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Handy Iterable For A Range Of Integers

04.10.2007

| 5135 views |

        Looping a specific number of times produces code a bit too verbose in Java.  The JDK5 enhanced for statement is a handy improvement, but you still have to fall back to the traditional for statement if you want to repeat a loop, say, 10 times.

Not with this implementation of Iterable.  You can use it like this:

for (int i : new Range(100))
    System.out.printf("I have said this %d times. Read dzone.com!%n", i);

class Range implements Iterable<Integer> {
    
    private final Integer stop;
    
    public Range(int stop) {
        this.stop = stop;
    }

    public Iterator<Integer> iterator() {
        return new Iterator<Integer>() {
            
            private Integer counter = 0;

            public boolean hasNext() {
                return (counter != stop);
            }

            public Integer next() {
                if (counter == stop)
                    throw new NoSuchElementException();
                return ++counter;
            }

            public void remove() {
            }};
    }
    
}

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Comments

Snippets Manager replied on Fri, 2007/04/20 - 4:31am

Sometime ago I realized the same problem. I posted the source code here; http://www.tutego.com/blog/javainsel/2007/04/range-in-java.html and for short:


package com.tutego;

import static com.tutego.Range.range;

public class RangeDemo
{
  public static void main( String[] args )
  {
    for ( int i : range( 10 ) )
      System.out.print( i + " " );

    System.out.println();

    for ( int i : range( 5, 10 ) )
      System.out.print( i + " " );

    System.out.println();

    for ( int i : range( 0, 10, 3 ) )
      System.out.print( i + " " );

    System.out.println();

    for ( int i : range( '0', '9' ) )
      System.out.print( (char) i  );

    System.out.println();

    String[] a = { "Mary", "had", "a", "little", "lamb" };
    for ( int i : range(a.length ) )
      System.out.printf( "%d %s%n", i, a[i] );
  }
}

And the main class Range


/*
 * This project is made available under the terms of the BSD license, more information can be found at
 * http://www.opensource.org/licenses/bsd-license.html
 *
 * Copyright (c) 2007. Christian Ullenboom (http://www.tutego.com/) and contributors. All rights reserved.
 */
package com.tutego;

import java.util.Iterator;

/**
 * Class that generates immutable sequences (ranges) as Iterable
 * objects. A range represents a start (0 if not given), an stop (mandatory) and
 * an optional step (1 by default). The start value is included in the range,
 * the stop value is exclusive. Every range is handled by an Iterable
 * which can by used in an extended for loop.
 * 
 *  * for ( int i : range( 0, 10, 3 ) )
 *   System.out.print( i + " " ); // 0 3 6 9
 * 
 * 
 * @author Christian Ullenboom (tutego)
 * @version 1.0
 */
public class Range
{
  public static Iterable range( final int start, final int stop, final int step )
  {
    if ( step <= 0 )
      throw new IllegalArgumentException( "step > 0 isrequired!" );

    return new Iterable()
    {
      public Iterator iterator()
      {
        return new Iterator()
        {
          private int counter = start;

          public boolean hasNext()
          {
            return counter < stop;
          }

          public Integer next()
          {
            try
            {
              return counter;
            }
            finally { counter += step; }
          }

          public void remove() { }
        };
      }
    };
  }

  public static Iterable range( final int start, final int stop )
  {
    return range( start, stop, 1 );
  }

  public static Iterable range( final int stop )
  {
    return range( 0, stop, 1 );
  }
}

Alan Moore replied on Sat, 2007/04/14 - 7:53pm

Nice tip, but there's a problem. Use an upper bound of 128 instead of 100, and you'll get an infinite loop. That's because you're doing an identity comparison on auto-boxed Integer objects using == and !=. You get away with it for values from -128 to 127 because they're cached, but they are objects, so you should be using equals(). However, a better solution would be to declare the "stop" and "count" fields as ints, not Integers; that will make it easier to add support for negative bounds and "step" increments greater than one.