Thread: how to write a function *method* in Java to reverse an Array using Groovy?

This is my solution:

/*3)    Write a function called reverse that takes a char array and returns the array in reverse order.*/
 
char [] band = char [4];
band [0] = "I";
band [1] = "N";
band [2] = "X";
band [3] = "S";
 
band [] = reverse (band []);
 
def char [] reverse (char [] favband) {
 //char [] favband = new char [band.length];
 
 char [] newarray = new char [band.length];
 
 for (int i = band.length; i >=0; i--) {
 
  for (int j =0; j < band.length; j++) {
  favband [i] = newarray [j];
  }
 }
  return favband;
}

I keep updating the code but nothing helps. Thx

This is a Java programming forum, not a Groovy programming forum. We can try to help you as far as your algorithm is concerned but understand that to someone like me (someone who knows Java but doesn't know Groovy), I can't help you figure out if you have the syntax and semantics right.

You may want to try your luck with a Groovy-specific forum or over at Stack Overflow where they are more likely to have someone more capable of helping you.

As far as general help goes, what is the program doing? Add a few print statements (or use a debugger) to figure out what the code is doing at each step along the way. Try pretending you are the computer and execute the program by hand on a piece of paper.

It does look vaguely familiar and we'll try to help you, but I just wanted you to understand that our advice won't necessarily be correct concerning semantics and such.

For example,

char [] band = char [4];

In Java this is a compile error. You must use the new keyword to create a new array. I have no idea if that's the same in Groovy or not. I'm not sure if this is what you meant by your program isn't working or not, you didn't say what is actually wrong with the code (are you getting compile errors? Runtime errors? Wrong results? Which ones?).

I can tell though that your algorithm for reversing the array isn't quite right. Try working out by hand with a piece of paper how you would reverse an array.

I keep updating the code but nothing helps

Can you clarify what you mean by this? Are there exceptions being thrown? There are syntax errors within the code you posted, which should result in an exception (for instance the code noted by helloworld)

Originally Posted by Cyclist

but we use Java..

First of all:
Groovy != Java

Then, looking at your "code"

1. There are, by my count, five lines that will cause groovy compiler errors. (Maybe more---I got tired of counting.)
   
   
2. After making corrections to eliminate compiler errors, there is a line that is guaranteed to cause a runtime error.
   
   
3. After making a simple change to eliminate the runtime error, I see that there is an assignment statement that results in the return value being an array of zero bytes.
   
   
4. If that line is corrected, the function returns the original array and not the new array.
   
   
5. If the return statement is corrected you will discover that the returned array is not the reverse of the original array, but has copied the last char of the original array to all positions of the return value.
   
   
6. Etc.

After correcting compiler errors, if you want to see why it is not working, put some println() and/or printf() statements in the code to see what is copying and where. If something happens that you don't understand, post the code and post the exact ouput or error message or whatever.

For example:

      System.out.printf("Setting favband[%d] = newarray[%d] = '%c' (%d decimal)\n", i, j, newarray[j], (int)newarray[j]);
      favband[i] = newarray[j];

Notes:

1. There is various "getting started with groovy" documentation and there are tutorials for groovy, all of which are linked from the groovy mother lode at groovy.codehaus.com

2. There is at least one active forum for Groovy. It's at nabble.com and is linked from the Groovy home page. Perhaps helpers there would be more tuned in to assisting Groovy beginners. I mean, it's OK to post here in the "Other Programming Languages" partition of this Java site, but you may find that there are more eager Groovy helpers on a Groovy forum than there are on a Java forum.

If you do post, then the best chances for getting assistance would be to follow form.

• Post the code. Post the entire code so that we can try to walk you through the debugging process.
  
  
• Tell us exactly what happened---compiler error(s), runtime error(s), not getting expected output, whatever...
  
  
• If there are compiler messages, paste the entire message into your post. Don't abbreviate; don't paraphrase.
  
  
• Tell us exactly what you don't understand about the behavior.

It's also a Good Idea to tell us about your setup: What version of Groovy? What Operating System (and what version)? Sometimes it makes a difference to people who would like to help.


Cheers,

Z

Originally Posted by Starstreak

Try creating a Java class with a main method. Then create a method to handle the reversal and call it from main.

Well, that might serve to test the algorithm, and might be appropriate for a Java programmer learning Groovy, but for Groovy beginners, that public class stuff and public static void main() stuff just serve to clutter up the landscape.

Originally Posted by Starstreak

Declare your array in main as follows: char[] band = { 'I', 'N', 'X', 'S' };

OK for Java; won't work in Groovy. (Have you tried it?)

Originally Posted by Starstreak

(Bear in mind that a char [] uses single quotes ' ' and not " " <-- those are for Strings.)

In Groovy, there are no character literals. Single quotes indicate String literals. Double quotes also indicate String literals. Strings with length = 1 are "coerced" into character constants. So your second example works in Groovy with single quotes or double quotes around the initialization array members. The code from the OP was missing the "new" and that's the only thing about his array declaration/initialization that needed fixing. I'll give the Groovy alternative that looks "almost" like your first example in a minute.

Originally Posted by Starstreak

Ideally you would need a loop to print the result to see if it is correct.

Why would you say that you need a loop to print an array of char?

In Groovy you don't need a loop. You don't even need System.out. You don't even need parentheses. Heck, you don't even need a semicolon

        // Groovy print statement to print out the contents of an array of char looking like a String
        println "band = " + band

I mean, you can put in all of that Java-looking stuff if you want to (as I showed in my example of debugging print statements). Many Java constructs and much Java syntax is "forward-compatible" with Groovy. (And that's not a coincidence.)

Semicolons at the end of statements are optional in Groovy. People with previous experience in languages like C and C++ and Java tend to put them there by force of habit, but they serve no purpose in Groovy itself.


Here's a complete Groovy program except I deleted the part in the function that does the work that assignment expects:

// Groovy program demonstrating two ways to reverse
// the elements of an array of char.
// First way uses List reverse method.
// Second way uses method defined in this program.
// 
//   Zaphod_b
//
 
// Note array initialization syntax differs from Java!
//
// Note that arrays of char get printed as if they
// contained a String.
char [] band = ["I", "N", "X", "S"]
println "Original            : band = " + band
 
// Not what the assignment asked for, but I'll show
// it just for the heck of it.  Don't turn this in!
// It is not what your instructor wants to see.
//
band = Arrays.asList(band).reverse()
println "After List.reverse(): band = " + band
 
// Without any extra class stuff, it uses the locally-defined
// function.
//
// If you wanted to emphasize the local-ness, you
// could write it like the following:
// band = this.reverse(band)
//
band = reverse(band)
println "After this.reverse(): band = " + band
 
//
// Locally-defined function to reverse elements in an array of chars
//
def char [] reverse (char [] favband) {
    //
    char [] whatever// Put your stuff here...
    // Put your stuff here
    // Put your stuff here
    return whatever
}

Output:

Original            : band = INXS
After List.reverse(): band = SXNI
After this.reverse(): band = INXS

Maybe that can be used as a starting point and a testbed.


Bottom line: Although Groovy accepts a lot of Java-looking syntax and some of it even works the way that Java programmers expect, Groovy != Java (If your grandad knows Java, and you show him Groovy, he may even say that it's "Groovy, man!" like we used to say about cool stuff in the 1960's.)

To the Original Poster: Sometimes the first one is the hardest. Once you get the program to compile and print some output, then, if the output is wrong, debugging becomes a "snap." The hard part is when it won't compile or it compiles but crashes before printing anything useful. Debugging "nothing" is harder than debugging "somthing."


Cheers!

Z