuing - the while condition
An update to execute after each iteration of the body - increments or decrements the loop control variable.

The advantage of the for statement is that everything you need to know to understand how many times the loop will run is explicitly included in the header line.

Example 1:

int number;
for(number = 1; number <= 10; number++){
System.out.println("The number is: " + number);
}
System.out.println("Done");

Note: When the loop control condition is evaluated and has value false, the loop is said to be satisfied and control passes to the statement following the for statement.

Example 2:

int number;
for(number = 10; number >= 1; number--){
System.out.println("The number is: " + number);
}
System.out.println("Done");

Example 3:

int sum = 0;
for(int n = 1; n <= 10; n++){
sum = sum + n;
}
System.out.println("Sum is: " + sum);

Equivalent to:

int sum = 0;
int n = 1;
while(n <= 10){
sum = sum + n;
n++;
}
System.out.println("Sum is: " + sum);

Be extra careful with these:

When writing a count-controlled loop with a for statement, don't overlook the fact that the loop control variable is incremented automatically (updated), so don't try to increment it again within the loop body.
for( ; ; ) is an infinite loop.
Initialization can be a declaration with initialization. Example:

for(int i = 1; i <= 10; i++)
System.out.println("Something ");

Note: iis a local variable whose scope will extend only to the end of the for statement. Being a local variable,iis inaccessible outside the for statement.

Like while loops, for loops can be nested. What is the output?

int i, j;
for(i = 1; i <= 5; i++){
for(j = 1; j <= i; j++)
System.out.println(j);
}

Comma can be used to allow more than one expression. Example:

int x, y;
for (x = 0, y = 0; x + y < 1000; x++, y++)
System.out.println("The sum is: " + (x + y));

Don't use semicolon after a for statement, unless you want a loop with a null body.

3. The break and continue statements

Both break and continue statements alter the flow of control. Use them with caution.
The break statement, when used in a while, do-while, for, or switch structure, causes immediate exit from that structure.

The execution of the program will continue with the first statement after the structure.
It is mostly used with switch statements, but can be also used to exit early from a loop.
If the condition is met, the loop is exited immediately.

Example:

   // Using the break statement in a for structure
   for (int x = 1; x <= 10; x++ ) {
      if ( x == 5 )
         break;    // break loop only if x is 5
      System.out.print(x + " ");
   }
   System.out.println();
   System.out.println("Broke out of loop at x = " + x );

Note: If break is located inside nested structures, control exits only the innermost structure containing it.

The continue statement, when used in a while,do-while, or forstructure, skips the remaining statements in the body of the structure.

The execution of the program will continue with the next iteration of the loop. In while and do-while structures, the loop test is evaluated immediately after the continue statement is executed. In a for structure, the increment is executed, then the loop test is evaluated.

Example:

// Using the continue statement in a for structure
int temp;
for (int x = 1; x <= 10; x++) {
      if (x == 5) {
         temp = x;
         continue; // skip remaining code in loop for x=5
      }
      System.out.print(x + " ");
}
System.out.println();
System.out.println("Used continue to skip printing the value " + temp);

Note: continue terminates the current loop iteration, NOT the entire loop

4. Some guidelines for choosing a looping statement:

Simple count-controlled loop ==> use for
Event-controlled loop whose body has to be executed at least once ==> use do-while
Event-controlled loop, first execution unknown ==> use while
When in doubt ==> use while
Avoid infinite loops with break statements ==> better to use while, do-while or for.

5. Generating random numbers

Java provides several mechanisms for obtaining pseudorandom numbers (seem to be random, but they are produced by an algoriyhm).

Option 1: Call the random method from the Math class --> obtain a random value of type double with the property 0.0 <= Math.random() < 1.0. This option provides a quick and easy way to get a random number that can be used in combination with multiplication to change the range of values.
Option 2: Objects of the class Random generate pseudorandom numbers. The class Random is found in the java.util package, so you need to include in your program the statement:

        import java.util.*;
Examples:
        Random rand = new Random();
        int randNumber = rand.nextInt(10); // randNumber has a random value between 0 and 9


Method Name	Description
nextInt()	Returns a random integer
nextInt(MAX)	Returns a random integer x; 0 <= x <= MAX - 1
nextDouble()	Returns a random real number x; 0.0 <= x < 1.0
nextBoolean()	Returns a random logical value (true OR false)

Common usage: to generate random integer numbers in a range. NOTE: use class constants for the MIN and MAX values.

