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program with java

Answer 1

+1 A:

It's one thing asking for help, quite another pasting a homework question in verbatim and expecting the community to do it for you! Don't expect a decent response if you're not even willing to put any effort in.

alasdairg 2009-11-06 21:08:08

Answer 2

+1 A:

I'd recommend having a look at this...

http://java.sun.com/docs/books/tutorial/

Mark Iliffe 2009-11-06 21:36:25

Answer 3

A:

This is actually a very instructive assignment. I will first provide the code that follows the given instructions almost exactly, and then discuss it part by part.

public class HarveysHomework {
   static void display(int number) {
      System.out.print(number + "\t");
      int mask = 1 << 31;
      for (int i = 1; i <= 32; i++) {
         System.out.print((mask & number) != 0 ? 1 : 0);
         mask >>>= 1;
         if (i % 4 == 0) {
            System.out.print(" ");
         }
      }
      System.out.println();
   }
   public static void main(String[] args) {
      int number = 128;
      while (number >= -128) {
         display(number);
         if (number > 0) {
            number >>= 1;
         } else if (number == 0) {
            number = ~number;
         } else {
            number <<= 1;
         }
      }

      System.out.print("Input any number: ");
      number = new java.util.Scanner(System.in).nextInt();
      display(number);
   }
}

Here's where the code differs from the specification:

the implied third if in the while from the specification is omitted since it's not necessary
usage of java.util.Scanner is not explicitly in the specification
the condition for the ternary operator is re-parenthesized to follow precedence rules
right shifting of mask was missing from the specification
the use of % is not from the specification

It's important to learn the process from doing this assignment. First and foremost, get Eclipse; it makes your life so much easier (many homework questions on stackoverflow could've been avoided if students would just use an IDE).

Then here's what I did:

stub class with empty display and main
stub display to only println(number);
write the while in main as specified
- RUN IT NOW make sure it's correct
write the last 3 lines of main
- RUN IT NOW make sure it's correct
now finish up display
- write everything but the for loop body
- now finish up the loop body
  - ignore the space issue for now, just write out the rest
    - RUN IT NOW make sure it's correct
  - OOOPS! I used arithmetic right shift on mask! Fix it to use logical right shift!
    - RUN IT NOW make sure it's correct
  - now add the spacing code
    - RUN IT NOW make sure it's correct
DONE!!!

It should be noted that what I did chronologically mimics the progression of details laid out in the specification.

Now let's talk about some bits not mentioned in the specification:

mask >>>= 1;

This is missing from the specification, but if you understood the assignment, you'd have figured it out for yourself. The idea is that mask is being used to test if each bit of number is 0 or 1, going from bit 31 to bit 0. An easy way to do this is to initialize mask = 1 << 31; as specified, then right shift it one at a time.

Logical right shift is necessary here because you do not want any sign extensions (note that bit 31 in a Java int is the sign bit). You want mask to have only one bit set, at the position you're testing for. This is why the logical right shift is used.

This bit of code is also not explicitly from the specification:

if (i % 4 == 0) //...

This is a standard trick for these scenarios. In the for loop, i goes from 1 to 32, and (i % 4 == 0) precisely when i == 4, 8, 16, 32. Not coincidentally, that's precisely when you want to insert the single spaces.

You could, in the spirit of the assignment, use bit masking operation instead:

if ((i & 3) == 0) //...

Here, we check the lowest 2 bits of i to see if they're both zeroes. This is a sufficient and necessary condition for a number to be divisible by 4.

Now let's address some of your remarks:

If I set int number = 128 and perform the while loop to run until >-128 I would have thought it would run the numbers 128 thru -128

There are some crucial mistakes in your statements. The problem statement asks that your program loop "while number is >= -128". This is not the same as performing the loop "until (the number is) >-128". It would've been correct had you said it performs the loop "until (the number is)<-128", but regardless, this is evidence why it's best not to think inuntil` logic in the first place.

Some programming languages have an until construct; Java only has while and for. while andforare similar in that they loop as long as the condition istrue. Anuntilis the exact opposite: it loops as long as the condition isfalse` instead.

until can be seen as a syntactic sugar for while with the terminating condition negated. That is, had Java had until, the following two loops would've been equivalent:

do {
   // something
} until (timeToStop);

do {
   // something
} while (!timeToStop);

Perhaps it was decided that until is therefore unnecessary, and is likely to cause confusion instead for being the odd one out.

Your second crucial mistake is thinking that the loop would "run the numbers 128 thru -128" because of the way it's set up. Keep in mind that the while loop at its core doesn't have a concept of having to go through a whole range of numbers, or repeating something some N number of times; it just does what it's asked to do while something is true! IT'S YOUR JOB to craft the loop so that it does what you want the number of times that you want, and that it terminates when it does.

Look at this simple example:

int number = 128;
while (number >= -128) {
   display(number);
   number = -1000;
}

Here just like you said, we initialize number = 128 and we loop while (number >= -128), but it doesn't run through all numbers from 128 to -128. It just displays one number, 128.

You can't analyze a loop just from its bits and pieces. You have to analyze it as a whole (and yes, sometimes this can be a very difficult task).

In this case, number actually takes these successive values:

128, 64, 32, 16, 8, 4, 2, 1, 0, -1, -2, -4, -8, -16, -32, -64, -128, -256
|_________________________|  |  |__________________________________|   |
       number >>= 1          |                number <<= 1             | 
                             |                                        exit
                      number = ~number
                      (see reference)

References:

Java Language Specification

JLS 15.19 Shift Operators
- "The value of n>>s is n right-shifted s bit positions with sign-extension"
- "The value of n>>>s is n right-shifted s bit positions with zero-extension"
JLS 15.17.3 Remainder Operator %
- "The binary % operator is said to yield the remainder of its operands from an implied division; the left-hand operand is the dividend and the right-hand operand is the divisor"
JLS 15.15.5 Bitwise Complement Operator
- "note that, in all cases, ~x equals (-x)-1"

On bit manipulations

"Why ~0 == -1 ?" (on stackoverflow)
Bit manipulation (on wikipedia)
Bit twiddling hacks (the one and only!)

polygenelubricants 2010-03-31 09:12:33

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program with java

References:

Java Language Specification

On bit manipulations

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