Big Java / Java Concepts Lab 7

Simple Loops

1.

It is often necessary to repeat a portion of code several times in a program. A simple loop can automate the repetition. Here is a program that computes the number of digits needed to represent a number in base 10.

/**
   Count number of digits needed to express an integer in base 10
   using multiple if statements.
*/
public static void main(String[] args)
{
   Scanner in = new Scanner(System.in);
   System.out.print("Input an integer between 1 and 9999: ");
   int n = in.nextInt();

   if (n < 1 || n > 9999) return;

   int temp = n;
   int d = 1;

   if (temp > 9)
   {
      temp = temp / 10;
      d++;
   }

   if (temp > 9)
   {
      temp = temp / 10;  
      d++;
   }

   if (temp > 9)
   {
      temp = temp / 10;
      d++;
   }

   if (temp > 9)
   {
      temp = temp / 10;
      d++;
   }

   System.out.println(n + " can be expressed in " + d + " digits");
}

Repeating code four times, even using copy/paste, is tedious, and the solution works only for n <= 9999.

Here is the same program, with a while loop:


/**
   Count number of digits needed to express an integer in base 10
   using while loop.
*/
public static void main(String[] args)
{
   Scanner in = new Scanner(System.in);
   System.out.print("Input an integer: ");
   int n = in.nextInt();

   int d = 1;
   int temp = n;

   while (temp > 9)
   {
      temp = temp / 10;
      d++;
   }

   System.out.println(n + " can be expressed in " + d + " digits");
}

The fractions 1/2, 1/4, 1/8 get closer and closer to 0 as they are divided by two. Change the previous program to count the number of divisions by two needed to be within 0.0001 of zero.

Loop Termination

2.

 Which values of year cause the following loop to terminate?

/**
   Count the number of years from a user-input year until the year 3000.
*/
public static void main(String[] args)
{
   int millennium = 3000;
   Scanner in = new Scanner(System.in);
   System.out.print("Please enter the current year: ");

   int year = in.nextInt();
   int nyear = year;

   while (nyear != millennium)
   {
      nyear++;
   }

   System.out.println("Another " + (nyear - year) + " years to the millennium.");
}


3.

Re-write the preceding while loop so that it will terminate for any integer input.

for Loops

4.

 A variable that counts the iterations of a loop is called a loop index or loop control variable. In the preceding examples nyear serves as an index, counting the number of years to the next millennium. This type of loop is frequently written using a for loop.
for (initialization; condition; update)
    statement

Write a program controlled by two (non-nested) for loops that produces the following listing of inclusive dates, from the fifth century B.C. through the fifth century A.D.


Century 5 BC 400-499
Century 4 BC 300-399
Century 3 BC 200-299
Century 2 BC 100-199
Century 1 BC 1-99
Century 1 AD 1-99
Century 2 AD 100-199
Century 3 AD 200-299
Century 4 AD 300-399
Century 5 AD 400-499


5.

Write the same program with a single loop for (i = -5 ; i <= 5 ; i++) and an if in the body of the loop.

Other Loops

6.

 One loop type might be better suited than another to a particular purpose. The following usages are idiomatic.

for

Known number of iterations

while

Unknown number of iterations

do/while

At least one iteration

Convert the following while loop to a do/while loop.


public static void main(String[] args)
{
    Scanner in = new Scanner(System.in);
    int sum = 0;
    int n = 1;

    while (n != 0)
    {
        System.out.print("Please enter a number, 0 to quit: ");
        n = in.nextInt();
        if (n != 0)
        {
            sum = sum + n;
            System.out.println("Sum = " + sum);
        }
    }
}



7.

Is this an improvement? Why or why not?



8.

Convert the while loop to a for loop.


/**
    Program to compute the first integral power to which 2 can be
    raised that is greater than that multiple of a given integer.
*/
public static void main(String[] args)
{
    Scanner in = new Scanner(System.in);
    System.out.print("Please enter a number, 0 to quit: ");
    int n = in.nextInt();

    int i = 1;
    while (n * n > Math.pow(2,i))
    {
        i++;
    }

    System.out.println("2 raised to " + i
            + " is the first power of two greater than " + n + " squared");
    }
}


9.

Convert to a while loop:


public static void main(String[] args)
{
    for (int i = 1; i <= 10; i++)
    {
        System.out.println(i + " squared equals " + i * i);
    }
}


Tracing Loops

10.

 What is the output of the following loop?

for (int i = 0; i < 5; i++)
{
    System.out.print(i + " ");
}



11.

Leave the loop as it is and change only the expression inside System.out.print so that the program will display "1 2 3 4 5 ".

What change do you make to the argument of System.out.print?



12.

What is the output of the following loop?


int decimals = 1;
while (decimals < 100000)
{
    System.out.print(decimals + " ");
    decimals = decimals * 10;
}



13.

Leave the loop as it is and change only the expression inside System.out.print so that the program will display "1 2 3 4 5 ".

What change do you make to the argument of System.out.print?



14.

What is the output of the following loop?

int i = 5;
do
{
    System.out.print(i + " ");
    i--;
}
while( i > 0 );


Nested Loops

15.

 Write a program to draw a top view of 24 soda cans, that is 24 circles, arranged in a 4 x 6 grid like this:

What is the code for the SodaCanComponent class?

Random Numbers

16.

 To generate random numbers, you construct an object of the Random class, and then apply one of the following methods:

nextInt(n): A random integer between the integers 0 (inclusive) and n (exclusive)

nextDouble(): A random floating-point number between 0 (inclusive) and 1 (exclusive)

Write a program that simulates the drawing of one playing card (for example, Ace of Spades).