1. Generate a random number x, in range [0, MAX] (0 <= x <= MAX):
int n = rand.nextInt(MAX + 1);
Examples:

A random integer between 0 and 20, inclusive: int n = rand.nextInt(21);
A random integer between 0 and 100, inclusive: int n = rand.nextInt(101);

2. Generate a random number x, in range [MIN, MAX] (MIN <= x <= MAX):
int n = rand.nextInt(MAX - MIN + 1) + MIN;
Examples:

A random integer between 1 and 20, inclusive: int n = rand.nextInt(20) + 1;
A random integer between 10 and 100, inclusive: int n = rand.nextInt(91) + 10;
A random integer between 1 and 100, inclusive: int n = rand.nextInt(100) + 1;
A random integer between 5 and 10, inclusive: int n = rand.nextInt(6) + 5;
A random integer between 7 and 63, inclusive: int n = rand.nextInt(57) + 7;
NOTE: Check the Dice program at the end of these notes for a sample Java program generating random numbers.

7. Some Exercises:

1. What is the output?

for (int i = 7; i > 3; i--)
System.out.print(i + " ");
System.out.println();

2. What is the output?

for (int i = 1; i < 10; i = i + 2)
System.out.print(i + " ");

3. What is the output?

int sum = 0;
for (int i = 1; i < 6; i++){
sum = sum + i;
System.out.println("Sum = " + sum);
}

4. What is the output now?

int sum = 0;
for (int i = 1; i < 6; i++)
sum = sum + i;
System.out.println("Sum = " + sum);

5. What is the output?

int x = 1;
for (int i = 1; i < 6; i++) {
System.out.print(x + " ");
x = 2 * x + 1;
}
System.out.println(x);

6. Find the errors:

1. for (j == 0, j < n, j ++) {...

2. for (j < n; j --);
     System.out.println(j);

3. for (int j = 7; j > 3; j--)
     k = k + j;
     System.out.println(j + " " + k);

7. What is the output?

int i, j;
for (i = 1 ; i <= 3 ; i++ ){
     System.out.print(i + " ");
     for (j = 1 ; j <= 5 ; j++ )
          System.out.print(j + " ");
     System.out.println();
}

8. What is the output?

//LECTURE #7: EXAMPLE #8
import java.util.*;
public class Ex8{
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int how_many;
        System.out.print("How many rows/columns box(>= 5): ");
        how_many = input.nextInt();
        while(how_many < 5) {
            System.out.print("ERROR! Should be >= 5. Reenter: ");
            how_many = input.nextInt();
        }
        for (int row = 1; row <= how_many; row++) {
            for (int column = 1; column <= how_many; column++) {
                System.out.print("*");
            }
            System.out.println();
        }
    }
}
OUTPUT:
How many rows/columns box(>= 5): 3
ERROR! Should be >= 5. Reenter: 6
******
******
******
******
******
******

9. What is the output?

//LECTURE #7: EXAMPLE #9
import java.util.*;
public class Ex9{
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int how_many;
        char what;
        System.out.print("How many rows/columns box(>= 5): ");
        how_many = input.nextInt();
        while(how_many < 5) {
            System.out.print("ERROR! Should be >= 5. Reenter: ");
            how_many = input.nextInt();
        }
        for (int row = 1; row <= how_many; row++) {
                if(row % 2 == 0)
                    what = '+';
                else
                    what = '-';
            for (int column = 1; column <= how_many; column++) {
                System.out.print(what);
            }
            System.out.println();
        }
    }
}
OUTPUT:
How many rows/columns box(>= 5): 4
ERROR! Should be >= 5. Reenter: 6
------
++++++
------
++++++
------
++++++

Change the program so that '+' and '-' alternate /each row.

10. What is the output?

//LECTURE #7: EXAMPLE #10
public class Ex10 {
    public static final int MAX = 5;
    public static void main(String[] args) {
        for (int row = 1; row <= MAX; row++) {
            for (int column = 1; column <= MAX; column++) {
                System.out.print("|");
                for (int dashes = 1; dashes <= 2; dashes++) {
                    System.out.print("_");
                }
            }
            System.out.println("|");
        }
    }
}
OUTPUT:
|__|__|__|__|__|
|__|__|__|__|__|
|__|__|__|__|__|
|__|__|__|__|__|
|__|__|__|__|__|

11. What is the output?

//LECTURE #7: EXAMPLE #11
public class Ex11 {
    public static void main(String[] args) {
        int j,k;
        int n = 0;
        for (j = 0; j < 6; j++) {
            System.out.println("j = " + j);
            for (k = 0; k <j; k++) {
                System.out.print("\tk = " + k + " ");
                n++;
                System.out.print("\tn = " + n + "\n ");
            }
        }
    }
}
OUTPUT:
j = 0
j = 1
    k = 0 n = 1
j = 2
    k = 0 n = 2
    k = 1 n = 3
j = 3
    k = 0 n = 4
    k = 1 n = 5
    k = 2 n = 6
j = 4
    k = 0 n = 7
    k = 1 n = 8
    k = 2 n = 9
    k = 3 n = 10
j = 5
    k = 0 n = 11
    k = 1 n = 12
    k = 2 n = 13
    k = 3 n = 14
    k = 4 n = 15

12. What is the output?

//LECTURE #7: EXAMPLE #12
//TEST FOR break
public class Ex12 {
    public static void main(String[] args) {
        int j,k;
        int n = 0;
        for (j = 0; j < 6; j++) {
            System.out.println("j = " + j);
            for (k = 0; k <j; k++) {
                System.out.print("\tk = " + k + " ");
                if(j == 3) {
                    System.out.print("\tbreak used here\n");
                    break;
                }
                n++;
                System.out.print("\tn = " + n + "\n");
            }
        }
        System.out.println();
    }
}
OUTPUT:
j = 0
j = 1
     k = 0 n = 1
j = 2
     k = 0 n = 2
     k = 1 n = 3
j = 3
     k = 0 break used here
j = 4
     k = 0 n = 4
     k = 1 n = 5
     k = 2 n = 6
     k = 3 n = 7
j = 5
     k = 0 n = 8
     k = 1 n = 9
     k = 2 n = 10
     k = 3 n = 11
     k = 4 n = 12

13. Example of a Java program using a switch nested in a while loop. Change the loop from while to do-while.

//LECTURE #7: EXAMPLE #13
//Counting letter grades (switch)
import java.util.*;
public class Ex13 {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        char grade;
        int countA = 0; // number of A's
        int countB = 0; // number of B's
        int countC = 0; // number of C's
        int countD = 0; // number of D's
        int countF = 0; // number of F's

        System.out.print("Enter the letter grades (A,B,C,D,F; S to stop): ");
        grade = input.next().charAt(0); //initialize: priming read
        while(grade != 'S' && grade != 's'){ //test sentinel
            switch (grade){
                case 'A':
                case 'a':
                    ++countA;
                    break;
                case 'B':
                case 'b':
                    ++countB;
                    break;
                case 'C':
                case 'c':

++countC;

                    break;
                case 'D':
                case 'd':
                    ++countD;
                    break;
                case 'F':
                case 'f':
                    ++countF;
                    break;

default:

                    System.out.println("Incorrect letter grade entered!");
           }
           System.out.print("Enter the letter grades (A,B,C,D,F; S to stop): )";
           grade = input.next().charAt(0);//update: next read
       }
       System.out.println("\nTotals for each letter grade are: ");
       System.out.println("A: " + countA);
       System.out.println("B: " + countB);
       System.out.println("C: " + countC);
       System.out.println("D: " + countD);
       System.out.println("F: " + countF);
    }
}

14. Example of a Java program using random numbers.

//LECTURE #7: EXAMPLE #14
//Random numbers
import java.util.*;

public class Dice {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        Random rand = new Random();
        int user_number;
        int count = 1;
        int dice_number;
        System.out.println("Choose a number between 1 and 6 ");
        System.out.print("I will tell you after how many die rolls it will come up: ");
        user_number = input.nextInt();
        while(user_number < 1 || user_number > 6) {
          System.out.print("ERROR! Out of range. Reenter: ");
          user_number = input.nextInt();
        }
        do{
          dice_number = rand.nextInt(6) + 1;
          System.out.println("Die roll # " + count + " --- Die number: " + dice_number);
          count++;
        }while(dice_number != user_number);
        System.out.println("Your number came up after " + (count - 1) + " die rolls");
    }
}
OUTPUT:
Choose a number between 1 and 6
I will tell you after how many die rolls it will come up: 8
ERROR! Out of range. Reenter: 7
ERROR! Out of range. Reenter: 6
Die roll # 1 --- Die number: 4
Die roll # 2 --- Die number: 3
Die roll # 3 --- Die number: 2
Die roll # 4 --- Die number: 5
Die roll # 5 --- Die number: 4
Die roll # 6 --- Die number: 2
Die roll # 7 --- Die number: 3
Die roll # 8 --- Die number: 6
Your number came up after 8 die rolls

References:
[1] Building Java Programs: A Back to Basics Approach, by Stuart Reges, and Marty Stepp, Addison Wesley, 2008.
[2] Java Programming: From Problem Analysis to Program Design, by D.S. Malik, Thomson Course Technology, 2008